# Commands

## Math Functions

atan2(Y,X)
Returns the arctangent of Y/X in the range
cos(EXPR)
cos EXPR
cos
Returns the cosine of EXPR (expressed in radians). If EXPR is omitted takes cosine of `\$_'.
exp(EXPR)
exp EXPR
exp
Returns `e' to the power of EXPR. If EXPR is omitted, gives `exp(\$_)`.
hex(EXPR)
hex EXPR
hex
Returns the decimal value of EXPR interpreted as an hex string. (To interpret strings that might start with `0' or `0x' see `oct()`.) If EXPR is omitted, uses `\$_'.
int(EXPR)
int EXPR
int
Returns the integer portion of EXPR. If EXPR is omitted, uses `\$_'.
log(EXPR)
log EXPR
log
Returns logarithm (base `e') of EXPR. If EXPR is omitted, returns log of `\$_'.
oct(EXPR)
oct EXPR
oct
Returns the decimal value of EXPR interpreted as an octal string. (If EXPR happens to start off with `0x', interprets it as a hex string instead.) The following will handle decimal, octal and hex in the standard notation:
```\$val = oct(\$val) if \$val =~ /^0/;
```
If EXPR is omitted, uses `\$_'.
sin(EXPR)
sin EXPR
sin
Returns the sine of EXPR (expressed in radians). If EXPR is omitted, returns sine of `\$_'.
sqrt(EXPR)
sqrt EXPR
sqrt
Return the square root of EXPR. If EXPR is omitted, returns square root of `\$_'.

## Structure Conversion

pack(TEMPLATE,LIST)
Takes an array or list of values and packs it into a binary structure, returning the string containing the structure. The TEMPLATE is a sequence of characters that give the order and type of values, as follows:
```A       An ascii string, will be space padded.
a       An ascii string, will be null padded.
c       A signed char value.
C       An unsigned char value.
s       A signed short value.
S       An unsigned short value.
i       A signed integer value.
I       An unsigned integer value.
l       A signed long value.
L       An unsigned long value.
n       A short in `network' order.
N       A long in `network' order.
f       A single-precision float in the native format.
d       A double-precision float in the native format.
p       A pointer to a string.
v       A short in `VAX' (little-endian) order.
V       A long in `VAX' (little-endian) order.
x       A null byte.
X       Back up a byte.
@       Null fill to absolute position.
u       A uuencoded string.
b       A bit string (ascending bit order, like vec()).
B       A bit string (descending bit order).
h       A hex string (low nybble first).
H       A hex string (high nybble first).
```
Each letter may optionally be followed by a number which gives a repeat count. With all types except `a', `A', `b', `B', `h', and `H', the pack function will gobble up that many values from the LIST. A `*' for the repeat count means to use however many items are left. The `a' and `A' types gobble just one value, but pack it as a string of length count, padding with nulls or spaces as necessary. (When unpacking, `A' strips trailing spaces and nulls, but `a' does not.) Likewise, the `b' and `B' fields pack a string that many bits long. The `h' and `H' fields pack a string that many nybbles long. Real numbers (floats and doubles) are in the native machine format only; due to the multiplicity of floating formats around, and the lack of a standard "network" representation, no facility for interchange has been made. This means that packed floating point data written on one machine may not be readable on another - even if both use IEEE floating point arithmetic (as the endian-ness of the memory representation is not part of the IEEE spec). Note that perl uses doubles internally for all numeric calculation, and converting from double to float back to double will lose precision (i.e. `unpack("f", pack("f", \$foo))' will not in general equal `\$foo'). Examples:
```\$foo = pack("cccc",65,66,67,68);
# foo eq "ABCD"
\$foo = pack("c4",65,66,67,68);
# same thing

\$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"

\$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian

\$foo = pack("a4","abcd","x","y","z");
# "abcd"

\$foo = pack("aaaa","abcd","x","y","z");
# "axyz"

\$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"

\$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)

sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
```
The same template may generally also be used in the `unpack` function.
unpack(TEMPLATE,EXPR)
`unpack` does the reverse of `pack`: it takes a string representing a structure and expands it out into an array value, returning the array value. (In a scalar context, it merely returns the first value produced.) The TEMPLATE has the same format as in the `pack` function. Here's a subroutine that does substring:
```sub substr {
local(\$what,\$where,\$howmuch) = @_;
unpack("x\$where a\$howmuch", \$what);
}
```
and then there's
```sub ord { unpack("c",\$_[0]); }
```
In addition, you may prefix a field with a `%<number>' to indicate that you want a <number>-bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. For example, the following computes the same number as the System V sum program:
```while (<>) {
\$checksum += unpack("%16C*", \$_);
}
\$checksum %= 65536;
```

## String Functions

chop(LIST)
chop(VARIABLE)
chop VARIABLE
chop
Chops off the last character of a string and returns the character chopped. It's used primarily to remove the newline from the end of an input record, but is much more efficient than `s/\n//' because it neither scans nor copies the string. If VARIABLE is omitted, chops `\$_'. Example:
```while (<>) {
chop;   # avoid \n on last field
@array = split(/:/);
...
}
```
You can actually chop anything that's an lvalue, including an assignment:
```chop(\$cwd = `pwd`);
```
If you chop a list, each element is chopped. Only the value of the last chop is returned.
crypt(PLAINTEXT,SALT)
Encrypts a string exactly like the `crypt()` function in the C library. Useful for checking the password file for lousy passwords. Only the guys wearing white hats should do this.
index(STR,SUBSTR,POSITION)
index(STR,SUBSTR)
Returns the position of the first occurrence of SUBSTR in STR at or after POSITION. If POSITION is omitted, starts searching from the beginning of the string. The return value is based at 0, or whatever you've set the `\$[' variable to. If the substring is not found, returns one less than the base, ordinarily -1.
length(EXPR)
length EXPR
length
Returns the length in characters of the value of EXPR. If EXPR is omitted, returns length of `\$_'.
rindex(STR,SUBSTR,POSITION)
rindex(STR,SUBSTR)
Works just like `index` except that it returns the position of the LAST occurrence of SUBSTR in STR. If POSITION is specified, returns the last occurrence at or before that position.
substr(EXPR,OFFSET,LEN)
substr(EXPR,OFFSET)
Extracts a substring out of EXPR and returns it. First character is at offset 0, or whatever you've set `\$[' to. If OFFSET is negative, starts that far from the end of the string. If LEN is omitted, returns everything to the end of the string. You can use the `substr()` function as an lvalue, in which case EXPR must be an lvalue. If you assign something shorter than LEN, the string will shrink, and if you assign something longer than LEN, the string will grow to accommodate it. To keep the string the same length you may need to pad or chop your value using `sprintf()`.

## Array and List Functions

delete \$ASSOC{KEY}
Deletes the specified value from the specified associative array. Returns the deleted value, or the undefined value if nothing was deleted. Deleting from \$ENV{} modifies the environment. Deleting from an array bound to a dbm file deletes the entry from the dbm file. The following deletes all the values of an associative array:
```foreach \$key (keys %ARRAY) {
delete \$ARRAY{\$key};
}
```
(But it would be faster to use the `reset` command. Saying `undef %ARRAY' is faster yet.)
each(ASSOC_ARRAY)
each ASSOC_ARRAY
Returns a 2 element array consisting of the key and value for the next value of an associative array, so that you can iterate over it. Entries are returned in an apparently random order. When the array is entirely read, a null array is returned (which when assigned produces a FALSE (0) value). The next call to `each()` after that will start iterating again. The iterator can be reset only by reading all the elements from the array. You must not modify the array while iterating over it. There is a single iterator for each associative array, shared by all `each()`, `keys()` and `values()` function calls in the program. The following prints out your environment like the `printenv` program, only in a different order:
```while ((\$key,\$value) = each %ENV) {
print "\$key=\$value\n";
}
```
See also `keys()` and `values()`.
grep(EXPR,LIST)
Evaluates EXPR for each element of LIST (locally setting `\$_' to each element) and returns the array value consisting of those elements for which the expression evaluated to true. In a scalar context, returns the number of times the expression was true.
```@foo = grep(!/^#/, @bar);    # weed out comments
```
Note that, since `\$_' is a reference into the array value, it can be used to modify the elements of the array. While this is useful and supported, it can cause bizarre results if the LIST is not a named array.
join(EXPR,LIST)
join(EXPR,ARRAY)
Joins the separate strings of LIST or ARRAY into a single string with fields separated by the value of EXPR, and returns the string. Example:
```\$_ = join(':', \$login,\$passwd,\$uid,\$gid,\$gcos,\$home,\$shell);
```
See `split` function.
keys(ASSOC_ARRAY)
keys ASSOC_ARRAY
Returns a normal array consisting of all the keys of the named associative array. The keys are returned in an apparently random order, but it is the same order as either the `values()` or `each()` function produces (given that the associative array has not been modified). Here is yet another way to print your environment:
```@keys = keys %ENV;
@values = values %ENV;
while (\$#keys >= 0) {
print pop(@keys), '=', pop(@values), "\n";
}
```
or how about sorted by key:
```foreach \$key (sort(keys %ENV)) {
print \$key, '=', \$ENV{\$key}, "\n";
}
```
pop(ARRAY)
pop ARRAY
Pops and returns the last value of the array, shortening the array by 1. Has the same effect as:
```\$tmp = \$ARRAY[\$#ARRAY--];
```
If there are no elements in the array, returns the undefined value.
push(ARRAY,LIST)
Treats ARRAY (`@' is optional) as a stack, and pushes the values of LIST onto the end of ARRAY. The length of ARRAY increases by the length of LIST. Has the same effect as:
```for \$value (LIST) {
\$ARRAY[++\$#ARRAY] = \$value;
}
```
but is more efficient.
reverse(LIST)
reverse LIST
In an array context, returns an array value consisting of the elements of LIST in the opposite order. In a scalar context, returns a string value consisting of the bytes of the first element of LIST in the opposite order.
shift(ARRAY)
shift ARRAY
shift
Shifts the first value of the array off and returns it, shortening the array by 1 and moving everything down. If there are no elements in the array, returns the undefined value. If ARRAY is omitted, shifts the `@ARGV' array in the main program, and the `@_' array in subroutines. (This is determined lexically.) See also `unshift()`, `push()` and `pop()`. `shift()` and `unshift()` do the same thing to the left end of an array that `push()` and `pop()` do to the right end.
sort(SUBROUTINE LIST)
sort(LIST)
sort SUBROUTINE LIST
sort BLOCK LIST
sort LIST
Sorts the LIST and returns the sorted array value. Nonexistent values of arrays are stripped out. If SUBROUTINE or BLOCK is omitted, sorts in standard string comparison order. If SUBROUTINE is specified, gives the name of a subroutine that returns an integer less than, equal to, or greater than 0, depending on how the elements of the array are to be ordered. (The `<=>` and `cmp` operators are extremely useful in such routines.) SUBROUTINE may be a scalar variable name, in which case the value provides the name of the subroutine to use. In place of a SUBROUTINE name, you can provide a BLOCK as an anonymous, in-line sort subroutine. In the interests of efficiency the normal calling code for subroutines is bypassed, with the following effects: the subroutine may not be a recursive subroutine, and the two elements to be compared are passed into the subroutine not via `@_' but as `\$a' and `\$b' (see example below). They are passed by reference so don't modify `\$a' and `\$b'. Examples:
```# sort lexically
@articles = sort @files;

# same thing, but with explicit sort routine
@articles = sort { \$a cmp \$b } @files;

# same thing in reversed order
@articles = sort { \$b cmp \$a } @files;

# sort numerically ascending
@articles = sort { \$a <=> \$b } @files;

# sort numerically descending
@articles = sort { \$b <=> \$a } @files;

# sort using explicit subroutine name
sub byage {
\$age{\$a} <=> \$age{\$b};    # presuming integers
}
@sortedclass = sort byage @class;

sub reverse { \$b cmp \$a; }
@harry = ('dog','cat','x','Cain','Abel');
@george = ('gone','chased','yz','Punished','Axed');
print sort @harry;
# prints AbelCaincatdogx
print sort reverse @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
```
splice(ARRAY,OFFSET,LENGTH,LIST)
splice(ARRAY,OFFSET,LENGTH)
splice(ARRAY,OFFSET)
Removes the elements designated by OFFSET and LENGTH from an array, and replaces them with the elements of LIST, if any. Returns the elements removed from the array. The array grows or shrinks as necessary. If LENGTH is omitted, removes everything from OFFSET onward. The following equivalencies hold (assuming `\$[ == 0`):
```push(@a,\$x,\$y)         splice(@a,\$#x+1,0,\$x,\$y)
pop(@a)                splice(@a,-1)
shift(@a)              splice(@a,0,1)
unshift(@a,\$x,\$y)      splice(@a,0,0,\$x,\$y)
\$a[\$x] = \$y            splice(@a,\$x,1,\$y);
```
Example, assuming array lengths are passed before arrays:
```sub aeq {      # compare two array values
local(@a) = splice(@_,0,shift);
local(@b) = splice(@_,0,shift);
return 0 unless @a == @b;     # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq(\$len,@foo[1..\$len],0+@bar,@bar)) { ... }
```
split(/PATTERN/,EXPR,LIMIT)
split(/PATTERN/,EXPR)
split(/PATTERN/)
split
Splits a string into an array of strings, and returns it. (If not in an array context, returns the number of fields found and splits into the `@_' array. (In an array context, you can force the split into `@_' by using `??' as the pattern delimiters, but it still returns the array value.)) If EXPR is omitted, splits the `\$_' string. If PATTERN is also omitted, splits on whitespace (`/[\t\n]+/'). Anything matching PATTERN is taken to be a delimiter separating the fields. (Note that the delimiter may be longer than one character.) If LIMIT is specified, splits into no more than that many fields (though it may split into fewer). If LIMIT is unspecified, trailing null fields are stripped (which potential users of `pop()` would do well to remember). A pattern matching the null string (not to be confused with a null pattern `//', which is just one member of the set of patterns matching a null string) will split the value of EXPR into separate characters at each point it matches that way. For example:
```print join(':', split(/ */, 'hi there'));
```
produces the output `h:i:t:h:e:r:e'. The LIMIT parameter can be used to partially split a line
```(\$login, \$passwd, \$remainder) = split(/:/, \$_, 3);
```
(When assigning to a list, if LIMIT is omitted, perl supplies a LIMIT one larger than the number of variables in the list, to avoid unnecessary work. For the list above LIMIT would have been 4 by default. In time critical applications it behooves you not to split into more fields than you really need.) If the PATTERN contains parentheses, additional array elements are created from each matching substring in the delimiter.
```split(/([,-])/,"1-10,20");
```
produces the array value
```(1,'-',10,',',20)
```
The pattern /PATTERN/ may be replaced with an expression to specify patterns that vary at runtime. (To do runtime compilation only once, use `/\$variable/o'.) As a special case, specifying a space (' ') will split on white space just as split with no arguments does, but leading white space does NOT produce a null first field. Thus, split(' ') can be used to emulate `awk`'s default behavior, whereas `split(/ /)' will give you as many null initial fields as there are leading spaces. Example:
```open(passwd, '/etc/passwd');
while (<passwd>) {
(\$login, \$passwd, \$uid, \$gid, \$gcos, \$home, \$shell)
= split(/:/);
...
}
```
(Note that `\$shell' above will still have a newline on it. See `chop()`.) See also `join`.
unshift(ARRAY,LIST)
Does the opposite of a `shift`. Or the opposite of a `push`, depending on how you look at it. Prepends list to the front of the array, and returns the number of elements in the new array.
```unshift(ARGV, '-e') unless \$ARGV[0] =~ /^-/;
```
values(ASSOC_ARRAY)
values ASSOC_ARRAY
Returns a normal array consisting of all the values of the named associative array. The values are returned in an apparently random order, but it is the same order as either the `keys()` or `each()` function would produce on the same array. See also `keys()` and `each()`.

## File Operations

chmod(LIST)
chmod LIST
Changes the permissions of a list of files. The first element of the list must be the numerical mode. Returns the number of files successfully changed.
```\$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
```
chown(LIST)
chown LIST
Changes the owner (and group) of a list of files. The first two elements of the list must be the NUMERICAL uid and gid, in that order. Returns the number of files successfully changed.
```\$cnt = chown \$uid, \$gid, 'foo', 'bar';
chown \$uid, \$gid, @filenames;
```
Here's an example that looks up non-numeric uids in the passwd file:
```print "User: ";
\$user = <STDIN>;
chop(\$user);
print "Files: "
\$pattern = <STDIN>;
chop(\$pattern);
open(pass, '/etc/passwd') || die "Can't open passwd: \$!\n";
while (<pass>) {
}
@ary = <\${pattern}>; # get filenames
if (\$uid{\$user} eq '') {
die "\$user not in passwd file";
}
else {
chown \$uid{\$user}, \$gid{\$user}, @ary;
}
```
fcntl(FILEHANDLE,FUNCTION,SCALAR)
Implements the `fcntl(2)' function. You'll probably have to say:
```require "fcntl.ph";   # probably /usr/local/lib/perl/fcntl.ph
```
first to get the correct function definitions. If `fcntl.ph' doesn't exist or doesn't have the correct definitions you'll have to roll your own, based on your C header files such as `<sys/fcntl.h>'. (There is a perl script called `h2ph' that comes with the perl kit which may help you in this.) Argument processing and value return works just like `ioctl` below. Note that `fcntl` will produce a fatal error if used on a machine that doesn't implement `fcntl(2)'.
fileno(FILEHANDLE)
fileno FILEHANDLE
Returns the file descriptor for a filehandle. Useful for constructing bitmaps for `select()`. If FILEHANDLE is an expression, the value is taken as the name of the filehandle.
flock(FILEHANDLE,OPERATION)
Calls `flock(2)' on FILEHANDLE. See manual page for `flock(2)' for definition of OPERATION. Returns true for success, false on failure. Will produce a fatal error if used on a machine that doesn't implement `flock(2)'. Here's a mailbox appender for BSD systems.
```\$LOCK_SH = 1;
\$LOCK_EX = 2;
\$LOCK_NB = 4;
\$LOCK_UN = 8;

sub lock {
flock(MBOX,\$LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}

sub unlock {
flock(MBOX,\$LOCK_UN);
}

open(MBOX, ">>/usr/spool/mail/\$ENV{'USER'}")
|| die "Can't open mailbox: \$!";

do lock();
print MBOX \$msg,"\n\n";
do unlock();
```
Creates a new filename linked to the old filename. Returns 1 for success, 0 otherwise.
lstat(FILEHANDLE)
lstat FILEHANDLE
lstat(EXPR)
lstat SCALARVARIABLE
Does the same thing as the `stat()` function, but stats a symbolic link instead of the file the symbolic link points to. If symbolic links are unimplemented on your system, a normal stat is done.
Returns the value of a symbolic link, if symbolic links are implemented. If not, gives a fatal error. If there is some system error, returns the undefined value and sets `\$!' (errno). If EXPR is omitted, uses `\$_'.
rename(OLDNAME,NEWNAME)
Changes the name of a file. Returns 1 for success, 0 otherwise. Will not work across filesystem boundaries.
stat(FILEHANDLE)
stat FILEHANDLE
stat(EXPR)
stat SCALARVARIABLE
Returns a 13-element array giving the statistics for a file, either the file opened via FILEHANDLE, or named by EXPR. Returns a null list if the `stat` fails. Typically used as follows:
```(\$dev,\$ino,\$mode,\$nlink,\$uid,\$gid,\$rdev,\$size,
\$atime,\$mtime,\$ctime,\$blksize,\$blocks)
= stat(\$filename);
```
If `stat` is passed the special filehandle consisting of an underline (`_'), no stat is done, but the current contents of the stat structure from the last stat or filetest are returned. Example:
```if (-x \$file && ((\$d) = stat(_)) && \$d < 0) {
print "\$file is executable NFS file\n";
}
```
(This only works on machines for which the device number is negative under NFS.)
Creates a new filename symbolically linked to the old filename. Returns 1 for success, 0 otherwise. On systems that don't support symbolic links, produces a fatal error at run time. To check for that, use eval:
```\$symlink_exists = (eval 'symlink("","");', \$@ eq '');
```
truncate(FILEHANDLE,LENGTH)
truncate(EXPR,LENGTH)
Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length. Produces a fatal error if `truncate` isn't implemented on your system.
Deletes a list of files. If EXPR is not specified, deletes file specified by `\$_'. Returns the number of files successfully deleted.
```\$cnt = unlink 'a', 'b', 'c';
```
Note: unlink will not delete directories unless you are superuser and the `-U' flag is supplied to perl. Even if these conditions are met, be warned that unlinking a directory can inflict damage on your filesystem. Use `rmdir` instead.
utime(LIST)
utime LIST
Changes the access and modification times on each file of a list of files. The first two elements of the list must be the NUMERICAL access and modification times, in that order. Returns the number of files successfully changed. The inode modification time of each file is set to the current time. Example of a "touch" command:
```#!/usr/bin/perl
\$now = time;
utime \$now, \$now, @ARGV;
```

chdir(EXPR)
chdir EXPR
chdir
Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to home directory. Returns 1 upon success, 0 otherwise. See example under `die`.
closedir(DIRHANDLE)
closedir DIRHANDLE
Closes a directory opened by `opendir()`.
mkdir(FILENAME,MODE)
Creates the directory specified by FILENAME, with permissions specified by MODE (as modified by `umask`). If it succeeds it returns 1, otherwise it returns 0 and sets `\$!' (errno).
opendir(DIRHANDLE,EXPR)
Opens a directory named EXPR for processing by `readdir()`, `telldir()`, `seekdir()`, `rewinddir()` and `closedir()`. Returns true if successful. DIRHANDLEs have their own namespace separate from FILEHANDLEs.
Returns the next directory entry for a directory opened by `opendir()`. If used in an array context, returns all the rest of the entries in the directory. If there are no more entries, returns an undefined value in a scalar context or a null list in an array context.
rewinddir(DIRHANDLE)
rewinddir DIRHANDLE
Sets the current position to the beginning of the directory for the `readdir()` routine on DIRHANDLE.
rmdir(FILENAME)
rmdir FILENAME
rmdir
Deletes the directory specified by FILENAME if it is empty. If it succeeds it returns 1, otherwise it returns 0 and sets `\$!' (errno). If FILENAME is omitted, uses `\$_'.
seekdir(DIRHANDLE,POS)
Sets the current position for the `readdir()` routine on DIRHANDLE. POS must be a value returned by `telldir()`. Has the same caveats about possible directory compaction as the corresponding system library routine.
telldir(DIRHANDLE)
telldir DIRHANDLE
Returns the current position of the `readdir()` routines on DIRHANDLE. Value may be given to `seekdir()` to access a particular location in a directory. Has the same caveats about possible directory compaction as the corresponding system library routine.

## Input/Output

binmode(FILEHANDLE)
binmode FILEHANDLE
Arranges for the file to be read in binary mode in operating systems that distinguish between binary and text files. Files that are not read in binary mode have CR LF sequences translated to LF on input and LF translated to CR LF on output. `binmode` has no effect under Unix. If FILEHANDLE is an expression, the value is taken as the name of the filehandle.
close(FILEHANDLE)
close FILEHANDLE
Closes the file or pipe associated with the file handle. You don't have to close FILEHANDLE if you are immediately going to do another open on it, since open will close it for you. (See `open`.) However, an explicit close on an input file resets the line counter (`\$.'), while the implicit close done by `open` does not. Also, closing a pipe will wait for the process executing on the pipe to complete, in case you want to look at the output of the pipe afterwards. Closing a pipe explicitly also puts the status value of the command into `\$?'. Example:
```open(OUTPUT, '|sort >foo');     # pipe to sort
...         # print stuff to output
close OUTPUT;           # wait for sort to finish
open(INPUT, 'foo');     # get sort's results
```
FILEHANDLE may be an expression whose value gives the real filehandle name.
eof(FILEHANDLE)
eof()
eof
Returns 1 if the next read on FILEHANDLE will return end of file, or if FILEHANDLE is not open. FILEHANDLE may be an expression whose value gives the real filehandle name. (Note that this function actually reads a character and the `ungetc`'s it, so it is not very useful in an interactive context.) An `eof` without an argument returns the eof status for the last file read. Empty parentheses `()' may be used to indicate the pseudo file formed of the files listed on the command line, i.e. `eof()' is reasonable to use inside a `while (<>)' loop to detect the end of only the last file. Use `eof(ARGV)' or `eof` without the parentheses to test EACH file in a `while (<>)' loop. Examples:
```# insert dashes just before last line of last file
while (<>) {
if (eof()) {
print "--------------\n";
}
print;
}

# reset line numbering on each input file
while (<>) {
print "\$.\t\$_";
if (eof) {     # Not eof().
close(ARGV);
}
}
```
getc(FILEHANDLE)
getc FILEHANDLE
getc
Returns the next character from the input file attached to FILEHANDLE, or a null string at EOF. If FILEHANDLE is omitted, reads from STDIN.
open(FILEHANDLE,EXPR)
open(FILEHANDLE)
open FILEHANDLE
Opens the file whose filename is given by EXPR, and associates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. If EXPR is omitted, the scalar variable of the same name as the FILEHANDLE contains the filename. If the filename begins with `<' or nothing, the file is opened for input. If the filename begins with `>', the file is opened for output. If the filename begins with `>>', the file is opened for appending. (You can put a `+' in front of the `>' or `<' to indicate that you want both read and write access to the file.) If the filename begins with `|', the filename is interpreted as a command to which output is to be piped, and if the filename ends with a `|', the filename is interpreted as command which pipes input to us. (You may not have a command that pipes both in and out.) Opening `-' opens `STDIN' and opening `>-' opens `STDOUT'. `open` returns non-zero upon success, the undefined value otherwise. If the open involved a pipe, the return value happens to be the pid of the subprocess. Examples:
```\$article = 100;
open article || die "Can't find article \$article: \$!\n";
while (<article>) {...

open(LOG, '>>/usr/spool/news/twitlog');
# (log is reserved)

open(article, "caesar <\$article |");
# decrypt article

open(extract, "|sort >/tmp/Tmp\$\$");
# \$\$ is our process#

# process argument list of files along with any includes

foreach \$file (@ARGV) {
do process(\$file, 'fh00');      # no pun intended
}

sub process {
local(\$filename, \$input) = @_;
\$input++;               # this is a string increment
unless (open(\$input, \$filename)) {
print STDERR "Can't open \$filename: \$!\n";
return;
}
while (<\$input>) {     # note the use of indirection
if (/^#include "(.*)"/) {
do process(\$1, \$input);
next;
}
...         # whatever
}
}
```
You may also, in the Bourne shell tradition, specify an EXPR beginning with `>&', in which case the rest of the string is interpreted as the name of a filehandle (or file descriptor, if numeric) which is to be duped and opened. You may use `&' after `>', `>>', `<', `+>', `+>>' and `+<'. The mode you specify should match the mode of the original filehandle. Here is a script that saves, redirects, and restores `STDOUT' and `STDERR':
```#!/usr/bin/perl
open(SAVEOUT, ">&STDOUT");
open(SAVEERR, ">&STDERR");

open(STDOUT, ">foo.out") || die "Can't redirect stdout";
open(STDERR, ">&STDOUT") || die "Can't dup stdout";

select(STDERR); \$| = 1;         # make unbuffered
select(STDOUT); \$| = 1;         # make unbuffered

print STDOUT "stdout 1\n";      # this works for
print STDERR "stderr 1\n";      # subprocesses too

close(STDOUT);
close(STDERR);

open(STDOUT, ">&SAVEOUT");
open(STDERR, ">&SAVEERR");

print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
```
If you open a pipe on the command `-', i.e. either `|-' or `-|', then there is an implicit fork done, and the return value of open is the pid of the child within the parent process, and 0 within the child process. (Use `defined(\$pid)' to determine if the `open` was successful.) The filehandle behaves normally for the parent, but i/o to that filehandle is piped from/to the `STDOUT'/`STDIN' of the child process. In the child process the filehandle isn't opened--i/o happens from/to the new `STDOUT' or `STDIN'. Typically this is used like the normal piped `open` when you want to exercise more control over just how the pipe command gets executed, such as when you are running setuid, and don't want to have to scan shell commands for metacharacters. The following pairs are equivalent:
```open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';

open(FOO, "cat -n '\$file'|");
open(FOO, "-|") || exec 'cat', '-n', \$file;
```
Explicitly closing any piped filehandle causes the parent process to wait for the child to finish, and returns the status value in `\$?'. Note: on any operation which may do a fork, unflushed buffers remain unflushed in both processes, which means you may need to set `\$|' to avoid duplicate output. The filename that is passed to open will have leading and trailing whitespace deleted. In order to open a file with arbitrary weird characters in it, it's necessary to protect any leading and trailing whitespace thusly:
```\$file =~ s#^(\s)#./\$1#;
open(FOO, "< \$file\0");
```
Opens a pair of connected pipes like the corresponding system call. Note that if you set up a loop of piped processes, deadlock can occur unless you are very careful. In addition, note that perl's pipes use stdio buffering, so you may need to set `\$|' to flush your WRITEHANDLE after each command, depending on the application.
[Requires version 3.0 patchlevel 9.]
print(FILEHANDLE LIST)
print(LIST)
print FILEHANDLE LIST
print LIST
print
Prints a string or a comma-separated list of strings. Returns non-zero if successful. FILEHANDLE may be a scalar variable name, in which case the variable contains the name of the filehandle, thus introducing one level of indirection. (NOTE: If FILEHANDLE is a variable and the next token is a term, it may be misinterpreted as an operator unless you interpose a `+' or put parens around the arguments.) If FILEHANDLE is omitted, prints by default to standard output (or to the last selected output channel--see `select()`).If LIST is also omitted, prints `\$_' to `STDOUT'. To set the default output channel to something other than `STDOUT' use the select operation. Note that, because `print` takes a LIST, anything in the LIST is evaluated in an array context, and any subroutine that you call will have one or more of its expressions evaluated in an array context. Also be careful not to follow the `print` keyword with a left parenthesis unless you want the corresponding right parenthesis to terminate the arguments to the `print`---interpose a `+' or put parens around all the arguments.
printf(FILEHANDLE LIST)
printf(LIST)
printf FILEHANDLE LIST
printf LIST
printf
Equivalent to a `print FILEHANDLE sprintf(LIST)'.
Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE. Returns the number of bytes actually read, or `undef` if there was an error. SCALAR will be grown or shrunk to the length actually read. An OFFSET may be specified to place the read data at some other place than the beginning of the string. This call is actually implemented in terms of stdio's `fread` call. To get a true `read` system call, see `sysread`.
select(RBITS,WBITS,EBITS,TIMEOUT)
This calls the select system call with the bitmasks specified, which can be constructed using `fileno()` and `vec()`, along these lines:
```\$rin = \$win = \$ein = '';
vec(\$rin,fileno(STDIN),1) = 1;
vec(\$win,fileno(STDOUT),1) = 1;
\$ein = \$rin | \$win;
```
If you want to select on many filehandles you might wish to write a subroutine:
```sub fhbits {
local(@fhlist) = split(' ',\$_[0]);
local(\$bits);
for (@fhlist) {
vec(\$bits,fileno(\$_),1) = 1;
}
\$bits;
}
\$rin = &fhbits('STDIN TTY SOCK');
```
The usual idiom is:
```(\$nfound,\$timeleft) =
select(\$rout=\$rin, \$wout=\$win, \$eout=\$ein, \$timeout);
```
or to block until something becomes ready:
```\$nfound = select(\$rout=\$rin, \$wout=\$win,
\$eout=\$ein, undef);
```
Any of the bitmasks can also be `undef`. The timeout, if specified, is in seconds, which may be fractional. NOTE: not all implementations are capable of returning the `\$timeleft'. If not, they always return `\$timeleft' equal to the supplied `\$timeout'.
seek(FILEHANDLE,POSITION,WHENCE)
Randomly positions the file pointer for FILEHANDLE, just like the `fseek()` call of stdio. FILEHANDLE may be an expression whose value gives the name of the filehandle. Returns 1 upon success, 0 otherwise.
select(FILEHANDLE)
select
Returns the currently selected filehandle. Sets the current default filehandle for output, if FILEHANDLE is supplied. This has two effects: first, a `write` or a `print` without a filehandle will default to this FILEHANDLE. Second, references to variables related to output will refer to this output channel. For example, if you have to set the top of form format for more than one output channel, you might do the following:
```select(REPORT1);
\$^ = 'report1_top';
select(REPORT2);
\$^ = 'report2_top';
```
FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus:
```\$oldfh = select(STDERR); \$| = 1; select(\$oldfh);
```
tell(FILEHANDLE)
tell FILEHANDLE
tell
Returns the current file position for FILEHANDLE. FILEHANDLE may be an expression whose value gives the name of the actual filehandle. If FILEHANDLE is omitted, assumes the file last read.
write(FILEHANDLE)
write(EXPR)
write
Writes a formatted record (possibly multi-line) to the specified file, using the format associated with that file. By default the format for a file is the one having the same name is the filehandle, but the format for the current output channel (see `select`) may be set explicitly by assigning the name of the format to the `\$~' variable. Top of form processing is handled automatically: if there is insufficient room on the current page for the formatted record, the page is advanced by writing a form feed, a special top-of-page format is used to format the new page header, and then the record is written. By default the top-of-page format is the name of the filehandle with `_TOP` appended, but it may be dynamically set to the format of your choice by assigning the name to the `\$^' variable while the filehandle is `select`ed. The number of lines remaining on the current page is in variable `\$-', which can be set to 0 to force a new page. If FILEHANDLE is unspecified, output goes to the current default output channel, which starts out as `STDOUT' but may be changed by the `select` operator. If the FILEHANDLE is an EXPR, then the expression is evaluated and the resulting string is used to look up the name of the FILEHANDLE at run time. See section Formats, for more info. Note that `write` is NOT the opposite of `read`.

## Search and Replace Functions

m/PATTERN/gio
/PATTERN/gio
Searches a string for a pattern match, and returns true (1) or false (''). If no string is specified via the `=~' or `!~' operator, the `\$_' string is searched. (The string specified with `=~' need not be an lvalue--it may be the result of an expression evaluation, but remember the `=~' binds rather tightly.) See section Regular Expressions, for more info. If `/' is the delimiter then the initial `m' is optional. With the `m' you can use any pair of non-alphanumeric characters as delimiters. This is particularly useful for matching Unix path names that contain `/'. If the final delimiter is followed by the optional letter `i', the matching is done in a case-insensitive manner. PATTERN may contain references to scalar variables, which will be interpolated (and the pattern recompiled) every time the pattern search is evaluated. (Note that `\$)' and `\$|' may not be interpolated because they look like end-of-string tests.) If you want such a pattern to be compiled only once, add an `o' after the trailing delimiter. This avoids expensive run-time recompilations, and is useful when the value you are interpolating won't change over the life of the script. If the PATTERN evaluates to a null string, the most recent successful regular expression is used instead. If used in a context that requires an array value, a pattern match returns an array consisting of the subexpressions matched by the parentheses in the pattern, i.e. `(\$1, \$2, \$3...)'. It does NOT actually set `\$1', `\$2', etc. in this case, nor does it set `\$+', `\$`', `\$&' or `\$''. If the match fails, a null array is returned. If the match succeeds, but there were no parentheses, an array value of (1) is returned. Examples:
```open(tty, '/dev/tty');
<tty> =~ /^y/i && do foo(); # do foo if desired

if (/Version: *([0-9.]*)/) { \$version = \$1; }

next if m#^/usr/spool/uucp#;

# poor man's grep
\$arg = shift;
while (<>) {
print if /\$arg/o;   # compile only once
}

if ((\$F1, \$F2, \$Etc) = (\$foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
```
This last example splits `\$foo' into the first two words and the remainder of the line, and assigns those three fields to `\$F1', `\$F2' and `\$Etc'. The conditional is true if any variables were assigned, i.e. if the pattern matched. The `g' modifier specifies global pattern matching--that is, matching as many times as possible within the string. How it behaves depends on the context. In an array context, it returns a list of all the substrings matched by all the parentheses in the regular expression. If there are no parentheses, it returns a list of all the matched strings, as if there were parentheses around the whole pattern. In a scalar context, it iterates through the string, returning TRUE each time it matches, and FALSE when it eventually runs out of matches. (In other words, it remembers where it left off last time and restarts the search at that point.) It presumes that you have not modified the string since the last match. Modifying the string between matches may result in undefined behavior. (You can actually get away with in-place modifications via `substr()` that do not change the length of the entire string. In general, however, you should be using `s///g` for such modifications.) Examples:
```# array context
(\$one,\$five,\$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);

# scalar context
\$/ = ""; \$* = 1;
while (\$paragraph = <>) {
while (\$paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
\$sentences++;
}
}
print "\$sentences\n";
```
?PATTERN?
This is just like the `/pattern/` search, except that it matches only once between calls to the `reset` operator. This is a useful optimization when you only want to see the first occurrence of something in each file of a set of files, for instance. Only `??' patterns local to the current package are reset.
s/PATTERN/REPLACEMENT/gieo
Searches a string for a pattern, and if found, replaces that pattern with the replacement text and returns the number of substitutions made. Otherwise it returns false (0). The `g' is optional, and if present, indicates that all occurrences of the pattern are to be replaced. The `i' is also optional, and if present, indicates that matching is to be done in a case-insensitive manner. The `e' is likewise optional, and if present, indicates that the replacement string is to be evaluated as an expression rather than just as a double-quoted string. Any non-alphanumeric delimiter may replace the slashes; if single quotes are used, no interpretation is done on the replacement string (the `e' modifier overrides this, however); if backquotes are used, the replacement string is a command to execute whose output will be used as the actual replacement text. If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own pair of quotes, which may or may not be bracketing quotes, e.g. `s(foo)(bar)` or `s<foo>/bar/`. If no string is specified via the `=~' or `!~' operator, the `\$_' string is searched and modified. (The string specified with `=~' must be a scalar variable, an array element, or an assignment to one of those, i.e. an lvalue.) If the pattern contains a `\$' that looks like a variable rather than an end-of-string test, the variable will be interpolated into the pattern at run-time. If you only want the pattern compiled once the first time the variable is interpolated, add an `o' at the end. If the PATTERN evaluates to a null string, the most recent successful regular expression is used instead. See section Regular Expressions, for more info. Examples:
```s/\bgreen\b/mauve/g;        # don't change wintergreen

\$path =~ s|/usr/bin|/usr/local/bin|;

(\$foo = \$bar) =~ s/bar/foo/;

\$_ = 'abc123xyz';
s/\d+/\$&*2/e;               # yields `abc246xyz'
s/\d+/sprintf("%5d",\$&)/e;  # yields `abc  246xyz'
s/\w/\$& x 2/eg;             # yields `aabbcc  224466xxyyzz'

s/([^ ]*) *([^ ]*)/\$2 \$1/;  # reverse 1st two fields
```
(Note the use of `\$' instead of `\' in the last example. See section Regular Expressions.)
study(SCALAR)
study SCALAR
study
Takes extra time to study SCALAR (`\$_' if unspecified) in anticipation of doing many pattern matches on the string before it is next modified. This may or may not save time, depending on the nature and number of patterns you are searching on, and on the distribution of character frequencies in the string to be searched--you probably want to compare runtimes with and without it to see which runs faster. Those loops which scan for many short constant strings (including the constant parts of more complex patterns) will benefit most. You may have only one study active at a time--if you study a different scalar the first is "unstudied". (The way study works is this: a linked list of every character in the string to be searched is made, so we know, for example, where all the `k' characters are. From each search string, the rarest character is selected, based on some static frequency tables constructed from some C programs and English text. Only those places that contain this "rarest" character are examined.) For example, here is a loop which inserts index producing entries before any line containing a certain pattern:
```while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
...
print;
}
```
In searching for `/\bfoo\b/', only those locations in `\$_' that contain `f' will be looked at, because `f' is rarer than `o'. In general, this is a big win except in pathological cases. The only question is whether it saves you more time than it took to build the linked list in the first place. Note that if you have to look for strings that you don't know till runtime, you can build an entire loop as a string and eval that to avoid recompiling all your patterns all the time. Together with undefining `\$/' to input entire files as one record, this can be very fast, often faster than specialized programs like `fgrep'. The following scans a list of files (`@files') for a list of words (`@words'), and prints out the names of those files that contain a match:
```\$search = 'while (<>) { study;';
foreach \$word (@words) {
\$search .= "++\\$seen{\\$ARGV} if /\\b\$word\\b/;\n";
}
\$search .= "}";
@ARGV = @files;
undef \$/
eval \$search;           # this screams
\$/ = "\n";              # put back to normal input delim
foreach \$file (sort keys(%seen)) {
print \$file, "\n";
}
```
tr/SEARCHLIST/REPLACEMENTLIST/cds
y/SEARCHLIST/REPLACEMENTLIST/cds
Translates all occurrences of the characters found in the search list with the corresponding character in the replacement list. It returns the number of characters replaced or deleted. If no string is specified via the `=~' or `!~' operator, the `\$_' string is translated. (The string specified with `=~' must be a scalar variable, an array element, or an assignment to one of those, i.e. an lvalue.) For `sed` devotees, `y` is provided as a synonym for `tr`. If the SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of quotes, which may or may not be bracketing quotes, e.g. `tr[A-Z][a-z]` or `tr(+-*/)/ABCD/`. If the `c' modifier is specified, the SEARCHLIST character set is complemented. If the `d' modifier is specified, any characters specified by SEARCHLIST that are not found in REPLACEMENTLIST are deleted. (Note that this is slightly more flexible than the behavior of some `tr` programs, which delete anything they find in the SEARCHLIST, period.) If the `s' modifier is specified, sequences of characters that were translated to the same character are squashed down to 1 instance of the character. If the `d' modifier was used, the REPLACEMENTLIST is always interpreted exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter than the SEARCHLIST, the final character is replicated till it is long enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated. The latter is useful for counting characters in a class, or for squashing character sequences in a class. Examples:
```\$ARGV[1] =~ y/A-Z/a-z/;          # canonicalize to lower case

\$cnt = tr/*/*/;                  # count the stars in \$_

\$cnt = tr/0-9//;                 # count the digits in \$_

tr/a-zA-Z//s;                    # bookkeeper -> bokeper

(\$HOST = \$host) =~ tr/a-z/A-Z/;

y/\001-@[-_{-\177/ /;            # change non-alphas to space
#   (before the c & s modifiers)
y/a-zA-Z/ /cs;                   # change non-alphas to single space
#   (version 3.0 patchlevel 40+)

tr/\200-\377/\0-\177/;           # delete 8th bit
```

## System Interaction

alarm(SECONDS)
alarm SECONDS
Arranges to have a `SIGALRM' delivered to this process after the specified number of seconds (minus 1, actually) have elapsed. Thus, `alarm(15)` will cause a `SIGALRM' at some point more than 14 seconds in the future. Only one timer may be counting at once. Each call disables the previous timer, and an argument of 0 may be supplied to cancel the previous timer without starting a new one. The returned value is the amount of time remaining on the previous timer.
chroot(FILENAME)
chroot FILENAME
chroot
Does the same as the system call of that name. If you don't know what it does, don't worry about it. If FILENAME is omitted, does `chroot` to `\$_'.
die(LIST)
die LIST
die
Outside of an `eval`, prints the value of LIST to `STDERR' and exits with the current value of `\$!' (errno). As of version 3.0 patchlevel 27, `die` without LIST specified is equivalent to
```die 'Died';
```
If `\$!' is 0, exits with the value of `(\$? >> 8)' (`command` status). If `(\$? >> 8)' is 0, exits with 255. Inside an `eval`, the error message is stuffed into `\$@' and the `eval` is terminated with the undefined value. Equivalent examples:
```die "Can't cd to spool: \$!\n" unless chdir '/usr/spool/news';

chdir '/usr/spool/news' || die "Can't cd to spool: \$!\n"
```
If the value of EXPR does not end in a newline, the current script line number and input line number (if any) are also printed, and a newline is supplied. Hint: sometimes appending ", stopped" to your message will cause it to make better sense when the string "at foo line 123" is appended. Suppose you are running script "canasta".
```die "/etc/games is no good";
die "/etc/games is no good, stopped";
```
produce, respectively
```/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
```
See also `exit`.
exec(LIST)
exec LIST
If there is more than one argument in LIST, or if LIST is an array with more than one value, calls `execvp()` with the arguments in LIST. If there is only one scalar argument, the argument is checked for shell metacharacters. If there are any, the entire argument is passed to `/bin/sh -c' for parsing. If there are none, the argument is split into words and passed directly to `execvp()`, which is more efficient. Note: `exec` (and `system`) do not flush your output buffer, so you may need to set `\$|' to avoid lost output. Examples:
```exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort \$outfile | uniq";
```
If you don't really want to execute the first argument, but want to lie to the program you are executing about its own name, you can specify the program you actually want to run by assigning that to a variable and putting the name of the variable in front of the LIST without a comma. (This always forces interpretation of the LIST as a multi-valued list, even if there is only a single scalar in the list.) Example:
```\$shell = '/bin/csh';
exec \$shell '-sh';              # pretend it's a login shell
```
exit(EXPR)
exit EXPR
Evaluates EXPR and exits immediately with that value. Example:
```\$ans = <STDIN>;
exit 0 if \$ans =~ /^[Xx]/;
```
See also `die`. If EXPR is omitted, exits with 0 status.
fork
Does a `fork()` call. Returns the child pid to the parent process and 0 to the child process. Note: unflushed buffers remain unflushed in both processes, which means you may need to set `\$|' to avoid duplicate output.
getpwnam(NAME)
getgrnam(NAME)
gethostbyname(NAME)
getnetbyname(NAME)
getprotobyname(NAME)
getpwuid(UID)
getgrgid(GID)
getservbyname(NAME,PROTO)
getprotobynumber(NUMBER)
getservbyport(PORT,PROTO)
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent(STAYOPEN)
setnetent(STAYOPEN)
setprotoent(STAYOPEN)
setservent(STAYOPEN)
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines perform the same functions as their counterparts in the system library. With an array context, the return values from the various get routines are as follows:
```(\$name,\$passwd,\$uid,\$gid,
\$quota,\$comment,\$gcos,\$dir,\$shell) = getpw...
(\$name,\$passwd,\$gid,\$members) = getgr...
(\$name,\$aliases,\$proto) = getproto...
(\$name,\$aliases,\$port,\$proto) = getserv...
```
(If the entry doesn't exist you get a null list.) Within a scalar context, you get the name, unless the function was a lookup by name, in which case you get the other thing, whatever it is. (If the entry doesn't exist you get the undefined value.) For example:
```\$uid = getpwnam
\$name = getpwuid
\$name = getpwent
\$gid = getgrnam
\$name = getgrgid
\$name = getgrent
etc.
```
The `\$members' value returned by `getgr...` is a space separated list of the login names of the members of the group. For the `gethost...` functions, if the `h_errno` variable is supported in C, it will be returned to you via `\$?' if the function call fails. The `@addrs' value returned by a successful call is a list of the raw addresses returned by the corresponding system library call. In the Internet domain, each address is four bytes long and you can unpack it by saying something like:
```(\$a,\$b,\$c,\$d) = unpack('C4',\$addr[0]);
```
Returns the current login from `/etc/utmp', if any. If null, use `getpwuid`.
```\$login = getlogin || (getpwuid(\$<))[0] || "Somebody";
```
getpgrp(PID)
getpgrp PID
getpgrp
Returns the current process group for the specified PID, 0 for the current process. Will produce a fatal error if used on a machine that doesn't implement `getpgrp(2)`. If PID is omitted, returns process group of current process. PID can be an expression.
getppid
Returns the process id of the parent process.
getpriority(WHICH,WHO)
Returns the current priority for a process, a process group, or a user. (See the `getpriority(2)` man page.) Will produce a fatal error if used on a machine that doesn't implement `getpriority(2)`.
ioctl(FILEHANDLE,FUNCTION,SCALAR)
Implements the `ioctl(2)` function. You'll probably have to say
```require "ioctl.ph";   # probably `/usr/local/lib/perl/ioctl.ph'
```
first to get the correct function definitions. If `ioctl.ph' doesn't exist or doesn't have the correct definitions you'll have to roll your own, based on your C header files such as `<sys/ioctl.h>'. (There is a perl script called `h2ph` that comes with the perl kit which may help you in this.) SCALAR will be read and/or written depending on the FUNCTION---a pointer to the string value of SCALAR will be passed as the third argument of the actual `ioctl` call. (If SCALAR has no string value but does have a numeric value, that value will be passed rather than a pointer to the string value. To guarantee this to be true, add a 0 to the scalar before using it.) The `pack()` and `unpack()` functions are useful for manipulating the values of structures used by `ioctl()`. The following example sets the erase character to DEL.
```require 'ioctl.ph';
\$sgttyb_t = "ccccs";            # 4 chars and a short
if (ioctl(STDIN,\$TIOCGETP,\$sgttyb)) {
@ary = unpack(\$sgttyb_t,\$sgttyb);
\$ary[2] = 127;
\$sgttyb = pack(\$sgttyb_t,@ary);
ioctl(STDIN,\$TIOCSETP,\$sgttyb)
|| die "Can't ioctl: \$!";
}
```
The return value of `ioctl` (and `fcntl`) is as follows:
```if OS returns:                  perl returns:
-1                              undefined value
0                               string "0 but true"
anything else                   that number
```
Thus perl returns true on success and false on failure, yet you can still easily determine the actual value returned by the operating system:
```(\$retval = ioctl(...)) || (\$retval = -1);
printf "System returned %d\n", \$retval;
```
kill(LIST)
kill LIST
Sends a signal to a list of processes. The first element of the list must be the signal to send. Returns the number of processes successfully signaled.
```\$cnt = kill 1, \$child1, \$child2;
kill 9, @goners;
```
If the signal is negative, kills process groups instead of processes. (On System V, a negative process number will also kill process groups, but that's not portable.) You may use a signal name in quotes.
setpgrp(PID,PGRP)
Sets the current process group for the specified PID, 0 for the current process. Will produce a fatal error if used on a machine that doesn't implement `setpgrp(2)`.
setpriority(WHICH,WHO,PRIORITY)
Sets the current priority for a process, a process group, or a user. (See the `setpriority(2)` man page.) Will produce a fatal error if used on a machine that doesn't implement `setpriority(2)`.
sleep(EXPR)
sleep EXPR
sleep
Causes the script to sleep for EXPR seconds, or forever if no EXPR. May be interrupted by sending the process a `SIGALRM'. Returns the number of seconds actually slept. You probably cannot mix `alarm()` and `sleep()` calls, since `sleep()` is often implemented using `alarm()`.
syscall(LIST)
syscall LIST
Calls the system call specified as the first element of the list, passing the remaining elements as arguments to the system call. If unimplemented, produces a fatal error. The arguments are interpreted as follows: if a given argument is numeric, the argument is passed as an int. If not, the pointer to the string value is passed. You are responsible to make sure a string is pre-extended long enough to receive any result that might be written into a string. If your integer arguments are not literals and have never been interpreted in a numeric context, you may need to add 0 to them to force them to look like numbers.
```require 'syscall.ph';         # may need to run h2ph
syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
```
Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE, using the system call `read(2)`. It bypasses stdio, so mixing this with other kinds of reads may cause confusion. Returns the number of bytes actually read, or `undef` if there was an error. SCALAR will be grown or shrunk to the length actually read. An OFFSET may be specified to place the read data at some other place than the beginning of the string.
syswrite(FILEHANDLE,SCALAR,LENGTH,OFFSET)
syswrite(FILEHANDLE,SCALAR,LENGTH)
Attempts to write LENGTH bytes of data from variable SCALAR to the specified FILEHANDLE, using the system call `write(2)`. It bypasses stdio, so mixing this with prints may cause confusion. Returns the number of bytes actually written, or `undef` if there was an error. An OFFSET may be specified to place the read data at some other place than the beginning of the string.
system(LIST)
system LIST
Does exactly the same thing as `exec LIST' except that a fork is done first, and the parent process waits for the child process to complete. Note that argument processing varies depending on the number of arguments. The return value is the exit status of the program as returned by the `wait()` call. To get the actual exit value divide by 256. See also `exec`.
times
Returns a four-element array giving the user and system times, in seconds, for this process and the children of this process.
```(\$user,\$system,\$cuser,\$csystem) = times;
```
Sets the umask for the process and returns the old one. If EXPR is omitted, merely returns current umask.
wait
Waits for a child process to terminate and returns the pid of the deceased process, or -1 if there are no child processes. The status is returned in `\$?'.
waitpid(PID,FLAGS)
Waits for a particular child process to terminate and returns the pid of the deceased process or -1 if there are no such child process. The status is returns in `\$?'. If you say
```require "sys/wait.ph";
...
waitpid(-1,&WNOHANG);
```
then you can do a non-blocking wait for any process. Non-blocking wait is only available on machines supporting either the `waitpid(2)` or `wait4(2)` system calls. However, waiting for a particular pid with FLAGS of 0 is implemented everywhere. (Perl emulates the system call by remembering the status values of processes that have exited but have not been harvested by the Perl script yet.)
warn(LIST)
warn LIST
Produces a message on `STDERR' just like `die`, but doesn't exit.

## Networking Functions - Interprocess Communication

accept(NEWSOCKET,GENERICSOCKET)
Does the same thing that the `accept` system call does. Returns true if it succeeded, false otherwise. See section Interprocess Communication, for an example.
bind(SOCKET,NAME)
Does the same thing that the `bind` system call does. Returns true if it succeeded, false otherwise. NAME should be a packed address of the proper type for the socket. See section Interprocess Communication, for an example.
connect(SOCKET,NAME)
Does the same thing that the `connect` system call does. Returns true if it succeeded, false otherwise. NAME should be a packed address of the proper type for the socket. See section Interprocess Communication, for an example.
getpeername(SOCKET)
Returns the packed sockaddr address of other end of the SOCKET connection.
```# An internet sockaddr
\$sockaddr = 'S n a4 x8';
```
getsockname(SOCKET)
Returns the packed sockaddr address of this end of the SOCKET connection.
```# An internet sockaddr
\$sockaddr = 'S n a4 x8';
```
getsockopt(SOCKET,LEVEL,OPTNAME)
Returns the socket option requested, or undefined if there is an error.
listen(SOCKET,QUEUESIZE)
Does the same thing that the `listen` system call does. Returns true if it succeeded, false otherwise. See section Interprocess Communication, for an example.
recv(SOCKET,SCALAR,LEN,FLAGS)
Receives a message on a socket. Attempts to receive LENGTH bytes of data into variable SCALAR from the specified SOCKET filehandle. Returns the address of the sender, or the undefined value if there's an error. SCALAR will be grown or shrunk to the length actually read. Takes the same flags as the system call of the same name.
send(SOCKET,MSG,FLAGS,TO)
send(SOCKET,MSG,FLAGS)
Sends a message on a socket. Takes the same flags as the system call of the same name. On unconnected sockets you must specify a destination to send TO. Returns the number of characters sent, or the undefined value if there is an error.
setsockopt(SOCKET,LEVEL,OPTNAME,OPTVAL)
Sets the socket option requested. Returns undefined if there is an error. OPTVAL may be specified as `undef` if you don't want to pass an argument.
shutdown(SOCKET,HOW)
Shuts down a socket connection in the manner indicated by HOW, which has the same interpretation as in the system call of the same name.
socket(SOCKET,DOMAIN,TYPE,PROTOCOL)
Opens a socket of the specified kind and attaches it to filehandle SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the system call of the same name. You may need to run `h2ph` on `sys/socket.h' to get the proper values handy in a perl library file. Return true if successful. See section Interprocess Communication, for an example.
socketpair(SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL)
Creates an unnamed pair of sockets in the specified domain, of the specified type. DOMAIN, TYPE and PROTOCOL are specified the same as for the system call of the same name. If unimplemented, yields a fatal error. Return true if successful.

## Networking Functions - System V IPC

msgctl(ID,CMD,ARG)
Calls the System V IPC function `msgctl`. If CMD is &IPC_STAT, then ARG must be a variable which will hold the returned msqid_ds structure. Returns like `ioctl`: the undefined value for error, "0 but true" for zero, or the actual return value otherwise.
msgget(KEY,FLAGS)
Calls the System V IPC function `msgget`. Returns the message queue id, or the undefined value if there is an error.
msgsnd(ID,MSG,FLAGS)
Calls the System V IPC function `msgsnd` to send the message MSG to the message queue ID. MSG must begin with the long integer message type, which may be created with `pack("L", \$type)'. Returns true if successful, or false if there is an error.
msgrcv(ID,VAR,SIZE,TYPE,FLAGS)
Calls the System V IPC function `msgrcv` to receive a message from message queue ID into variable VAR with a maximum message size of SIZE. Note that if a message is received, the message type will be the first thing in VAR, and the maximum length of VAR is SIZE plus the size of the message type. Returns true if successful, or false if there is an error.
semctl(ID,SEMNUM,CMD,ARG)
Calls the System V IPC function `semctl`. If CMD is &IPC_STAT or &GETALL, then ARG must be a variable which will hold the returned semid_ds structure or semaphore value array. Returns like `ioctl`: the undefined value for error, "0 but true" for zero, or the actual return value otherwise.
semget(KEY,NSEMS,SIZE,FLAGS)
Calls the System V IPC function `semget`. Returns the semaphore id, or the undefined value if there is an error.
semop(KEY,OPSTRING)
Calls the System V IPC function `semop` to perform semaphore operations such as signaling and waiting. OPSTRING must be a packed array of semop structures. Each semop structure can be generated with `pack("sss", \$semnum, \$semop, \$semflag)'. The number of semaphore operations is implied by the length of OPSTRING. Returns true if successful, or false if there is an error. As an example, the following code waits on semaphore \$semnum of semaphore id \$semid:
```\$semop = pack("sss", \$semnum, -1, 0);
die "Semaphore trouble: \$!\n" unless semop(\$semid, \$semop);
```
To signal the semaphore, replace `-1' with `1'.
shmctl(ID,CMD,ARG)
Calls the System V IPC function `shmctl`. If CMD is &IPC_STAT, then ARG must be a variable which will hold the returned shmid_ds structure. Returns like `ioctl`: the undefined value for error, "0 but true" for zero, or the actual return value otherwise.
shmget(KEY,SIZE,FLAGS)
Calls the System V IPC function `shmget`. Returns the shared memory segment id, or the undefined value if there is an error.
shmwrite(ID,STRING,POS,SIZE)
Reads or writes the System V shared memory segment ID starting at position POS for size SIZE by attaching to it, copying in/out, and detaching from it. When reading, VAR must be a variable which will hold the data read. When writing, if STRING is too long, only SIZE bytes are used; if STRING is too short, nulls are written to fill out SIZE bytes. Returns true if successful, or false if there is an error.

## Time Related Functions

gmtime(EXPR)
gmtime EXPR
gmtime
Converts a time as returned by the `time` function to a 9-element array with the time analyzed for the Greenwich timezone. Typically used as follows:
```(\$sec,\$min,\$hour,\$mday,\$mon,\$year,\$wday,\$yday,\$isdst)
= gmtime(time);
```
All array elements are numeric, and come straight out of a `struct tm'. In particular this means that `\$mon` has the range `0..11' and `\$wday` has the range `0..6'. If EXPR is omitted, does `gmtime(time)'.
localtime(EXPR)
localtime EXPR
localtime
Converts a time as returned by the time function to a 9-element array with the time analyzed for the local timezone. Typically used as follows:
```(\$sec,\$min,\$hour,\$mday,\$mon,\$year,\$wday,\$yday,\$isdst)
= localtime(time);
```
All array elements are numeric, and come straight out of a `struct tm'. In particular this means that \$mon has the range `0..11' and \$wday has the range `0..6'. If EXPR is omitted, does `localtime(time)`.
time
Returns the number of non-leap seconds since 00:00:00 UTC, January 1, 1970. Suitable for feeding to `gmtime()` and `localtime()`.

## DBM Functions

dbmclose(ASSOC_ARRAY)
dbmclose ASSOC_ARRAY
Breaks the binding between a dbm file and an associative array. The values remaining in the associative array are meaningless unless you happen to want to know what was in the cache for the dbm file. This function is only useful if you have ndbm.
dbmopen(ASSOC,DBNAME,MODE)
This binds a dbm or ndbm file to an associative array. ASSOC is the name of the associative array. (Unlike normal open, the first argument is NOT a filehandle, even though it looks like one). DBNAME is the name of the database (without the `.dir' or `.pag' extension). If the database does not exist, it is created with protection specified by MODE (as modified by the umask). If your system only supports the older dbm functions, you may perform only one `dbmopen` in your program. If your system has neither dbm nor ndbm, calling `dbmopen` produces a fatal error. Values assigned to the associative array prior to the `dbmopen` are lost. A certain number of values from the dbm file are cached in memory. By default this number is 64, but you can increase it by preallocating that number of garbage entries in the associative array before the `dbmopen`. You can flush the cache if necessary with the reset command. If you don't have write access to the dbm file, you can only read associative array variables, not set them. If you want to test whether you can write, either use file tests or try setting a dummy array entry inside an eval, which will trap the error. Note that functions such as `keys()` and `values()` may return huge array values when used on large dbm files. You may prefer to use the `each()` function to iterate over large dbm files. Example:
```# print out history file offsets
dbmopen(HIST,'/usr/lib/news/history',0666);
while ((\$key,\$val) = each %HIST) {
print \$key, ' = ', unpack('L',\$val), "\n";
}
dbmclose(HIST);
```

## Flow Control Functions

do BLOCK
Returns the value of the last command in the sequence of commands indicated by BLOCK. When modified by a loop modifier, executes the BLOCK once before testing the loop condition. (On other statements the loop modifiers test the conditional first.)
goto LABEL
Finds the statement labeled with LABEL and resumes execution there. Currently you may only go to statements in the main body of the program that are not nested inside a `do {}' construct. This statement is not implemented very efficiently, and is here only to make the sed-to-perl translator easier. I may change its semantics at any time, consistent with support for translated `sed` scripts. Use it at your own risk. Better yet, don't use it at all.
last LABEL
last
The `last` command is like the `break` statement in C (as used in loops); it immediately exits the loop in question. If the LABEL is omitted, the command refers to the innermost enclosing loop. The `continue` block, if any, is not executed:
```line: while (<STDIN>) {
last line if /^\$/;      # exit when done with header
...
}
```
next LABEL
next
The `next` command is like the `continue` statement in C; it starts the next iteration of the loop:
```line: while (<STDIN>) {
...
}
```
Note that if there were a `continue` block on the above, it would get executed even on discarded lines. If the LABEL is omitted, the command refers to the innermost enclosing loop.
redo LABEL
redo
The `redo` command restarts the loop block without evaluating the conditional again. The `continue` block, if any, is not executed. If the LABEL is omitted, the command refers to the innermost enclosing loop. This command is normally used by programs that want to lie to themselves about what was just input:
```# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
line: while (<STDIN>) {
while (s|({.*}.*){.*}|\$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
\$front = \$_;
while (<STDIN>) {
if (/}/) {    # end of comment?
s|^|\$front{|;
redo line;
}
}
}
print;
}
```

## Perl Library Functions

require(EXPR)
require EXPR
require
Includes the library file specified by EXPR, or by `\$_' if EXPR is not supplied. Has semantics similar to the following subroutine:
```sub require {
local(\$filename) = @_;
return 1 if \$INC{\$filename};
local(\$realfilename,\$result);
ITER: {
foreach \$prefix (@INC) {
\$realfilename = "\$prefix/\$filename";
if (-f \$realfilename) {
\$result = do \$realfilename;
last ITER;
}
}
die "Can't find \$filename in \@INC";
}
die \$@ if \$@;
die "\$filename did not return true value" unless \$result;
\$INC{\$filename} = \$realfilename;
\$result;
}
```
Note that the file will not be included twice under the same specified name. The file must return true as the last statement to indicate successful execution of any initialization code, so it's customary to end such a file with `1;` unless you're sure it'll return true otherwise.
do EXPR
Uses the value of EXPR as a filename and executes the contents of the file as a perl script. Its primary use is to include subroutines from a perl subroutine library.
```do 'stat.pl';
```
is just like
```eval `cat stat.pl`;
```
except that it's more efficient, more concise, keeps track of the current filename for error messages, and searches all the `-I' libraries if the file isn't in the current directory (see section Predefined Names, for more info). It's the same, however, in that it does reparse the file every time you call it, so if you are going to use the file inside a loop you might prefer to use `-P' and `#include', at the expense of a little more startup time. (The main problem with `#include' is that cpp doesn't grok `#' comments--a workaround is to use `;#' for standalone comments.) Note that the following are NOT equivalent:
```do \$foo;        # eval a file
do \$foo();      # call a subroutine
```
Note that inclusion of library routines is better done with the `require` operator.

## Subroutine Functions

caller(EXPR)
caller
Returns the context of the current subroutine call:
```(\$package,\$filename,\$line) = caller;
```
With EXPR, returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one.
do SUBROUTINE (LIST)
Executes a SUBROUTINE declared by a `sub` declaration, and returns the value of the last expression evaluated in SUBROUTINE. If there is no subroutine by that name, produces a fatal error. (You may use the `defined` operator to determine if a subroutine exists.) If you pass arrays as part of LIST you may wish to pass the length of the array in front of each array. (See section Subroutines.) The parentheses are required to avoid confusion with the `do EXPR' form. SUBROUTINE may also be a single scalar variable, in which case the name of the subroutine to execute is take from the variable. As an alternate (and preferred) form, you may call a subroutine by prefixing the name with an ampersand: `&foo(@args)'. If you aren't passing any arguments, you don't have to use parentheses. If you omit the parentheses, no `@_' array is passed to the subroutine. The `&' form is also used to specify subroutines to the `defined` and `undef` operators.
```if (defined &\$var) { &\$var(\$parm); undef &\$var; }
```
local(LIST)
Declares the listed variables to be local to the enclosing block, subroutine, `eval` or `do`. All the listed elements must be legal lvalues. This operator works by saving the current values of those variables in LIST on a hidden stack and restoring them upon exiting the block, subroutine or `eval`. This means that called subroutines can also reference the local variable, but not the global one. The LIST may be assigned to if desired, which allows you to initialize your local variables. (If no initializer is given for a particular variable, it is created with an undefined value.) Commonly this is used to name the parameters to a subroutine. Examples:
```sub RANGEVAL {
local(\$min, \$max, \$thunk) = @_;
local(\$result) = '';
local(\$i);

# Presumably \$thunk makes reference to \$i

for (\$i = \$min; \$i < \$max; \$i++) {
\$result .= eval \$thunk;
}

\$result;
}

if (\$sw eq '-v') {
# init local array with global array
local(@ARGV) = @ARGV;
unshift(@ARGV,'echo');
system @ARGV;
}
# @ARGV restored

# temporarily add to digits associative array
if (\$base12) {
# (NOTE: not claiming this is efficient!)
local(%digits) = (%digits,'t',10,'e',11);
do parse_num();
}
```
Note that `local()` is a run-time command, and so gets executed every time through a loop, using up more stack storage each time until it's all released at once when the loop is exited.
return LIST
Returns from a subroutine with the value specified. (Note that a subroutine can automatically return the value of the last expression evaluated. That's the preferred method--use of an explicit `return` is a bit slower.)
wantarray
Returns true if the context of the currently executing subroutine is looking for an array value. Returns false if the context is looking for a scalar.
```return wantarray ? () : undef;
```

## Variable Functions

defined(EXPR)
defined EXPR
Returns a boolean value saying whether the lvalue EXPR has a real value or not. Many operations return the undefined value under exceptional conditions, such as end of file, uninitialized variable, system error and such. This function allows you to distinguish between an undefined null string and a defined null string with operations that might return a real null string, in particular referencing elements of an array. You may also check to see if arrays or subroutines exist. Use on predefined variables is not guaranteed to produce intuitive results. Examples:
```print if defined \$switch{'D'};
print "\$val\n" while defined(\$val = pop(@ary));
eval '@foo = ()' if defined(@foo);
die "No XYZ package defined" unless defined %_XYZ;
sub foo { defined &\$bar ? &\$bar(@_) : die "No bar"; }
```
See also `undef`.
reset(EXPR)
reset EXPR
reset
Generally used in a `continue` block at the end of a loop to clear variables and reset `??' searches so that they work again. The expression is interpreted as a list of single characters (hyphens allowed for ranges). All variables and arrays beginning with one of those letters are reset to their pristine state. If the expression is omitted, one-match searches (`?pattern?') are reset to match again. Only resets variables or searches in the current package. Always returns 1. Examples:
```reset 'X';          # reset all X variables
reset 'a-z';        # reset lower case variables
reset;              # just reset `??' searches
```
Note: resetting `A-Z' is not recommended since you'll wipe out your `ARGV' and `ENV' arrays. The use of reset on dbm associative arrays does not change the dbm file. (It does, however, flush any entries cached by perl, which may be useful if you are sharing the dbm file. Then again, maybe not.)
scalar(EXPR)
Forces EXPR to be interpreted in a scalar context and returns the value of EXPR.
undef(EXPR)
undef EXPR
undef
Undefines the value of EXPR, which must be an lvalue. Use only on a scalar value, an entire array, or a subroutine name (using `&'). (`undef` will probably not do what you expect on most predefined variables or dbm array values.) Always returns the undefined value. You can omit the EXPR, in which case nothing is undefined, but you still get an undefined value that you could, for instance, return from a subroutine. Examples:
```undef \$foo;
undef \$bar{'blurfl'};
undef @ary;
undef %assoc;
undef &mysub;
return (wantarray ? () : undef) if \$they_blew_it;
```

## Miscellaneous Functions

dump LABEL
dump
This causes an immediate core dump. Primarily this is so that you can use the `undump' program to turn your core dump into an executable binary after having initialized all your variables at the beginning of the program. When the new binary is executed it will begin by executing a `goto LABEL' (with all the restrictions that `goto` suffers). Think of it as a `goto` with an intervening core dump and reincarnation. If LABEL is omitted, restarts the program from the top. WARNING: any files opened at the time of the dump will NOT be open any more when the program is reincarnated, with possible resulting confusion on the part of perl. See also `-u'. Example:
```#!/usr/bin/perl
require 'getopt.pl';
require 'stat.pl';
%days = (
'Sun',1,
'Mon',2,
'Tue',3,
'Wed',4,
'Thu',5,
'Fri',6,
'Sat',7);

dump QUICKSTART if \$ARGV[0] eq '-d';

QUICKSTART:
do Getopt('f');
...
```
eval(EXPR)
eval EXPR
eval BLOCK
eval
EXPR is parsed and executed as if it were a little perl program. It is executed in the context of the current perl program, so that any variable settings, subroutine or format definitions remain afterwards. The value returned is the value of the last expression evaluated, just as with subroutines. If there is a syntax error or runtime error, or a `die` statement is executed, an undefined value is returned by `eval`, and `\$@' is set to the error message. If there was no error, `\$@' is guaranteed to be a null string. If EXPR is omitted, evaluates `\$_'. The final semicolon, if any, may be omitted from the expression. Note that, since `eval` traps otherwise-fatal errors, it is useful for determining whether a particular feature (such as `dbmopen` or `symlink`) is implemented. If is also Perl's exception trapping mechanism, where the `die` operator is used to raise exceptions. If the code to be executed doesn't vary, you may use the ```eval BLOCK``` form to trap run-time errors without incurring the penalty of recompiling each time. The error, if any, is still returned in `\$@'. Evaluating a single-quoted string (as EXPR) has the same effect, except that the `eval EXPR` form reports syntax errors at run time via `\$@', whereas the `eval BLOCK` form reports syntax errors at compile time. The `eval EXPR` form is optimized to `eval BLOCK` the first time it succeeds. (Since the replacement side of a substitution is considered a single-quoted string when you use the `e' modifier, the same optimization occurs there.) Examples:
```# make divide-by-zero non-fatal
eval { \$answer = \$a / \$b; }; warn \$@ if \$@;

# optimized to same thing after first use
eval '\$answer = \$a / \$b'; warn \$@ if \$@;

# a compile-time error

# a run-time error
eval '\$answer =';       # sets \$@
```
ord(EXPR)
ord EXPR
ord
Returns the numeric ascii value of the first character of EXPR. If EXPR is omitted, uses `\$_'.
q/STRING/
qq/STRING/
qx/STRING/
These are not really functions, but simply syntactic sugar to let you avoid putting too many backslashes into quoted strings. The `q` operator is a generalized single quote, and the `qq` operator a generalized double quote. The `qx` operator is a generalized backquote. Any non-alphanumeric delimiter can be used in place of `/', including newline. If the delimiter is an opening bracket or parenthesis, the final delimiter will be the corresponding closing bracket or parenthesis. (Embedded occurrences of the closing bracket need to be backslashed as usual.) Examples:
```\$foo = q!I said, "You said, 'She said it.'"!;
\$bar = q('This is it.');
\$today = qx{ date };
\$_ .= qq
*** The previous line contains the naughty word "\$&".\n
if /(ibm|apple|awk)/;      # :-)
```
rand(EXPR)
rand EXPR
rand
Returns a random fractional number between 0 and the value of EXPR. (EXPR should be positive.) If EXPR is omitted, returns a value between 0 and 1. See also `srand()`.
srand(EXPR)
srand EXPR
srand
Sets the random number seed for the `rand` operator. If EXPR is omitted, does `srand(time)`.
sprintf(FORMAT,LIST)
Returns a string formatted by the usual `printf` conventions. The `*' character is not supported.
vec(EXPR,OFFSET,BITS)
Treats a string as a vector of unsigned integers, and returns the value of the bitfield specified. May also be assigned to. BITS must be a power of two from 1 to 32. Vectors created with `vec()` can also be manipulated with the logical operators `|', `&' and `^', which will assume a bit vector operation is desired when both operands are strings. This interpretation is not enabled unless there is at least one `vec()` in your program, to protect older programs. To transform a bit vector into a string or array of 0's and 1's, use these:
```\$bits = unpack("b*", \$vector);
@bits = split(//, unpack("b*", \$vector));
```
If you know the exact length in bits, it can be used in place of the *.