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Conditions

This section describes commands that are primarily useful for their exit status, rather than their output. Thus, they are often used as the condition of shell if statements, or as the last command in a pipeline.

false: Do nothing, unsuccessfully

false does nothing except return an exit status of 1, meaning failure. It can be used as a place holder in shell scripts where an unsuccessful command is needed.

Any arguments are ignored, except for a lone `--help' or `--version' (see section Common options).

true: Do nothing, successfully

true does nothing except return an exit status of 0, meaning success. It can be used as a place holder in shell scripts where a successful command is needed, although the shell built-in command : (colon) may be faster.

Any arguments are ignored, except for a lone `--help' or `--version' (see section Common options).

test: Check file types and compare values

test returns a status of 0 (true) or 1 (false) depending on the evaluation of the conditional expression expr. Each part of the expression must be a separate argument.

test has file status checks, string operators, and numeric comparison operators.

Because most shells have a built-in command by the same name, using the unadorned command name in a script or interactively may get you different functionality than that described here.

Besides the options below, test accepts a lone `--help' or `--version'. See section Common options. A single non-option argument is also allowed: test returns true if the argument is not null.

File type tests

These options test for particular types of files. (Everything's a file, but not all files are the same!)

`-b file'
True if file exists and is a block special device.
`-c file'
True if file exists and is a character special device.
`-d file'
True if file exists and is a directory.
`-f file'
True if file exists and is a regular file.
`-h file'
`-L file'
True if file exists and is a symbolic link.
`-p file'
True if file exists and is a named pipe.
`-S file'
True if file exists and is a socket.
`-t [ fd ]'
True if fd is opened on a terminal. If fd is omitted, it defaults to 1 (standard output).

Access permission tests

These options test for particular access permissions.

`-g file'
True if file exists and has its set-group-id bit set.
`-k file'
True if file has its sticky bit set.
`-r file'
True if file exists and is readable.
`-u file'
True if file exists and has its set-user-id bit set.
`-w file'
True if file exists and is writable.
`-x file'
True if file exists and is executable.
`-O file'
True if file exists and is owned by the current effective user id.
`-G file'
True if file exists and is owned by the current effective group id.

File characteristics tests

These options test other file characteristics.

`-e file'
True if file exists.
`-s file'
True if file exists and has a size greater than zero.
`file1 -nt file2'
True if file1 is newer (according to modification date) than file2.
`file1 -ot file2'
True if file1 is older (according to modification date) than file2.
`file1 -ef file2'
True if file1 and file2 have the same device and inode numbers, i.e., if they are hard links to each other.

String tests

These options test string characteristics. Strings are not quoted for test, though you may need to quote them to protect characters with special meaning to the shell, e.g., spaces.

`-z string'
True if the length of string is zero.
`-n string'
`string'
True if the length of string is non-zero.
`string1 = string2'
True if the strings are equal.
`string1 != string2'
True if the strings are not equal.

Numeric tests

Numeric relationals. The arguments must be entirely numeric (possibly negative), or the special expression -l string, which evaluates to the length of string.

`arg1 -eq arg2'
`arg1 -ne arg2'
`arg1 -lt arg2'
`arg1 -le arg2'
`arg1 -gt arg2'
`arg1 -ge arg2'
These arithmetic binary operators return true if arg1 is equal, not-equal, less-than, less-than-or-equal, greater-than, or greater-than-or-equal than arg2, respectively.

For example: 

test -1 -gt -2 && echo yes
=> yes
test -l abc -gt 1 && echo yes
=> yes
test 0x100 -eq 1
error--> test: integer expression expected before -eq

Connectives for test

The usual logical connectives.

`! expr'
True if expr is false.
`expr1 -a expr2'
True if both expr1 and expr2 are true.
`expr1 -o expr2'
True if either expr1 or expr2 is true.

expr: Evaluate expressions

expr evaluates an expression and writes the result on standard output. Each token of the expression must be a separate argument.

Operands are either numbers or strings. expr coerces anything appearing in an operand position to an integer or a string depending on the operation being applied to it.

Strings are not quoted for expr, though you may need to quote them to protect characters with special meaning to the shell, e.g., spaces.

Operators may given as infix symbols or prefix keywords. Parentheses may be used for grouping in the usual manner (you must quote parentheses to avoid the shell evaluating them, however).

Exit status: 

0 if the expression is neither null nor 0,
1 if the expression is null or 0,
2 for invalid expressions.

Relations for expr

The usual logical connectives and relations, in order of precedence.

`|'
Yields its first argument if it is neither null nor 0, otherwise its second argument.
`&'
Yields its first argument if neither argument is null or 0, otherwise 0.
`< <= = == != >= >'
Compare the arguments and return 1 if the relation is true, 0 otherwise. == is a synonym for =. expr first tries to coerce both arguments to numbers and do a numeric comparison; if either coercion fails, it does a lexicographic comparison.

Numeric expressions

Numeric operators, in order of increasing precedence. The connectives (previous section) have higher precedence, the string operators (following section) have lower.

`+ -'
Addition and subtraction. Both arguments are coerced to numbers; an error occurs if this cannot be done.
`* / %'
Multiplication, division, remainder. Both arguments are coerced to numbers; an error occurs if this cannot be done.

String expressions

String operators. These have lowest precedence.

`string : regex'
Perform pattern matching. The arguments are coerced to strings and the second is considered to be a (basic, a la grep) regular expression, with a ^ implicitly prepended. The first argument is then matched against this regular expression. If the match succeeds and regex uses `\(' and `\)', the : expression returns the part of string that matched the subexpression; otherwise, it returns the number of characters matched. If the match fails, the : operator returns the null string if `\(' and `\)' are used in regex, otherwise 0. Only the first `\( ... \)' pair is relevant to the return value; additional pairs are meaningful only for grouping the regular expression operators. See section `Regular Expression Library' in Regex, for details of regular expression syntax.
`match string regex'
An alternative way to do pattern matching. This is the same as `string : regex'.
`substr string position length'
Returns the substring of string beginning at position with length at most length. If either position or length is negative or non-numeric, returns the null string.
`index string character-class'
Returns the first position in string where the first character in charset was found. If no character in charset is found in string, return 0.
`length string'
Returns the length of string.

The keywords cannot be used as strings.

Examples of expr

Here are a few examples, including quoting for shell metacharacters.

To add 1 to the shell variable foo, in Bourne-compatible shells: 

foo=`expr $foo + 1`

To print the non-directory part of the file name stored in $fname, which need not contain a /. 

expr $fname : '.*/\(^.*\)' '^|' $fname

expr abc : 'a\(.\)c'
=> b
expr index abcdef cz
=> 3
expr index index a
error--> expr: syntax error
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