Traps

This chapter points out traps and pitfalls you may run into if you are used to awk, C, sed or shell programming.

• Awk Traps: Notes for awk users.
• C Traps: Notes for C programmers.
• Sed Traps: Notes for sed programmers.
• Shell Traps: Notes for shell programmers.

Awk Traps

Accustomed awk users should take special note of the following:

• Semicolons are required after all simple statements in perl (except at the end of a block). Newline is not a statement delimiter.
• Curly brackets are required on ifs and whiles.
• Variables begin with `$' or `@' in perl.
• Arrays index from 0 unless you set `$['. Likewise string positions in substr() and index().
• You have to decide whether your array has numeric or string indices.
• Associative array values do not spring into existence upon mere reference.
• You have to decide whether you want to use string or numeric comparisons.
• Reading an input line does not split it for you. You get to split it yourself to an array. And the split operator has different arguments.
• The current input line is normally in `$_', not `$0'. It generally does not have the newline stripped. (`$0' is the name of the program executed.)
• `$<digit>' does not refer to fields--it refers to substrings matched by the last match pattern.
• The print statement does not add field and record separators unless you set `$,' and `$\'.
• You must open your files before you print to them.
• The range operator is `..', not comma. (The comma operator works as in C.)
• The match operator is `=~', not `~'. (`~' is the one's complement operator, as in C.)
• The exponentiation operator is `**', not `^'. (`^' is the XOR operator, as in C.)
• The concatenation operator is `.', not the null string. (Using the null string would render `/pat/ /pat/' unparsable, since the third slash would be interpreted as a division operator--the tokener is in fact slightly context sensitive for operators like `/', `?', and `<'. And in fact, `.' itself can be the beginning of a number.)
• next, exit and continue work differently.
• The following variables work differently

  Awk                 Perl
  ARGC                  $#ARGV
  ARGV[0]               $0
  FILENAME              $ARGV
  FNR                   $. - something
  FS                    (whatever you like)
  NF                    $#Fld, or some such
  NR                    $.
  OFMT                  $#
  OFS                   $,
  ORS                   $\
  RLENGTH               length($&)
  RS                    $/
  RSTART                length($`)
  SUBSEP                $;
  

• When in doubt, run the awk construct through a2p and see what it gives you (see section a2p - Awk to Perl Translator for more info).

C Traps

Cerebral C programmers should take note of the following:

• Curly brackets are required on ifs and whiles.
• You should use elsif rather than else if
• break and continue become last and next, respectively.
• There's no switch statement.
• Variables begin with `$', `@' or `%' in perl.
• printf does not implement `*'.
• Comments begin with `#', not `/*'.
• You can't take the address of anything.
• `ARGV' must be capitalized.
• The "system" calls link, unlink, rename, etc. return nonzero for success, not zero (0).
• Signal handlers deal with signal names, not numbers.

Sed Traps

Seasoned sed programmers should take note of the following:

• Backreferences in substitutions use `$' rather than `\'.
• The pattern matching metacharacters `(', `)', and `|' do not have backslashes in front.
• The range operator is `..' rather than comma.

Shell Traps

Sharp shell programmers should take note of the following:

• The backtick operator does variable interpretation without regard to the presence of single quotes in the command.
• The backtick operator does no translation of the return value, unlike csh.
• Shells (especially csh) do several levels of substitution on each command line. Perl does substitution only in certain constructs such as double quotes, backticks, angle brackets and search patterns.
• Shells interpret scripts a little bit at a time. Perl compiles the whole program before executing it.
• The arguments are available via `@ARGV', not `$1', `$2', etc.
• The environment is not automatically made available as variables.