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.

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 0.
  • Signal handlers deal with signal names, not numbers.

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.

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.