Programming Perl, 4th Edition

Shell Traps

Sharp shell programmers should take note of the following:

• Variables are prefixed with $, @, or % on the left side of the assignment as well as the right. A shellish assignment like:

  camel="dromedary";      # WRONG

  won’t be parsed the way you expect. You need:

  $camel="dromedary";     # ok

• The loop variable of a foreach also requires a $. Although csh likes:

  foreach hump (one two)
      stuff_it $hump
  end

  In Perl, this is written as:

  foreach $hump ("one", "two") {
      stuff_it($hump);
  }

• The backtick operator does variable interpolation without regard to the presence of single quotes in the command.

• The backtick operator does no translation of the return value. In Perl, you have to trim the newline explicitly, like this:

  chomp($thishost = `hostname`);

• Shells (especially csh) do several levels of substitution on each command line. Perl does interpolation only within certain constructs such as double quotes, backticks, angle brackets, and search patterns.

• Shells tend to interpret scripts a little bit at a time. Perl compiles the entire program before executing it (except for BEGIN blocks, which execute before the compilation is done).

• Program arguments are available via @ARGV, not $1, $2, and so on.

• The environment is not automatically made available as individual scalar variables. Use the standard Env module if you want that to happen.