• *nix shells such as sh, csh, tcsh, bash, etc. store their PID in $$.

  • perl, borrowing from Unix (as it is wont to do), also stores its PID in the $$ variable. However, this is not $$'s only use in perl. It also is used when resolving a reference to a scalar. It can be used to resolve a reference to an array or hash, but the -> notation is more common. Also, please use your references intelligently and only when necessary. Nobody wants to see 'print "$$$$$my_ref"' in the code s/he inherits.

    # Hello, World! in perl with a reference
    $greeting = 'Hello, World!';
    $ref = \$greeting;
    print "$$ref\n";
    # Hello, World! in perl with a referenced array
    @greeting = ('Hello,', 'World!');
    $ref = \@greeting;
    print "$$ref[0] $ref->[1]\n";

  • Outside the realm of computer programming, $$ is used to denote double dollars,the currency of Yasuhiro Nightow's manga cum anime, Trigun.