PERL Conditions

Placed in () when used with if, while, etc...

Any expression is considered true as long as it does not evaluate to the null string ("") or 0 or "0".

Common: http://www.perldoc.com/perl5.6/pod/perlop.html

Relational Operators

<: Returns true if the left argument is numerically less than the right argument.
>: Returns true if the left argument is numerically greater than the right argument.
<=: Returns true if the left argument is numerically less than or equal to the right argument.
>=: Returns true if the left argument is numerically greater than or equal to the right argument.
lt: Returns true if the left argument is stringwise less than the right argument.
gt: Returns true if the left argument is stringwise greater than the right argument.
le: Returns true if the left argument is stringwise less than or equal to the right argument.
ge: Returns true if the left argument is stringwise greater than or equal to the right argument.

Equality Operators

==: Returns true if the left argument is numerically equal to the right argument. Compares the length when used with strings (use eq)
!=: Returns true if the left argument is numerically not equal to the right argument.
<=>: Returns -1, 0, or 1 depending on whether the left argument is numerically less than, equal to, or greater than the right argument. Often used with Sort.
eq: Returns true if the left argument is stringwise equal to the right argument.
ne: Returns true if the left argument is stringwise not equal to the right argument.
cmp: returns -1, 0, or 1 depending on whether the left argument is stringwise less than, equal to, or greater than the right argument. Often used with Sort
! or not: Logical Not. The not form has a lower precedent and is therefor "safer"
&& or and: Logical AND. Short circuit. The and form has a lower precendent and is therefor "safer"
|| or or: Logical OR. The or form has a lower precendent and is therefor "safer". Short circuit evaluation applies; as in: open(IN, "infile.txt") or die "can't open"; where the die doesn't happen unless the open returns 0. (neat huh?) Also $foo = $bar or $this or $that; will return the first of $bar, $this, or $that which contain a value.

``lt'', ``le'', ``ge'', ``gt'' and ``cmp'' use the collation (sort) order specified by the current locale if use locale is in effect. See the perllocale manpage.

File-test operators http://www.perldoc.com/perl5.6/pod/func/X.html

-e filename: Test for the existance of a file or directory
-r: File is readable
-w: File is writable
-x: File is executable
-o: File is owned by effective uid.
-z: File has zero size.
-s: File has nonzero size (returns size).
-f: File is a plain file.
-d: File is a directory.
-M: Modified. Age of file in days when script started. e.g. if (-M $src <= -M $dest) {print "Backup already up to date";}
-A: Accessed. Same for access time.
-C: Created. Same for inode change time.

Example:

    while (<>) {
        chop;
        next unless -f $_;      # ignore specials
        #...
    }

Example:

    print "Can do.\n" if -r $a || -w _ || -x _;

    stat($filename);
    print "Readable\n" if -r _;
    print "Writable\n" if -w _;
    print "Executable\n" if -x _;