An if statement in Perl has the following structure.
if (comparison) {
--do something--
}
elsif (comparison) {
--do something--
}
else {
--do something
}
For example.
if ($foo == 10) {
print "foo equals 10.";
}
elsif ($foo == 20) {
print "foo equals 20.";
}
else {
print "foo does not equal 10 or 20.";
}
Following are common if statements.
| Equals |
if ($foo == $bar) # integer if ($foo eq $bar) # string |
| Does not equal |
if ($foo != $bar) # integer if ($foo ne $bar) # string |
| Greater than |
if ($foo > $bar) # integer if ($foo gt $bar) # string |
| Less than |
if ($foo < $bar) # integer if ($foo lt $bar) # string |
| Greater than or equal to |
if ($foo >= $bar) # integer if ($foo ge $bar) # string |
| Less than or equal to |
if ($foo <= $bar) # integer if ($foo le $bar) # string |
| If file or directory exists |
if ( -e "/path/to/file.txt") if ( -e "/path/to/directory") |
| If file or directory does not exist |
if ( ! -e "/path/to/file.txt") if ( ! -e "/path/to/directory") |
| If variable exists | if (defined($foo)) |
| If variable is empty (null) | if ($foo eq "") |
| If variable is not empty (not null) | if ($fee ne "") |
| If array is empty (null) | if (@foo == 0) |
| If array is not empty (not null) | if (@foo != 0) |
| If variable contains | if ($foo =~ /bar/) |
| If variable does not contain | if ($foo !~ /bar/) |
| If array contains | if (grep( /bar/, @foo )) |
| If array does not contains | if (!grep( /bar/, @foo )) |
| If array contains exact match | if (grep{ $_ eq $bar} @foo) |
| If array does not contain exact match | if (grep{ $_ ne $bar} @foo) |
| If file contains / does not contain | refer to this article |
| If string begins with | if ($foo =~ /^bar/) |
| If string does not begin with | if ($foo !~ /^bar/) |
| If string ends with | if ($foo =~ /bar$/) |
| If string does not end with | if ($foo !~ /bar$/) |
If directory is empty (care of this Stack Overflow post).
sub directory_is_empty {
my $dirname = shift;
opendir(my $dh, $dirname);
return scalar(grep { $_ ne "." && $_ ne ".." } readdir($dh)) == 0;
}
Use the subroutine.
if (directory_is_empty("/path/to/directory")) {
....
}
Ignore case
The i option can be used to ignore case.
if ($foo =! /bar/i)
Multiple conditions
Multiple conditions can be evaluated by separating each condition with and or or.
if (--condition-- and --condition-- or --condition--)
More precise conditions can be strung together by evaluating certain conditions together. The possible arrangement of evaluations is literally limitless.
if( (--condition and --condition) or --condition)
Regular Expressions
Following are commonly used regular expressions.
| Single integer (0 through 9) |
[0-9] |
| Single integer followed by anything | [0-9]* |
| Anything followed by a single integer | *[0-9] |
| Single lower case alpha character | [a-z] |
| Single lower case alpha character followed by anything | [a-z]* |
| Anything followed by single lower case alpha character | *[a-z] |
| Single upper case character | [A-Z] |
| Single upper case alpha character followed by anything | [A-Z]* |
| Anything followed by single upper case alpha character | *[A-Z] |
| Single alpha character | [a-zA-Z] |
| Single alpha character followed by anything | [a-zA-Z]* |
| Anything followed by alpha character | *[a-zA-Z] |
| Any alpha-numeric character | [0-9a-zA-Z] |
| Any alpha-numeric character followed by anything | [0-9a-zA-Z]* |
| Anything followed by any alpha-numeric character | *[0-9a-zA-Z] |
| Any single character except new line | . |
| Any single character except new line followed by anything | .* |
| Space, tab, or newline | \s |
| Not a space, tab, or newline | \S |