Perl

Warnings

Put a ; at the end of each line

This is still what I forget most.

autovivification

If testing for existence of a hash key beware that all intermediate keys will be created by the test. In below example $hash1($key1} will be created if it does not exist yet.

if (exists $hash1{$key1}{$key2) {
    ...
} else {
    ...
}
)

Things that do work

Searching and matching is explained in Regular Expressions.

perl -e '<commands>'

Execute <commnands>

Print out the contents of $variable in a structured format

use Data::Dumper;
print Dumper($variable);

if-then-else:

if < expr > {
 <codeblock>
} elsif < expr ) {
 <codeblock>
} else {
 <codeblock>
}

$var += <value>

Add <value> to $var (works for -, *, / too)

$string .= $addstring

Concatenate $string and $addstring

$string =~ s/<regexp>/<newvalue>/[g]

Substitute <regexp> with <newvalue> in $string. The g modifier makes the all occurrences of <regexp> are substituted.

$newstring = substr($string,start,length)

Return substring of $string

sprintf(format,$string)

Return formatted string. format is e.g. "%.3f". Check Python:Strings#Advanced for all formats.

$exitcode = system("a command")

Executed the command in a subprocess and return the exitcode

$output = `a command`

Execute the command and return all command output

sleep(x)

Sleep for x seconds

$|++

Set standard output to unbuffered, every is put on screen immediately.

$username = getpwuid($<)

Get the username of the one executing the script. $< is the uid of the current user.

Hashes

%newhash = (%oldhas1,%oldhash3)

$newhashref = {$oldhasref1,$oldhashref3}

Merge 2 hashes, works if all keys are unique

Arrays

arr1 = split(<sep>,$string)

Split a string in to an array on <sep>

a = arr1[0]

First element in arr1

a = arr1[-1]

Last element in arr1

if ( grep(/^<value>$/,@arr1) )

Check if <value> exist in arr1 (Note the ^ and $ for an exact match

Files

open (FH, "< filename") || die("Open failed for filename")

Open filename for reading on filehandle FH. Exit the program when the open failed

@array = <FH>

Read the file opened on FH into @array (1 element per line

while ( line = (<FH>) )

Read the file opened on FH line by line

Flow control

The usual works (if-then-else, for , foreach)

||die("message")

|| causes the statement only to be executed if the previous failed. Stop the program and write messaged to standard error.

|| do {codeblock};

Like die but not exiting the program, use to do additional actions on the failure of the previous command

Compare

==, !=, >=, >, <, etc

Compare numeric data

eq, ne, ge, gt, lt, etc

Compare strings