# perl
# The Perl 5 language interpreter.

# Parse and execute a Perl script:
perl script.pl

# Check syntax errors on a Perl script:
perl -c script.pl

# Parse and execute a perl statement:
perl -e perl_statement

# Import module before execution of a perl statement:
perl -Mmodule -e perl_statement

# Run a Perl script in debug mode, using perldebug:
perl -d script.pl

# Loo[p] over all lines of a file, editing them [i]n-place
# using a find/replace [e]xpression
perl -p -i -e 's/find/replace/g' filename

# Run a find/replace expression on a file,
# saving the original file with a given extension:
perl -p -i'.old' -e 's/find/replace/g' filename

# See also:
#   Perl language cheat sheets at /perl/
#   list of pages:      /perl/:list
#   learn perl:         /perl/:learn
#   perl one-liners:    /perl/1line
#   search in pages:    /perl/~keyword

# Perl method of listing out the environment variables, sans values.
perl -e 'print("$_\n") foreach keys(%ENV)'

# Output the columns and lines of the current terminal.
perl -e 'use Term::ReadKey "GetTerminalSize"; my ($Cols, $Lines) = GetTerminalSize(); print("${Cols}x$Lines\n")'

# List out all of the aliases within the provided file. This works by iterating
# over each line of the file, displaying only those lines matching the REGEX.
# Before printing out the relevant lines, all tabs are removed.
perl -ne '/^[[:space:]]+alias/ and print(tr/\t//dr)' "$HOME/.bash_aliases"
# Alternative logic approach:
perl -ne 'print(tr/\t//dr) if /^[[:space:]]+alias/' "$HOME/.bash_aliases"

# See if the current user has a non-empty password value. This may not work for
# systems set up with shadow passwords, however.
perl -ne '/^$ARGV[0]::$</ and print(STDERR "WARNING: User has an empty password.\n")' /etc/passwd ichy

# Display the current user's UID and GID in a format ideal for chmod(1).
perl -e 'print("$<:" . (split(" ", $)))[0] . "\n")'

# Permissions allowing, output the first 512 bytes of an MBR storage device, -
# where the partition table should be stored, using Perl's read() function.
#
# Note that the 2-argument style open() should not be used unless the file is
# static; no chance of changing to something potentially problematic.
#
# You might spot some printable strings, such as 'LILO' or 'GRUB', if you've
# also stored your bootloader on the MBR of the storage device (non-UEFI).
perl -e 'open(my $FH, "</dev/sda"); read($FH, my $Data, 512); close($FH); print("$Data\n")'

# Create a simple table of users within your system. This example demonstrates
# the use of the `getpwent()` subroutine, which, upon each execution, holds an
# array of each field in each line within the '/etc/passwd' file.
printf("%-15s %-5s %-5s %-s\n", 'USERNAME', 'UID', 'GID', 'HOME');
while (my @Data = getpwent()) {
	printf("%-15s %-5s %-5s %-s\n", $Data[0], $Data[2], $Data[3], $Data[7])
}