PERL ONE-LINERS - THE BASICS

Why Perl One-Liners?

Perl was designed for text processing. One-liners let you transform text without writing a script file. They're perfect for:

• Quick find-and-replace
• Extracting data from files
• Reformatting text on the fly
• Piping between commands

Same regex syntax as ack and rename. Learn once, use everywhere.

The Basic Structure

perl [options] 'code' [files]

Most common options:

-e    Execute code from command line
-n    Loop over input lines (don't print by default)
-p    Loop over input lines (print by default)
-i    Edit files in place
-l    Automatic newline handling

The -e flag is almost always used. It says "here comes the code."

Hello World

echo "hello world" | perl -pe 's/world/universe/'
Output: hello universe

The -p flag loops over input and prints each line. The s/// does our substitution.

Processing Files

You can give perl files directly:

perl -pe 's/old/new/g' file.txt

Or pipe from another command:

cat file.txt | perl -pe 's/old/new/g'

Both do the same thing.

The -n vs -p Difference

-p    Read lines, run code, print result (most common)
-n    Read lines, run code, don't print automatically

With -n you control what gets printed:

perl -ne 'print if /pattern/' file.txt

This prints only lines matching the pattern (like grep).

In-Place Editing

The -i flag modifies files directly:

perl -i -pe 's/old/new/g' file.txt

WARNING: This changes the file! No undo!

Safer: create a backup:

perl -i.bak -pe 's/old/new/g' file.txt

This saves the original as file.txt.bak before modifying.

Basic Substitution

Find and replace across a file:

perl -pe 's/foo/bar/g' input.txt > output.txt

Breaking it down:

-p          Print each line after processing
-e          Code follows
s/foo/bar/  Replace foo with bar
/g          Global (all occurrences)

Case-Insensitive Replacement

perl -pe 's/error/warning/gi' log.txt

The /i makes it case-insensitive. The /g makes it global.

Print Matching Lines Only

Like grep, but with Perl regex:

perl -ne 'print if /pattern/' file.txt

Or the inverse (non-matching lines):

perl -ne 'print unless /pattern/' file.txt

Extract Just the Match

Print only what the regex matched:

perl -ne 'print "$1\n" if /(\d+)/' file.txt

This prints captured numbers, one per line.

Without capture group, use $& for the whole match:

perl -ne 'print "$&\n" if /\d+/' file.txt

Multiple Files

Perl processes multiple files in sequence:

perl -pe 's/old/new/g' file1.txt file2.txt file3.txt

With -i, modifies all of them:

perl -i -pe 's/old/new/g' *.txt

The Default Variable $_

In Perl one-liners, $_ is the "current line." Most operations work on $_ by default:

perl -ne 'print if /pattern/'

Is shorthand for:

perl -ne 'print $_ if $_ =~ /pattern/'

You rarely need to write $_ explicitly.

Modifying and Printing

Transform each line:

perl -pe '$_ = uc'        # Uppercase everything
perl -pe '$_ = lc'        # Lowercase everything
perl -pe '$_ = reverse'   # Reverse each line

Or use substitution:

perl -pe 's/^\s+//'       # Remove leading whitespace
perl -pe 's/\s+$//'       # Remove trailing whitespace
perl -pe 's/^\s+|\s+$//g' # Trim both ends

Numbering Lines

Add line numbers:

perl -pe '$_ = "$. $_"' file.txt

$. is the current line number. Output:

1 First line
2 Second line
3 Third line

Quick Field Extraction

Split lines on whitespace and print specific fields:

perl -ane 'print "$F[0]\n"' file.txt

The -a flag auto-splits into @F array. Fields are zero-indexed.

Print first and third fields:

perl -ane 'print "$F[0] $F[2]\n"' file.txt

Custom Field Separator

Split on commas (CSV):

perl -F',' -ane 'print "$F[0]\n"' data.csv

The -F flag sets the field separator.

Sum Numbers

Add up all numbers in a file:

perl -ne '$sum += $_; END { print "$sum\n" }' numbers.txt

The END block runs after all lines are processed.