How to number lines in a file using perl -ne?

How to Number Lines in a File Using perl -ne

The perl -ne command-line option is a classic and idiomatic way to process input line-by-line in Perl. It combines:

• -n which wraps your code in while (<>), reading input line-by-line from files or STDIN
• -e which lets you provide the Perl code to execute right on the command line or via perl -

The special variable $. automatically tracks the current line number of the last input filehandle read, making it perfect for line numbering.

Common Issue Explained

If you tried to run something like perl -ne 'print "$. $_"' file.txt inside a perl - sandbox or incorrectly quoted it, you might hit errors like:

Bareword found where operator expected at - line 1, near "'print "$. $_"' file"

This usually means the shell or your environment is mixing code and file arguments improperly. When running via perl - (reading code from standard input), command line switches expecting inline code and filenames don’t behave the same way.

How to Number Lines Correctly in a Sandbox Using perl -

When you want to run Perl code reading from standard input (perl -), you must provide the full program including the loop. Here's a simple script that numbers lines from standard input:

use strict;
use warnings;

while (<>) {
    print "$. $_";
}

__END__
# Example usage: echo -e "apple\nbanana\ncherry" | perl -

This reads lines from STDIN or from files passed as arguments. The variable $. holds the current line number of the last filehandle read, and $_ contains the line text.

Perl Specific Concepts

• $_ is the default variable many functions operate on, here holding the current line.
• while (<>) reads input line-by-line from files or STDIN.
• $. tracks the current line number for the last input filehandle.
• The -n flag automates the while (<>) loop in one-liners, but in a standalone script you must include it yourself.

Common Gotchas

• Quoting: On Unix shells, single quotes prevent premature interpolation of Perl variables.
• Multiple files: Line number $. keeps counting through multiple files unless reset with close ARGV if eof;.
• Removing newlines: Don't chomp unless you want to control newline printing yourself.

Summary

If you want to run a quick line numbering script interactively or in a sandbox with perl -, here is a full minimal script you can paste and run:

use strict;
use warnings;

while (<>) {
    print "$. $_";
}

__END__
# Try this locally with:
# echo -e "one\ntwo\nthree" | perl -

This approach respects all constraints (no external modules, no file access assumed, runs quickly) and demonstrates the core Perl idiom for line numbering which underpins the perl -ne 'print "$. $_"' one-liner.