How to shuffle lines randomly with perl one-liner?

To shuffle lines randomly in Perl with a one-liner, the key is to read all lines into an array first, then perform an in-memory Fisher-Yates shuffle. This algorithm swaps elements from the end to the beginning, picking a random index each time. A common error when running inline Perl is to include the literal command perl -e inside your code block, causing syntax errors. The one-liner itself should start after the perl -e part; the shell command invokes Perl with the given code.

Fixed and Runnable Perl One-Liner to Shuffle Lines

@lines = <>;
for ($i = $#lines; $i > 0; $i--) {
    $j = int rand($i + 1);
    @lines[$i, $j] = @lines[$j, $i];
}
print @lines;

You can run this code as a one-liner from the shell like this (without embedding perl -e inside):

• perl -e '@lines = <>; for ($i = $#lines; $i > 0; $i--) { $j = int rand($i+1); @lines[$i,$j] = @lines[$j,$i]; } print @lines;' filename.txt
• cat filename.txt | perl -e '@lines = <>; for ($i = $#lines; $i > 0; $i--) { $j = int rand($i+1); @lines[$i,$j] = @lines[$j,$i]; } print @lines;'

Explanation

• @lines = <>; reads all input lines (including trailing newlines) from standard input or files.
• $#lines is the last index of the array @lines.
• The loop counts backward from the last index to 1, ensuring every element has a chance to be swapped.
• int rand($i+1) generates a random index from 0 to $i, inclusive.
• Swapping uses array slices @lines[$i,$j] to exchange elements efficiently.
• print @lines; outputs all shuffled lines, preserving newlines.

Common Pitfalls

• Do not include perl -e inside your Perl code block; this is part of the shell command.
• One-liners inside single quotes may need escaping if you add semicolons or special characters.
• Using -n or -p modifiers won't work here because lines are processed one at a time rather than accumulating all lines.
• Ensure your input file is small enough to fit into memory.
• No need to explicitly seed rand() in Perl 5.004+ because it is auto-seeded.

Summary

This approach leverages Perl's flexible syntax and built-in array features to implement an efficient Fisher-Yates shuffle in a concise and idiomatic one-liner. Using core Perl functions only, it adapts easily for files or piped input and demonstrates Perl's "There's More Than One Way To Do It" philosophy at its finest.