File Renaming One-Liners

2026-04-07

Batch renaming files is a common task that Raku handles elegantly from the command line. With its powerful regex engine and clean string manipulation, you can rename hundreds of files with a single line.

Basic File Renaming

Rename a single file:

raku -e '"old-name.txt".IO.rename("new-name.txt")'

Batch Rename with a Pattern

Rename all .txt files to .md:

raku -e 'for dir(test => /\.txt$/) -> $f { $f.rename($f.Str.subst(/\.txt$/, ".md")) }'

Lowercase All Filenames

raku -e 'for dir() -> $f { $f.rename($f.Str.lc) if $f.Str ne $f.Str.lc }'

Uppercase All Filenames

raku -e 'for dir() -> $f { $f.rename($f.Str.uc) if $f.Str ne $f.Str.uc }'

Replace Spaces with Hyphens

raku -e 'for dir(test => /\s/) -> $f { $f.rename($f.Str.subst(/\s+/, "-", :g)) }'

Replace Spaces with Underscores

raku -e 'for dir(test => /\s/) -> $f { $f.rename($f.Str.subst(/\s+/, "_", :g)) }'

Add a Prefix

Add "backup_" to all .conf files:

raku -e 'for dir(test => /\.conf$/) -> $f { $f.rename("backup_" ~ $f.Str) }'

Add a Suffix (Before Extension)

Add "v2" before the file extension:
raku -e 'for dir(test => /\.txt$/) -> $f { $f.rename($f.Str.subst(/\.txt$/, "_v2.txt")) }'

Remove a Prefix
Remove "temp" from filenames:

raku -e 'for dir(test => /^temp_/) -> $f { $f.rename($f.Str.subst(/^temp_/, "")) }'

Sequential Numbering

Rename files to a numbered sequence:

raku -e 'my $n = 1; for dir(test => /\.jpg$/).sort -> $f { $f.rename(sprintf("photo_%03d.jpg", $n++)) }'

This renames anything.jpg to photo_001.jpg, photo_002.jpg, etc.

Date-Based Renaming

Prepend today's date:

raku -e 'my $d = Date.today; for dir(test => /\.log$/) -> $f { $f.rename("{$d}_{$f.Str}") }'

Strip Special Characters

Remove everything except alphanumeric, dots, and hyphens:

raku -e 'for dir() -> $f { my $clean = $f.Str.subst(/<-[a..zA..Z0..9.\-]>/, "_", :g); $f.rename($clean) if $clean ne $f.Str }'

Change Extension Based on Content

Rename .txt files to .csv if they contain commas:

raku -e 'for dir(test => /\.txt$/) -> $f { $f.rename($f.Str.subst(/\.txt$/, ".csv")) if $f.slurp.contains(",") }'

Recursive Renaming

Rename files in subdirectories too:

raku -e 'for "." .IO.dir(:recurse).grep(*.f) -> $f { my $new = $f.Str.subst(/\.jpeg$/, ".jpg"); $f.rename($new) if $new ne $f.Str }'

Note: .dir(:recurse) is not built-in. For recursive traversal, use a helper:

raku -e '
    sub walk($dir) {
        for $dir.dir -> $entry {
            if $entry.d { walk($entry) }
            elsif $entry.Str ~~ /\.jpeg$/ {
                $entry.rename($entry.Str.subst(/\.jpeg$/, ".jpg"));
                say "Renamed: $entry";
            }
        }
    }
    walk(".".IO);
'

Dry Run (Preview Changes)

Always preview before renaming. Just replace .rename with a say:

raku -e 'for dir(test => /\.txt$/) -> $f { say "{$f} -> {$f.Str.subst(/\.txt$/, ".md")}" }'

This shows you what would happen without actually renaming anything. Once you are satisfied, change say back to $f.rename(...).

Undo Preparation

Save the mapping before renaming, so you can undo if needed:

raku -e '
    my @log;
    for dir(test => /\.txt$/) -> $f {
        my $new = $f.Str.subst(/\.txt$/, ".md");
        @log.push("{$new}\t{$f}");
        $f.rename($new);
    }
    "rename-log.txt".IO.spurt(@log.join("\n"));
    say "Renamed {+@log} files. Undo log saved.";
'

To undo:

raku -ne 'my ($new, $old) = .split("\t"); $new.IO.rename($old); say "Restored: $old"' rename-log.txt

Regex-Based Renaming

Use full regex power for complex transformations:

# CamelCase to snake_case
raku -e '
    for dir(test => /\.rb$/) -> $f {
        my $new = $f.Str.subst(/ (<[A..Z]>) /, { "_{$0.lc}" }, :g).subst(/^_/, "");
        $f.rename($new) if $new ne $f.Str;
        say "$f -> $new";
    }
'

# snake_case to CamelCase
raku -e '
    for dir(test => /\.py$/) -> $f {
        my $new = $f.Str.subst(/ "_" (.) /, { $0.Str.uc }, :g);
        say "$f -> $new";
    }
'

Sanitize Downloaded Filenames

Clean up messy filenames from downloads:

raku -e '
    for dir() -> $f {
        next if $f.d;  # Skip directories
        my $new = $f.Str
            .subst(/\%20/, " ", :g)        # URL-encoded spaces
            .subst(/\s+/, " ", :g)         # Multiple spaces
            .subst(/^\s|\s$/, "", :g)      # Leading/trailing spaces
            .subst(/<[\[\]\(\)]>/, "", :g); # Brackets
        if $new ne $f.Str {
            $f.rename($new);
            say "Cleaned: {$f.Str} -> $new";
        }
    }
'

Tips

Always do a dry run first (use say instead of .rename)
Save an undo log for batch operations
Be careful with .rename in the current directory -- you might process already-renamed files
Use .sort to ensure consistent ordering for numbered renames
Test your regex on a few filenames before applying to the whole directory

File renaming is one of those tasks where Raku's combination of regex power and clean IO operations really pays off. A one-liner that would take a bash script with sed, awk, and a for loop becomes a single readable expression.