The Fastest Way to Back Up a File in Bash (You’re Probably Not Using It)

When working in the terminal, backing up a file before editing it is a habit that saves time, nerves, and sometimes entire systems. There’s a Bash trick that makes this almost effortless:

cp filename{,.bak}

How It Works

This command relies on brace expansion, a shell feature that generates multiple strings from a single pattern.

filename{,.bak}

expands to:

filename filename.bak

So Bash actually executes:

cp filename filename.bak

You type the filename once, and Bash handles the rest.

Why This Is Better Than the “Normal” Way

Most people write:

cp filename filename.bak

That works — but brace expansion has advantages:

• Faster typing
• Fewer mistakes, especially with long filenames
• Cleaner commands
• Easier to scan in scripts and terminal history

Once you get used to it, going back feels slow.

Real-World Examples

Before editing a configuration file:

cp nginx.conf{,.bak}

Before changing a script:

cp deploy.sh{,.bak}

Need to roll back?

mv filename.bak filename

Instant undo.

Useful Variations

Different suffix:

cp file{,.old} 
cp file{,.backup}

Timestamped backup:

cp file{,.$(date +%F)}

Result:

file.2025-12-15

Interactive mode (asks before overwriting):

cp -i filename{,.bak}

Important Notes

• Works in Bash, Zsh, and most modern shells
• Does not work in plain sh (POSIX shell)
• Overwrites existing .bak files unless -i is used

Why This Matters

This is a small trick, but it encourages a safer workflow:

• Backups become automatic
• You’re more likely to make one
• Mistakes become reversible

Over time, these tiny habits add up to fewer outages, fewer panics, and cleaner shell usage.

Final Thought

If you use the terminal daily, this one-liner deserves a permanent spot in your muscle memory:

cp filename{,.bak}

Small command. Big payoff.