Protect yourself from overwriting files with the noclobber option

Zettel#55

zettelkasten
Published

2023-04-02

Using the noclobber option:

noclobber is a shell option in Unix-like systems that prevents accidental overwriting of existing files when using output redirection with the > operator. This option is supported only in interactive shells, however. Keep in mind that your scripts will still overwrite files even with noclobber enabled.

To enable the noclobber option, use the set command:

set -o noclobber

Add this line to your .bashrc or .zshrc for permanent effect. Zsh uses setopt and unsetopt, but it also supports set -o and set +o.

If you want the correct zsh option, add this instead:

unsetopt clobber

Example of noclobber preventing file overwriting

Output redirection allows you to save the output of a command to a file. However, using the > operator can accidentally overwrite existing files. With noclobber enabled, this won’t happen:

echo "Hello, world!" > file.txt
echo "This will not overwrite." > file.txt
# Error: cannot overwrite existing file

The >| operator

You can still overwrite the file while the noclobber option is enabled. You just have to use the >| operator.

Example:

echo "This will overwrite." >| file.txt

Disabling noclobber

Bash:

set +o noclobber