bash Cookbook

Keeping Files Safe from Accidental Overwriting

Problem

You don’t want to delete the contents of a file by mistake. It can be too easy to mistype a filename and find that you’ve redirected output into a file that you meant to save.

Solution

Tell the shell to be more careful, as follows:

$ set -o noclobber
$

If you decide you don’t want to be so careful after all, then turn the option off:

$ set +o noclobber
$

Discussion

The noclobber option tells bash not to overwrite any existing files when you redirect output. If the file to which you redirect output doesn’t (yet) exist, everything works as normal, with bash creating the file as it opens it for output. If the file already exists, however, you will get an error message.

Here it is in action. We begin by turning the option off, just so that your shell is in a known state, regardless of how your particular system may be configured.

$ set +o noclobber
$ echo something > my.file
$ echo some more > my.file
$ set -o noclobber
$ echo something > my.file
bash: my.file: cannot overwrite existing file
$ echo some more >> my.file
$

The first time we redirect output to my.file the shell will create it for us. The second time we redirect, bash overwrites the file (it truncates the file to 0 bytes and starts writing from there). Then we set the noclobber option and we get an error message when we try to write to that file. As we show in the last part of this example, we can append to the file (using >>) just fine.

Warning

Beware! The noclobber option only ...