How to keep files safe from accidental overwriting with noclobber under BASH shell

This happens many times. You accidentally use redirecting output using > operator.

For example you type a command:
$ ls -l *.c > output.txt

If file output.txt exists and is a regular file it will be overwritten. Just imagine as root user, typing somecommand > /etc/passwd instead of somecommand < /etc/passwd.

Or you used > when they meant to use >> (append).

So how do you tell shell not to delete file data / contents by mistake?

You need to set noclobber variable. It can keep you from accidentally destroying your existing files by redirecting input over an already-existing file.

Task: Set noclobber i.e. prevent overwriting

Type the command:
$ set -o noclobber

Now try to write to a file called output.txt
$ cat > output.txt
OR
ls -l > output.txt
Output:

bash: output.txt: cannot overwrite existing file

Add set -o noclobber to your ~.bashrc file:
$ echo ‘set -o noclobber’ >> ~.bashrc

Task: Turn off noclobber

Type the following command:
$ set +o noclobber

Using + rather than - causes these flags to be turned off.

Task: Temporary turn off noclobber

Sometime you just need to turn off noclobber for single operation. Use >| operator to force the file to be overwritten:
$ ls /etc >| output.txt
$ less output.txt


Read bash man page for more information:
$ man bash
$ help set

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