HowTo Keep Files Safe From Accidental Overwriting With noclobber BASH Shell Option

Posted on in Categories File system, Linux, Shell scripting, UNIX last updated February 23, 2007

The general format for redirecting output on Bash/ksh shell is:

command > output.txt
command > /dev/device

However, you may end up overwriting file accidentally using > operator. For example:

$ ls -l *.c > output.txt

If file output.txt exists and is a regular file it will be overwritten. Say as a root user you typed “command > /etc/passwd” instead of “command < /etc/passwd". This can spell disaster for /etc/passwd file. On one occasion I had used > when I meant to use >> (append) operator.

How do I avoid accidental overwriting of a file on bash shell?

You can tell bash shell not to delete file data / contents by mistake by setting noclobber variable. It can keep you from accidentally destroying your existing files by redirecting input over an already-existing file.

How do I set noclobber option to prevent overwriting files on bash shell?

Open the Terminal and type the following command:
$ set -o noclobber
Create a test file:
$ echo "foo bar"> output.txt
Next, try to write to a file called output.txt:
$ cat > output.txt
OR
ls -l > output.txt
Sample outputs:

bash: output.txt: cannot overwrite existing file

Add set -o noclobber to your ~/.bashrc file:
$ echo 'set -o noclobber' >> ~/.bashrc

How do I turn off noclobber on bash shell?

Type the following command:
$ set +o noclobber

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

How do I temporary turn off noclobber on bash shell?

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

Putting it all together

Here is a quick demo about avoiding unintentional clobbering (overwrting) a file on bash and ksh shell:

Animated gif 01:  Bash / ksh prevent unintentional clobbering / overwriting a file
Animated gif 01: Bash / ksh prevent unintentional clobbering / overwriting a file

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Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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4 comment

  1. There is also chattr utility that is very useful to prevent files being overwritten and/or modified even under root. In order to protect critically important file just execute chattr +i /path/to/file. To disable protection execute chattr -i /path/to/file.

  2. @Artem

    The utiltiy chattr is not only used to prevent overwritten but also can make a file undeletable

    To view a file which has this attribute set we use the command

    lsattr

    I hope you know this command but to refresh i gave this command

    Thanks and Regards

    V.Balaviswanathan

  3. I know this an ancient post, but the command under “Add set -o noclobber to your ~.bashrc file” has two problems: The ‘smart’ quotes need replace by standard quotes, and the ~ should have a / after it.

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