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

by on February 23, 2007 · 4 comments· LAST UPDATED October 18, 2013

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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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{ 4 comments… read them below or add one }

1 Artem Nosulchik September 11, 2007 at 8:34 am

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.

Reply

2 V.Balaviswanathan May 12, 2009 at 8:47 am

@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

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3 Andy April 28, 2013 at 8:26 pm

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.

Reply

4 nixCraft October 18, 2013 at 6:51 am

The post has been updated to include your response. I appreciate your post and feedback!

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