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:

Recommended readings

• Chapter 6: Shell Redirection from Linux shell scripting wiki.
• Also, read bash man page for more information:
  $ bash(1)
$ help set