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
ls -l > output.txt
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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