It is possible (and very easy) to delete a file's content just by a simple output redirection to that file.

# echo "Some output" > output.o

If output.o is a regular file and it contained some information before this, you can say "Good bye" to it, unless you have another copy stored somewhere on the filesystems/backup media.

Let's say that output.o already contains some information and we do the redirection:

# cat output.o something # echo "another text" >| output.o # cat output.o another text

I've just overwritten the file. The information stored is gone.

To prevent accidental file overwrites from output redirection, use the noclobber variable (type: set -o noclobber in your shell). It will keep you from accidentally overwrite files.

# set -o noclobber # cat output.o another text # echo "something back" > output.o output.o: cannot overwrite existing file

... and there you have it.. :)

Appending output to an existing file

Same as redirecting, only that instead of ">" we use ">>"

# echo "something back" >> output.o && cat output.o another text something back

Overwrite a file's content even if noclobber variable is set

(instead of command > file , use command >| file )

# echo "something back" >| output.o && cat output.o something back

To have noclobber variable set all the time add the line set -o noclobber line to your shell's profile or /etc/profile so that all users will have it (of course they can overwrite it by deactivating noclobber: set +o noclobber).

Pay great attention. NOCLOBBER variable can save you from accidental file overwrites from output redirection, but not from forced redirection (>|)

Also read: How to empty/delete file content in Linux/BSD/Unix

Bookmarks:

Echo "How to prevent accidental file overwrites from output redirection" around:

-------------------advertising-----------------