Advanced Bash−Scripting Guide

Chapter 29. Of Zeros and Nulls
/dev/zero and /dev/null
Uses of /dev/null
Think of /dev/null as a "black hole". It is the nearest equivalent to a write−only file. Everything
written to it disappears forever. Attempts to read or output from it result in nothing. Nevertheless,
/dev/null can be quite useful from both the command line and in scripts.
Suppressing stdout.
cat $filename >/dev/null
# Contents of the file will not list to stdout.
Suppressing stderr (from Example 12−2).
rm $badname 2>/dev/null
# So error messages [stderr] deep−sixed.
Suppressing output from both stdout and stderr.
cat $filename 2>/dev/null >/dev/null
# If "$filename" does not exist, there will be no error message output.
# If "$filename" does exist, the contents of the file will not list to stdout.
# Therefore, no output at all will result from the above line of code.
#
# This can be useful in situations where the return code from a command
#+ needs to be tested, but no output is desired.
#
# cat $filename &>/dev/null
# also works, as Baris Cicek points out.
Deleting contents of a file, but preserving the file itself, with all attendant permissions (from Example
2−1 and Example 2−2):
cat /dev/null > /var/log/messages
# : > /var/log/messages has same effect, but does not spawn a new process.
cat /dev/null > /var/log/wtmp
Automatically emptying the contents of a logfile (especially good for dealing with those nasty
"cookies" sent by Web commercial sites):
Example 29−1. Hiding the cookie jar
if [ −f ~/.netscape/cookies ] # Remove, if exists.
then
rm −f ~/.netscape/cookies
fi
ln −s /dev/null ~/.netscape/cookies
Chapter 29. Of Zeros and Nulls 316