3.25. Of Zeros and Nulls

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 or stderr (from Example 3-98):

rm $badname 2>/dev/null
#           So error messages [stderr] deep-sixed.

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
cat /dev/null > /var/log/wtmp

Automatically emptying the contents of a log file (especially good for dealing with those nasty "cookies" sent by Web commercial sites):

rm -f ~/.netscape/cookies
ln -s /dev/null ~/.netscape/cookies
# All cookies now get sent to a black hole, rather than saved to disk.

Like /dev/null, /dev/zero is a pseudo file, but it actually contains nulls (numerical zeros, not the ASCII kind). Output written to it disappears, and it is fairly difficult to actually read the nulls in /dev/zero, though it can be done with od or a hex editor. The chief use for /dev/zero is in creating an initialized dummy file of specified length intended as a temporary swap file.

#!/bin/bash

# Creating a swapfile.
# This script must be run as root.

FILE=/swap
BLOCKSIZE=1024
PARAM_ERROR=33
SUCCESS=0


if [ -z $1 ]
then
  echo "Usage: `basename $0` swapfile-size"
  # Must be at least 40 blocks.
  exit $PARAM_ERROR
fi
    
dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$1

echo "Creating swapfile of size $1 blocks (KB)."

mkswap $FILE $1
swapon $FILE

echo "Swapfile activated."

exit $SUCCESS