Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
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Uses of /dev/zero
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.
Example 29−2. Setting up a swapfile using /dev/zero
#!/bin/bash
# Creating a swapfile.
# This script must be run as root.
ROOT_UID=0 # Root has $UID 0.
E_WRONG_USER=65 # Not root?
FILE=/swap
BLOCKSIZE=1024
MINBLOCKS=40
SUCCESS=0
if [ "$UID" −ne "$ROOT_UID" ]
then
echo; echo "You must be root to run this script."; echo
exit $E_WRONG_USER
fi
blocks=${1:−$MINBLOCKS}
# Set to default of 40 blocks,
#+ if nothing specified on command line.
# This is the equivalent of the command block below.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# if [ −n "$1" ]
# then
# blocks=$1
# else
# blocks=$MINBLOCKS
# fi
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
if [ "$blocks" −lt $MINBLOCKS ]
then
blocks=$MINBLOCKS
fi
# Must be at least 40 blocks long.
echo "Creating swap file of size $blocks blocks (KB)."
dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$blocks # Zero out file.
mkswap $FILE $blocks
swapon $FILE
# Designate it a swap file.
# Activate swap file.
echo "Swap file created and activated."
exit $SUCCESS
Chapter 29. Of Zeros and Nulls 317