Chapter 31. Of Zeros and Nulls

/dev/zero ... /dev/null

Uses of /dev/null

Think of /dev/null as a blackhole. It is essentially the equivalent ofa write-only file. Everything written to it disappears.Attempts to read or output from it result in nothing. Allthe same, /dev/null can be quiteuseful 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 16-3).

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 2-3):

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 commercial Web sites):

Example 31-1. Hiding the cookie jar

# Obsolete Netscape browser.# Same principle applies to newer browsers.if [ -f ~/.netscape/cookies ] # Remove, if exists.then rm -f ~/.netscape/cookiesfiln -s /dev/null ~/.netscape/cookies# All cookies now get sent to a black hole, rather than saved to disk.

Uses of /dev/zero

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

Example 31-2. Setting up a swapfile using /dev/zero

#!/bin/bash# Creating a swap file.# A swap file provides a temporary storage cache#+ which helps speed up certain filesystem operations.ROOT_UID=0 # Root has $UID 0.E_WRONG_USER=85 # Not root?FILE=/swapBLOCKSIZE=1024MINBLOCKS=40SUCCESS=0# This script must be run as root.if [ "$UID" -ne "$ROOT_UID" ]then echo; echo "You must be root to run this script."; echo exit $E_WRONG_USERfi blocks=${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 # Must be at least 40 blocks long.fi ######################################################################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 # Designate it a swap file.swapon $FILE # Activate swap file.retcode=$? # Everything worked?# Note that if one or more of these commands fails,#+ then it could cause nasty problems.####################################################################### Exercise:# Rewrite the above block of code so that if it does not execute#+ successfully, then:# 1) an error message is echoed to stderr,# 2) all temporary files are cleaned up, and# 3) the script exits in an orderly fashion with an#+ appropriate error code.echo "Swap file created and activated." exit $retcode

Another application of /dev/zero is to "zero out" a file of a designated size for a special purpose, such as mounting a filesystem on a loopback device (see Example 17-8) or "securely" deleting a file (see Example 16-60).

Example 31-3. Creating a ramdisk

#!/bin/bash# ramdisk.sh# A "ramdisk" is a segment of system RAM memory#+ which acts as if it were a filesystem.# Its advantage is very fast access (read/write time).# Disadvantages: volatility, loss of data on reboot or powerdown,#+ less RAM available to system.## Of what use is a ramdisk?# Keeping a large dataset, such as a table or dictionary on ramdisk,#+ speeds up data lookup, since memory access is much faster than disk access.E_NON_ROOT_USER=70 # Must run as root.ROOTUSER_NAME=rootMOUNTPT=/mnt/ramdisk # Create with mkdir /mnt/ramdisk.SIZE=2000 # 2K blocks (change as appropriate)BLOCKSIZE=1024 # 1K (1024 byte) block sizeDEVICE=/dev/ram0 # First ram deviceusername=`id -nu`if [ "$username" != "$ROOTUSER_NAME" ]then echo "Must be root to run "`basename $0`"." exit $E_NON_ROOT_USERfiif [ ! -d "$MOUNTPT" ] # Test whether mount point already there,then #+ so no error if this script is run mkdir $MOUNTPT #+ multiple times.fi##############################################################################dd if=/dev/zero of=$DEVICE count=$SIZE bs=$BLOCKSIZE # Zero out RAM device. # Why is this necessary?mke2fs $DEVICE # Create an ext2 filesystem on it.mount $DEVICE $MOUNTPT # Mount it.chmod 777 $MOUNTPT # Enables ordinary user to access ramdisk. # However, must be root to unmount it.############################################################################### Need to test whether above commands succeed. Could cause problems otherwise.# Exercise: modify this script to make it safer.echo ""$MOUNTPT" now available for use." # The ramdisk is now accessible for storing files, even by an ordinary user.# Caution, the ramdisk is volatile, and its contents will disappear#+ on reboot or power loss.# Copy anything you want saved to a regular directory.# After reboot, run this script to again set up ramdisk.# Remounting /mnt/ramdisk without the other steps will not work.# Suitably modified, this script can by invoked in /etc/rc.d/rc.local,#+ to set up ramdisk automatically at bootup.# That may be appropriate on, for example, a database server.exit 0

In addition to all the above, /dev/zero is needed by ELF (Executable and Linking Format) UNIX/Linux binaries.