Chapter 28. 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 12-3).

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 28-1. Hiding the cookie jar

if [ -f ~/.netscape/cookies ] # Remove, if exists.
then
 rm -f ~/.netscape/cookies
fi
ln -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 file, but it actually produces a stream of nulls (binary zeros, not the ASCII kind). Output written to it disappears, and it is fairly difficult to actually read the nulls from /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 28-2. Setting up a swapfile using /dev/zero

#!/bin/bash
# Creating a swapfile.
ROOT_UID=0 # Root has $UID 0.
E_WRONG_USER=65 # Not root?
FILE=/swap
BLOCKSIZE=1024
MINBLOCKS=40
SUCCESS=0
# This script must be run as root.
if [ "$UID" -ne "$ROOT_UID" ]
then
 echo; echo "You must be root to run this script."; echo
 exit $E_WRONG_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ドル" ]
# then
# blocks=1ドル
# 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.
echo "Swap file created and activated."
exit $SUCCESS

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 13-8) or "securely" deleting a file (see Example 12-55).

Example 28-3. 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=root
MOUNTPT=/mnt/ramdisk
SIZE=2000 # 2K blocks (change as appropriate)
BLOCKSIZE=1024 # 1K (1024 byte) block size
DEVICE=/dev/ram0 # First ram device
username=`id -nu`
if [ "$username" != "$ROOTUSER_NAME" ]
then
 echo "Must be root to run \"`basename 0ドル`\"."
 exit $E_NON_ROOT_USER
fi
if [ ! -d "$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 $MOUNTPT # Enables ordinary user to access ramdisk.
 # However, must be root to unmount it.
echo "\"$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 binaries.