Of Zeros and Nulls

Advanced Bash-Scripting Guide:

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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

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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

# 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 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
if [ -n "$1" ]
then
blocks=$1
else
blocks=$MINBLOCKS	# Set to default of 40 blocks
fi	# if nothing specified on command line.
if [ "$blocks" -lt $MINBLOCKS ]
then
blocks=$MINBLOCKS	# Must be at least 40 blocks long.
fi

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Of Zeros and Nulls

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 13-6) or securely deleting a file (see Example 12-41).

Example 29-3. Creating a ramdisk

#!/bin/bash

#ramdisk.sh

#A "ramdisk" is a segment of system RAM memory #+ that 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.
#

#What good 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`\"." 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.
mke2fs $DEVICE	# Create an ext2 filesystem on it.
mount $DEVICE $MOUNTPT	# Mount it.
chmod 777 $MOUNTPT	# So ordinary user can	access ramdisk.

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# 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 again to set up ramdisk.

#Remounting /mnt/ramdisk without the other steps will not work.

exit 0

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