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Faultily faultless, icily regular, splendidly null Dead perfection; no more. --Alfred Lord Tennyson |
Think of /dev/null as a black hole. It is essentially the equivalent of a write-only file. Everything written to it disappears. Attempts to read or output from it result in nothing. All the same, /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.
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Suppressing stderr (from Example 16-3).
rm $badname 2>/dev/null
# So error messages [stderr] deep-sixed.
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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.
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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
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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/cookies
fi
ln -s /dev/null ~/.netscape/cookies
# All cookies now get sent to a black hole, rather than saved to disk.
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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 $UID 0.
E_WRONG_USER=85 # Not root?
FILE=/swap
BLOCKSIZE=1024
MINBLOCKS=40
SUCCESS=0
# This script must be run as root.
if [ "$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" ]
# 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 $retcode
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Another application of /dev/zero is to "zero out" a file of a designated size for a special purpose, such as mounting a filesystem on a loopback device (see Example 17-8) or "securely" deleting a file (see Example 16-61).
Example 31-3. Creating a ramdisk
#!/bin/bash
# ramdisk.sh
# A "ramdisk" is a segment of system RAM memory
#+ which acts as if it were a filesystem.
# Its advantage is very fast access (read/write time).
# Disadvantages: volatility, loss of data on reboot or powerdown,
#+ less RAM available to system.
#
# Of what use is a ramdisk?
# Keeping a large dataset, such as a table or dictionary on ramdisk,
#+ speeds up data lookup, since memory access is much faster than disk access.
E_NON_ROOT_USER=70 # Must run as root.
ROOTUSER_NAME=root
MOUNTPT=/mnt/ramdisk # Create with mkdir /mnt/ramdisk.
SIZE=2000 # 2K blocks (change as appropriate)
BLOCKSIZE=1024 # 1K (1024 byte) block size
DEVICE=/dev/ram0 # First ram device
username=`id -nu`
if [ "$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.
# 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
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In addition to all the above, /dev/zero is needed by ELF (Executable and Linking Format) UNIX/Linux binaries.