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.
Suppressing stderr
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 :
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
# 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.
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 28-2. 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 $UID 0. E_WRONG_USER=65 # Not root?
FILE=/swap BLOCKSIZE=1024 MINBLOCKS=40 SUCCESS=0
# This script must be run as root. if [ "$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" ] # 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 $retcode
Another application of /dev/zero is to "zero out" a file of a designated size for a special purpose, such as mounting a filesystem on a loopback device or "securely" deleting a file.
Example 28-3. Creating a ramdisk
#!/bin/bash # ramdisk.sh
# A "ramdisk" is a segment of system RAM memory #+ which acts as if it were a filesystem. # Its advantage is very fast access (read/write time). # Disadvantages: volatility, loss of data on reboot or powerdown. #+ less RAM available to system. # # Of what use is a ramdisk? # Keeping a large dataset, such as a table or dictionary on ramdisk, #+ speeds up data lookup, since memory access is much faster than disk access.
E_NON_ROOT_USER=70 # Must run as root. ROOTUSER_NAME=root
MOUNTPT=/mnt/ramdisk SIZE=2000 # 2K blocks (change as appropriate) BLOCKSIZE=1024 # 1K (1024 byte) block size DEVICE=/dev/ram0 # First ram device
username=`id -nu` if [ "$username" != "$ROOTUSER_NAME" ] then echo "Must be root to run \"`basename $0`\"." 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
In addition to all the above, /dev/zero is needed by ELF (Executable and Linking Format) UNIX/Linux binaries.
