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 or stderr (from Example 12-2):
rm $badname 2>/dev/null # So error messages [stderr] deep-sixed. |
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 |
Automatically emptying the contents of a logfile (especially good for dealing with those nasty "cookies" sent by Web commercial sites):
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 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 $SUCCESS |
Another application of /dev/zero is to "zero out" a file of a designated size for a special purpose, such as mounting a filesystem on a loopback device (see Example 13-6) or securely deleting a file (see Example 12-31).
Example 29-3. Creating a ramdisk
#!/bin/bash
# ramdisk.sh
# A "ramdisk" is a segment of system RAM memory
#+ that acts as if it were a filesystem.
# Its advantage is very fast access (read/write time).
# Disadvantages: volatility, loss of data on reboot or powerdown.
# less RAM available to system.
#
# What good is a ramdisk?
# Keeping a large dataset, such as a table or dictionary on ramdisk
#+ speeds up data lookup, since memory access is much faster than disk access.
E_NON_ROOT_USER=70 # Must run as root.
ROOTUSER_NAME=root
MOUNTPT=/mnt/ramdisk
SIZE=2000 # 2K blocks (change as appropriate)
BLOCKSIZE=1024 # 1K (1024 byte) block size
DEVICE=/dev/ram0 # First ram device
username=`id -nu`
if [ "$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 $MOUNTPT # So ordinary user can access ramdisk.
# However, must be root to unmount it.
echo "\"$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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