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.
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