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.
1 cat $filename >/dev/null 2 # Contents of the file will not list to stdout. |
Suppressing stderr (from Example 15-3).
1 rm $badname 2>/dev/null 2 # So error messages [stderr] deep-sixed. |
Suppressing output from both stdout and stderr.
1 cat $filename 2>/dev/null >/dev/null 2 # If "$filename" does not exist, there will be no error message output. 3 # If "$filename" does exist, the contents of the file will not list to stdout. 4 # Therefore, no output at all will result from the above line of code. 5 # 6 # This can be useful in situations where the return code from a command 7 #+ needs to be tested, but no output is desired. 8 # 9 # cat $filename &>/dev/null 10 # 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 2-3):
1 cat /dev/null > /var/log/messages 2 # : > /var/log/messages has same effect, but does not spawn a new process. 3 4 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):
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 from there, 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 predetermined length intended as a temporary swap file.
Example 28-2. Setting up a swapfile using /dev/zero
1 #!/bin/bash
2 # Creating a swapfile.
3
4 ROOT_UID=0 # Root has $UID 0.
5 E_WRONG_USER=65 # Not root?
6
7 FILE=/swap
8 BLOCKSIZE=1024
9 MINBLOCKS=40
10 SUCCESS=0
11
12
13 # This script must be run as root.
14 if [ "$UID" -ne "$ROOT_UID" ]
15 then
16 echo; echo "You must be root to run this script."; echo
17 exit $E_WRONG_USER
18 fi
19
20
21 blocks=${1:-$MINBLOCKS} # Set to default of 40 blocks,
22 #+ if nothing specified on command line.
23 # This is the equivalent of the command block below.
24 # --------------------------------------------------
25 # if [ -n "$1" ]
26 # then
27 # blocks=$1
28 # else
29 # blocks=$MINBLOCKS
30 # fi
31 # --------------------------------------------------
32
33
34 if [ "$blocks" -lt $MINBLOCKS ]
35 then
36 blocks=$MINBLOCKS # Must be at least 40 blocks long.
37 fi
38
39
40 ######################################################################
41 echo "Creating swap file of size $blocks blocks (KB)."
42 dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$blocks # Zero out file.
43 mkswap $FILE $blocks # Designate it a swap file.
44 swapon $FILE # Activate swap file.
45 # Note that if one or more of these commands fails,
46 #+ then it could cause nasty problems.
47 ######################################################################
48
49 # Exercise:
50 # Rewrite the above block of code so that if it does not execute
51 #+ successfully, then:
52 # 1) an error message is echoed to stderr,
53 # 2) all temporary files are cleaned up, and
54 # 3) the script exits in an orderly fashion with an
55 #+ appropriate error code.
56
57 echo "Swap file created and activated."
58
59 exit $SUCCESS |
Another application of /dev/zero is to "zero out" a file of a designated size for a special purpose, such as mounting a filesystem on a loopback device (see Example 16-8) or "securely" deleting a file (see Example 15-56).
Example 28-3. Creating a ramdisk
1 #!/bin/bash 2 # ramdisk.sh 3 4 # A "ramdisk" is a segment of system RAM memory 5 #+ which acts as if it were a filesystem. 6 # Its advantage is very fast access (read/write time). 7 # Disadvantages: volatility, loss of data on reboot or powerdown. 8 #+ less RAM available to system. 9 # 10 # Of what use is a ramdisk? 11 # Keeping a large dataset, such as a table or dictionary on ramdisk, 12 #+ speeds up data lookup, since memory access is much faster than disk access. 13 14 15 E_NON_ROOT_USER=70 # Must run as root. 16 ROOTUSER_NAME=root 17 18 MOUNTPT=/mnt/ramdisk 19 SIZE=2000 # 2K blocks (change as appropriate) 20 BLOCKSIZE=1024 # 1K (1024 byte) block size 21 DEVICE=/dev/ram0 # First ram device 22 23 username=`id -nu` 24 if [ "$username" != "$ROOTUSER_NAME" ] 25 then 26 echo "Must be root to run \"`basename $0`\"." 27 exit $E_NON_ROOT_USER 28 fi 29 30 if [ ! -d "$MOUNTPT" ] # Test whether mount point already there, 31 then #+ so no error if this script is run 32 mkdir $MOUNTPT #+ multiple times. 33 fi 34 35 ############################################################################## 36 dd if=/dev/zero of=$DEVICE count=$SIZE bs=$BLOCKSIZE # Zero out RAM device. 37 # Why is this necessary? 38 mke2fs $DEVICE # Create an ext2 filesystem on it. 39 mount $DEVICE $MOUNTPT # Mount it. 40 chmod 777 $MOUNTPT # Enables ordinary user to access ramdisk. 41 # However, must be root to unmount it. 42 ############################################################################## 43 # Need to test whether above commands succeed. Could cause problems otherwise. 44 # Exercise: modify this script to make it safer. 45 46 echo "\"$MOUNTPT\" now available for use." 47 # The ramdisk is now accessible for storing files, even by an ordinary user. 48 49 # Caution, the ramdisk is volatile, and its contents will disappear 50 #+ on reboot or power loss. 51 # Copy anything you want saved to a regular directory. 52 53 # After reboot, run this script to again set up ramdisk. 54 # Remounting /mnt/ramdisk without the other steps will not work. 55 56 # Suitably modified, this script can by invoked in /etc/rc.d/rc.local, 57 #+ to set up ramdisk automatically at bootup. 58 # That may be appropriate on, for example, a database server. 59 60 exit 0 |
In addition to all the above, /dev/zero is needed by ELF binaries.