Of Zeros and Nulls

Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting
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# Chapter 31. Of Zeros and Nulls

| Faultily faultless, icily regular, splendidly null Dead perfection; no more. --Alfred Lord Tennyson

`/dev/zero` ... `/dev/null`

Uses of `/dev/null`

  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`.

         1 cat $filename >/dev/null
         2 # Contents of the file will not list to stdout.

  Suppressing `stderr` (from Example 16-3).

         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):

  Example 31-1. Hiding the cookie jar

         1 # Obsolete Netscape browser.
         2 # Same principle applies to newer browsers.
         3
         4 if [ -f ~/.netscape/cookies ]  # Remove, if exists.
         5 then
         6   rm -f ~/.netscape/cookies
         7 fi
         8
         9 ln -s /dev/null ~/.netscape/cookies
        10 # 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 31-2. Setting up a swapfile using `/dev/zero`

         1 #!/bin/bash
         2 # Creating a swap file.
         3
         4 #  A swap file provides a temporary storage cache
         5 #+ which helps speed up certain filesystem operations.
         6
         7 ROOT_UID=0         # Root has $UID 0.
         8 E_WRONG_USER=85    # Not root?
         9
        10 FILE=/swap
        11 BLOCKSIZE=1024
        12 MINBLOCKS=40
        13 SUCCESS=0
        14
        15
        16 # This script must be run as root.
        17 if [ "$UID" -ne "$ROOT_UID" ]
        18 then
        19   echo; echo "You must be root to run this script."; echo
        20   exit $E_WRONG_USER
        21 fi
        22
        23
        24 blocks=${1:-$MINBLOCKS}          #  Set to default of 40 blocks,
        25                                  #+ if nothing specified on command-line.
        26 # This is the equivalent of the command block below.
        27 # --------------------------------------------------
        28 # if [ -n "$1" ]
        29 # then
        30 #   blocks=$1
        31 # else
        32 #   blocks=$MINBLOCKS
        33 # fi
        34 # --------------------------------------------------
        35
        36
        37 if [ "$blocks" -lt $MINBLOCKS ]
        38 then
        39   blocks=$MINBLOCKS              # Must be at least 40 blocks long.
        40 fi
        41
        42
        43 ######################################################################
        44 echo "Creating swap file of size $blocks blocks (KB)."
        45 dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$blocks  # Zero out file.
        46 mkswap $FILE $blocks             # Designate it a swap file.
        47 swapon $FILE                     # Activate swap file.
        48 retcode=$?                       # Everything worked?
        49 #  Note that if one or more of these commands fails,
        50 #+ then it could cause nasty problems.
        51 ######################################################################
        52
        53 #  Exercise:
        54 #  Rewrite the above block of code so that if it does not execute
        55 #+ successfully, then:
        56 #    1) an error message is echoed to stderr,
        57 #    2) all temporary files are cleaned up, and
        58 #    3) the script exits in an orderly fashion with an
        59 #+      appropriate error code.
        60
        61 echo "Swap file created and activated."
        62
        63 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 (see Example 17-8) or "securely" deleting a file (see Example 16-61).

  Example 31-3. 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           # Create with mkdir /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 (Executable and Linking Format) UNIX/Linux binaries.

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