Of Zeros and Nulls

Advanced Bash-Scripting Guide:
Prev |  | Next

# 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`.

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

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

      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 (from Example 2-1 and Example 2-3):

      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 31-1. 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 31-2. 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=85    # 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 (see Example 17-8) or "securely" deleting a file (see Example 16-61).

  Example 31-3. 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           # Create with mkdir /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.

Prev | Home | Next
Network Programming | Up | Debugging