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>Of Zeros and Nulls</TITLE
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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><DIV
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><H1
><A
NAME="ZEROS"
></A
>Chapter 31. Of Zeros and Nulls</H1
><TABLE
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><TR
><TD
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>&nbsp;</TD
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><I
><P
><I
>Faultily faultless, icily regular, splendidly null</I
></P
><P
><I
>Dead perfection; no more.</I
></P
><P
><I
>--Alfred Lord Tennyson</I
></P
></I
></TD
></TR
></TABLE
><P
><A
NAME="ZEROSREF"
></A
></P
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="ZERONULL1"
></A
><TT
CLASS="FILENAME"
>/dev/zero</TT
>
          ... <TT
CLASS="FILENAME"
>/dev/null</TT
></B
></P
><DL
><DT
><A
NAME="DEVNULLREF"
></A
>Uses of
	    <TT
CLASS="FILENAME"
>/dev/null</TT
></DT
><DD
><P
>Think of <TT
CLASS="FILENAME"
>/dev/null</TT
> as a <I
CLASS="FIRSTTERM"
>black
		hole</I
>. 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, <TT
CLASS="FILENAME"
>/dev/null</TT
> can be quite
		useful from both the command-line and in scripts.</P
><P
>Suppressing <TT
CLASS="FILENAME"
>stdout</TT
>.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;cat $filename &#62;/dev/null
   2&nbsp;# Contents of the file will not list to stdout.</PRE
></TD
></TR
></TABLE
>
            </P
><P
>Suppressing <TT
CLASS="FILENAME"
>stderr</TT
>
	      (from <A
HREF="moreadv.html#EX57"
>Example 16-3</A
>).
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;rm $badname 2&#62;/dev/null
   2&nbsp;#           So error messages [stderr] deep-sixed.</PRE
></TD
></TR
></TABLE
>
	    </P
><P
>Suppressing output from <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>both</I
></SPAN
>
	      <TT
CLASS="FILENAME"
>stdout</TT
> and <TT
CLASS="FILENAME"
>stderr</TT
>.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;cat $filename 2&#62;/dev/null &#62;/dev/null
   2&nbsp;# If "$filename" does not exist, there will be no error message output.
   3&nbsp;# If "$filename" does exist, the contents of the file will not list to stdout.
   4&nbsp;# Therefore, no output at all will result from the above line of code.
   5&nbsp;#
   6&nbsp;#  This can be useful in situations where the return code from a command
   7&nbsp;#+ needs to be tested, but no output is desired.
   8&nbsp;#
   9&nbsp;# cat $filename &#38;&#62;/dev/null
  10&nbsp;#     also works, as Baris Cicek points out.</PRE
></TD
></TR
></TABLE
>
	    </P
><P
>Deleting contents of a file, but preserving the file itself, with
	      all attendant permissions (from <A
HREF="sha-bang.html#EX1"
>Example 2-1</A
> and <A
HREF="sha-bang.html#EX2"
>Example 2-3</A
>):
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;cat /dev/null &#62; /var/log/messages
   2&nbsp;#  : &#62; /var/log/messages   has same effect, but does not spawn a new process.
   3&nbsp;
   4&nbsp;cat /dev/null &#62; /var/log/wtmp</PRE
></TD
></TR
></TABLE
>
	      </P
><P
>Automatically emptying the contents of a logfile
	      (especially good for dealing with those nasty
	      <SPAN
CLASS="QUOTE"
>"cookies"</SPAN
> sent by commercial Web sites):</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="COOKIES"
></A
><P
><B
>Example 31-1. Hiding the cookie jar</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# Obsolete Netscape browser.
   2&nbsp;# Same principle applies to newer browsers.
   3&nbsp;
   4&nbsp;if [ -f ~/.netscape/cookies ]  # Remove, if exists.
   5&nbsp;then
   6&nbsp;  rm -f ~/.netscape/cookies
   7&nbsp;fi
   8&nbsp;
   9&nbsp;ln -s /dev/null ~/.netscape/cookies
  10&nbsp;# All cookies now get sent to a black hole, rather than saved to disk.</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="ZEROSREF1"
></A
>Uses of <TT
CLASS="FILENAME"
>/dev/zero</TT
></DT
><DD
><P
>Like <TT
CLASS="FILENAME"
>/dev/null</TT
>,
	      <TT
CLASS="FILENAME"
>/dev/zero</TT
> is a pseudo-device file, but
	      it actually produces a stream of nulls
	      (<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>binary</I
></SPAN
> zeros, not the <A
HREF="special-chars.html#ASCIIDEF"
>ASCII</A
> kind). Output written
	      to <TT
CLASS="FILENAME"
>/dev/zero</TT
> disappears, and it is
	      fairly difficult to actually read the nulls emitted there,
	      though it can be done with <A
HREF="extmisc.html#ODREF"
>od</A
>
	      or a hex editor. <A
NAME="SWAPFILEREF"
></A
>The chief use of
	      <TT
CLASS="FILENAME"
>/dev/zero</TT
> is creating an initialized
	      dummy file of predetermined length intended as a temporary
	      swap file.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX73"
></A
><P
><B
>Example 31-2. Setting up a swapfile using <TT
CLASS="FILENAME"
>/dev/zero</TT
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Creating a swap file.
   3&nbsp;
   4&nbsp;#  A swap file provides a temporary storage cache
   5&nbsp;#+ which helps speed up certain filesystem operations.
   6&nbsp;
   7&nbsp;ROOT_UID=0         # Root has $UID 0.
   8&nbsp;E_WRONG_USER=85    # Not root?
   9&nbsp;
  10&nbsp;FILE=/swap
  11&nbsp;BLOCKSIZE=1024
  12&nbsp;MINBLOCKS=40
  13&nbsp;SUCCESS=0
  14&nbsp;
  15&nbsp;
  16&nbsp;# This script must be run as root.
  17&nbsp;if [ "$UID" -ne "$ROOT_UID" ]
  18&nbsp;then
  19&nbsp;  echo; echo "You must be root to run this script."; echo
  20&nbsp;  exit $E_WRONG_USER
  21&nbsp;fi  
  22&nbsp;  
  23&nbsp;
  24&nbsp;blocks=${1:-$MINBLOCKS}          #  Set to default of 40 blocks,
  25&nbsp;                                 #+ if nothing specified on command-line.
  26&nbsp;# This is the equivalent of the command block below.
  27&nbsp;# --------------------------------------------------
  28&nbsp;# if [ -n "$1" ]
  29&nbsp;# then
  30&nbsp;#   blocks=$1
  31&nbsp;# else
  32&nbsp;#   blocks=$MINBLOCKS
  33&nbsp;# fi
  34&nbsp;# --------------------------------------------------
  35&nbsp;
  36&nbsp;
  37&nbsp;if [ "$blocks" -lt $MINBLOCKS ]
  38&nbsp;then
  39&nbsp;  blocks=$MINBLOCKS              # Must be at least 40 blocks long.
  40&nbsp;fi  
  41&nbsp;
  42&nbsp;
  43&nbsp;######################################################################
  44&nbsp;echo "Creating swap file of size $blocks blocks (KB)."
  45&nbsp;dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$blocks  # Zero out file.
  46&nbsp;mkswap $FILE $blocks             # Designate it a swap file.
  47&nbsp;swapon $FILE                     # Activate swap file.
  48&nbsp;retcode=$?                       # Everything worked?
  49&nbsp;#  Note that if one or more of these commands fails,
  50&nbsp;#+ then it could cause nasty problems.
  51&nbsp;######################################################################
  52&nbsp;
  53&nbsp;#  Exercise:
  54&nbsp;#  Rewrite the above block of code so that if it does not execute
  55&nbsp;#+ successfully, then:
  56&nbsp;#    1) an error message is echoed to stderr,
  57&nbsp;#    2) all temporary files are cleaned up, and
  58&nbsp;#    3) the script exits in an orderly fashion with an
  59&nbsp;#+      appropriate error code.
  60&nbsp;
  61&nbsp;echo "Swap file created and activated."
  62&nbsp;
  63&nbsp;exit $retcode</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>Another application of <TT
CLASS="FILENAME"
>/dev/zero</TT
>
	      is to <SPAN
CLASS="QUOTE"
>"zero out"</SPAN
> a file of a designated
	      size for a special purpose, such as mounting a filesystem
	      on a <A
HREF="devproc.html#LOOPBACKREF"
>loopback device</A
>
	      (see <A
HREF="system.html#CREATEFS"
>Example 17-8</A
>) or <SPAN
CLASS="QUOTE"
>"securely"</SPAN
>
	      deleting a file (see <A
HREF="extmisc.html#BLOTOUT"
>Example 16-61</A
>).</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="RAMDISK"
></A
><P
><B
>Example 31-3. Creating a ramdisk</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# ramdisk.sh
   3&nbsp;
   4&nbsp;#  A "ramdisk" is a segment of system RAM memory
   5&nbsp;#+ which acts as if it were a filesystem.
   6&nbsp;#  Its advantage is very fast access (read/write time).
   7&nbsp;#  Disadvantages: volatility, loss of data on reboot or powerdown,
   8&nbsp;#+                less RAM available to system.
   9&nbsp;#
  10&nbsp;#  Of what use is a ramdisk?
  11&nbsp;#  Keeping a large dataset, such as a table or dictionary on ramdisk,
  12&nbsp;#+ speeds up data lookup, since memory access is much faster than disk access.
  13&nbsp;
  14&nbsp;
  15&nbsp;E_NON_ROOT_USER=70             # Must run as root.
  16&nbsp;ROOTUSER_NAME=root
  17&nbsp;
  18&nbsp;MOUNTPT=/mnt/ramdisk           # Create with mkdir /mnt/ramdisk.
  19&nbsp;SIZE=2000                      # 2K blocks (change as appropriate)
  20&nbsp;BLOCKSIZE=1024                 # 1K (1024 byte) block size
  21&nbsp;DEVICE=/dev/ram0               # First ram device
  22&nbsp;
  23&nbsp;username=`id -nu`
  24&nbsp;if [ "$username" != "$ROOTUSER_NAME" ]
  25&nbsp;then
  26&nbsp;  echo "Must be root to run \"`basename $0`\"."
  27&nbsp;  exit $E_NON_ROOT_USER
  28&nbsp;fi
  29&nbsp;
  30&nbsp;if [ ! -d "$MOUNTPT" ]         #  Test whether mount point already there,
  31&nbsp;then                           #+ so no error if this script is run
  32&nbsp;  mkdir $MOUNTPT               #+ multiple times.
  33&nbsp;fi
  34&nbsp;
  35&nbsp;##############################################################################
  36&nbsp;dd if=/dev/zero of=$DEVICE count=$SIZE bs=$BLOCKSIZE  # Zero out RAM device.
  37&nbsp;                                                      # Why is this necessary?
  38&nbsp;mke2fs $DEVICE                 # Create an ext2 filesystem on it.
  39&nbsp;mount $DEVICE $MOUNTPT         # Mount it.
  40&nbsp;chmod 777 $MOUNTPT             # Enables ordinary user to access ramdisk.
  41&nbsp;                               # However, must be root to unmount it.
  42&nbsp;##############################################################################
  43&nbsp;# Need to test whether above commands succeed. Could cause problems otherwise.
  44&nbsp;# Exercise: modify this script to make it safer.
  45&nbsp;
  46&nbsp;echo "\"$MOUNTPT\" now available for use."
  47&nbsp;# The ramdisk is now accessible for storing files, even by an ordinary user.
  48&nbsp;
  49&nbsp;#  Caution, the ramdisk is volatile, and its contents will disappear
  50&nbsp;#+ on reboot or power loss.
  51&nbsp;#  Copy anything you want saved to a regular directory.
  52&nbsp;
  53&nbsp;# After reboot, run this script to again set up ramdisk.
  54&nbsp;# Remounting /mnt/ramdisk without the other steps will not work.
  55&nbsp;
  56&nbsp;#  Suitably modified, this script can by invoked in /etc/rc.d/rc.local,
  57&nbsp;#+ to set up ramdisk automatically at bootup.
  58&nbsp;#  That may be appropriate on, for example, a database server.
  59&nbsp;
  60&nbsp;exit 0</PRE
></TD
></TR
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><HR></DIV
><P
>In addition to all the above,
	      <TT
CLASS="FILENAME"
>/dev/zero</TT
> is needed by ELF
	      (<I
CLASS="FIRSTTERM"
>Executable and Linking Format</I
>)
	      UNIX/Linux binaries.</P
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