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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML
><HEAD
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>Of Zeros and Nulls</TITLE
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><DIV
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><H1
><A
NAME="ZEROS"
></A
>Chapter 31. Of Zeros and Nulls</H1
><TABLE
BORDER="0"
WIDTH="100%"
CELLSPACING="0"
CELLPADDING="0"
CLASS="EPIGRAPH"
><TR
><TD
WIDTH="45%"
>&nbsp;</TD
><TD
WIDTH="45%"
ALIGN="LEFT"
VALIGN="TOP"
><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
><P
></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
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>cat $filename &#62;/dev/null
# Contents of the file will not list to stdout.</PRE
></FONT
></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
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>rm $badname 2&#62;/dev/null
# So error messages [stderr] deep-sixed.</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
>Suppressing output from <EM
>both</EM
>
<TT
CLASS="FILENAME"
>stdout</TT
> and <TT
CLASS="FILENAME"
>stderr</TT
>.
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>cat $filename 2&#62;/dev/null &#62;/dev/null
# If "$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 &#38;&#62;/dev/null
# also works, as Baris Cicek points out.</PRE
></FONT
></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
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>cat /dev/null &#62; /var/log/messages
# : &#62; /var/log/messages has same effect, but does not spawn a new process.
cat /dev/null &#62; /var/log/wtmp</PRE
></FONT
></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"
><A
NAME="COOKIES"
></A
><P
><B
>Example 31-1. Hiding the cookie jar</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
># 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.</PRE
></FONT
></TD
></TR
></TABLE
></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
(<EM
>binary</EM
> 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"
><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
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/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 $retcode</PRE
></FONT
></TD
></TR
></TABLE
></DIV
><P
>Another application of <TT
CLASS="FILENAME"
>/dev/zero</TT
>
is to <SPAN
CLASS="QUOTE"
>"zero out"</SPAN
> a file of a designated
size for a special purpose, such as mounting a filesystem
on a <A
HREF="devref1.html#LOOPBACKREF"
>loopback device</A
>
(see <A
HREF="system.html#CREATEFS"
>Example 17-8</A
>) or <SPAN
CLASS="QUOTE"
>"securely"</SPAN
>
deleting a file (see <A
HREF="extmisc.html#BLOTOUT"
>Example 16-61</A
>).</P
><DIV
CLASS="EXAMPLE"
><A
NAME="RAMDISK"
></A
><P
><B
>Example 31-3. Creating a ramdisk</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# ramdisk.sh
# A "ramdisk" is a segment of system RAM memory
#+ which acts as if it were a filesystem.
# Its advantage is very fast access (read/write time).
# Disadvantages: volatility, loss of data on reboot or powerdown,
#+ less RAM available to system.
#
# Of what use is a ramdisk?
# Keeping a large dataset, such as a table or dictionary on ramdisk,
#+ speeds up data lookup, since memory access is much faster than disk access.
E_NON_ROOT_USER=70 # Must run as root.
ROOTUSER_NAME=root
MOUNTPT=/mnt/ramdisk # Create with mkdir /mnt/ramdisk.
SIZE=2000 # 2K blocks (change as appropriate)
BLOCKSIZE=1024 # 1K (1024 byte) block size
DEVICE=/dev/ram0 # First ram device
username=`id -nu`
if [ "$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</PRE
></FONT
></TD
></TR
></TABLE
></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
></DD
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