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Of Zeros and Nulls

Chapter 28. Of Zeros and Nulls

/dev/zero and /dev/null

Uses of /dev/null

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.

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

Suppressing stderr (from Example 15-3).

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 28-1. Hiding the cookie jar

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 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

#!/bin/bash
# Creating a swapfile.

ROOT_UID=0         # Root has $UID 0.
E_WRONG_USER=65    # 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.
#  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 $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 15-55).

Example 28-3. 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
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 binaries.