Shell Programming and Scripting

hi guys

Need some help on my below script

#!/bin/sh
if [ -f /root/joy/inittab ]
then
echo "~~:S:wait:/sbin/sulogin" >> /root/joy/inittab
else
echo "/root/joy/inittab does not exist"
fi

now the problem is that when i run the above script it runs successfully
but when i run it repeatedly the word echo "~~:S:wait:/sbin/sulogin" gets added to the file irrespectively the no of times i run the script.
I jst want the line created jst once in the file irrespective of times i run the script.How do i do pls help and thanks in advance

reg:ash

Hope this helps:

#!/bin/bash

if [ -f /root/joy/inittab ]
then
# check to see if inittab already contains this line
grep "~~:S:wait:/sbin/sulogin" /root/joy/inittab >/dev/null

# check return code from grep 0 line was found 1 or higher it wasn't
errorcode=`echo $?`
if [ $errorcode -ne 0 ]
then
echo "~~:S:wait:/sbin/sulogin" >> /root/joy/inittab
fi
else
echo "inittab does not exist"
fi

Just as a stylistic comment, "if" looks at the error code from whatever command you run it on; "if test $?" is a rather roundabout way of saying it.

Code:

#!/bin/bash

if [ -f /root/joy/inittab ]
then
  # check to see if inittab already contains this line
  if ! grep "~~:S:wait:/sbin/sulogin" /root/joy/inittab >/dev/null
  then
    echo "~~:S:wait:/sbin/sulogin" >> /root/joy/inittab
  fi
else
  echo "inittab does not exist"
fi

In theory, there is a race condition here; you should take care not to end up running many of these at the same time, or they will stumble over each other.