Exclusive locks in Bourne shell scripts

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Been writing some stuff in shell that needs to be able to ensure it doesn’t start itself twice, so a bit of Googling for exclusive locks in shell scripts brought me to this page on “advanced shell scripting” which had something that looked good.

Problem is that when you put it into a script you’ll find it doesn’t actually work because of some trivial coding issues.

So here’s the fixed version of that function..

# Function to do atomic locking.
# Original found at http://members.toast.net/art.ross/rute/node24.html
# Fixed by Chris Samuel http://www.csamuel.org/

function my_lockfile ()
 {
         TEMPFILE="$1.$$"
         LOCKFILE="$1.lock"
         ( echo $$ > $TEMPFILE ) >& /dev/null || (
                 echo "You don't have permission to access `dirname $TEMPFILE`"
                 return 1
         )
         ln $TEMPFILE $LOCKFILE >& /dev/null && {
                 rm -f $TEMPFILE
                 return 0
         }
         kill -0 `cat $LOCKFILE` >& /dev/null && {
                 rm -f $TEMPFILE
                 return 1
         }
         echo "Removing stale lock file"
         rm -f $LOCKFILE
         ln $TEMPFILE $LOCKFILE >& /dev/null && {
                 rm -f $TEMPFILE
                 return 0
         }
         rm -f $TEMPFILE
         return 1
 }

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