if mkdir $LOCKDIR
then
    # Do important, exclusive stuff
    if rmdir $LOCKDIR
    then
        echo "Victory is mine"
    else
        echo "Could not remove lock dir" >&2
    fi
else
    # Handle error condition
    ...
fi

#Remove the lock directory
function cleanup {
    if rmdir $LOCKDIR; then
        echo "Finished"
    else
        echo "Failed to remove lock directory '$LOCKDIR'"
        exit 1
    fi
}

if mkdir $LOCKDIR; then
    #Ensure that if we "grabbed a lock", we release it
    #Works for SIGTERM and SIGINT(Ctrl-C)
    trap "cleanup" EXIT

    echo "Acquired lock, running"

    # Processing starts here
else
    echo "Could not create lock directory '$LOCKDIR'"
    exit 1
fi

#!/bin/bash

# Exit if /tmp/lock.file exists
[ -f /tmp/lock.file ] && exit

# Create lock file, sleep 1 sec and verify lock
echo $$ > /tmp/lock.file
sleep 1
[ "x$(cat /tmp/lock.file)" == "x"$$ ] || exit

# Do stuff
sleep 60

# Remove lock file
rm /tmp/lock.file

for LINE in `lsof -c <your_script> -F p`; do
    if [ $$ -gt ${LINE#?} ] ; then
        echo "'$0' is already running" 1>&2
        exit 1;
    fi
done

#!/bin/bash

me=$(basename $0);
running=$(ps h -C $me | grep -v $$ | wc -l);
[[ $running > 1 ]] && exit;

# do stuff below this comment

#!/bin/sh

[ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock "$0" "$0" "$@" || exit 1

echo "servus!"
sleep 10

#!/bin/bash

# Check if another instance of script is running
pidof -x $0 >/dev/null && echo "ERROR: Script $0 already running" && exit 1

...