Using a lockfile containing PID (do this as first step in your script)...

myLF=/var/lock/myscript.lock

touch ${myLF}

read myPID < ${myLF}

[ ! -z "${myPID}" -a -d /proc/${myPID} ] && exit

echo $$ > ${myLF}

...

# Set variable with lockfile name

# Create an empty lockfile if it doesn't exist

# Read any PID from the lockfile

# If PID is not null and the process exists, then exit

# Add the PID of the current script to the lockfile

# Your script steps go here

Using an empty lockfile...

myLF=/var/lock/myscript.lock

trap " [ -f ${myLF} ] && /bin/rm -f ${myLF}" 0 1 2 3 13 15 

[ -f ${myLF} ] && exit

touch ${myLF}

...

rm -f ${myLF}

# Set variable with lockfile name

# Delete the lockfile when the script terminates

# Check if the lockfile exists and exit if it does

# Create the lockfile

# Your script steps go here

# Delete the file (but trap should handle it if you don't)

Using an empty diretory as a lockfile...

myLF=/var/lock/myscript.lock

trap " [ -d ${myLF} ] && /bin/rmdir ${myLF}" 0 1 2 3 13 15 

[ -d ${myLF} ] && exit

mkdir ${myLF} || exit

...

Works in a similar way to the empty lockfile above but avoids the theoretical situation where two scripts run at almost the same instant, both think the file does not exist, and both touch the same file (which is a valid operation and will not throw an error).

Using lockfile...

myLF=/var/lock/myscript.lock

trap " [ -f ${myLF} ] && /bin/rm -f ${myLF}" 0 1 2 3 13 15 

lockfile -r 0 ${myLF} || exit 1

...

# Set variable with lockfile name

# Delete the lockfile when the script terminates

# Check if the lockfile exists and exit if it does

# Your script steps go here

# Delete the file (but trap should handle it if you don't)

Using flock...

flock -n /var/lock/myscript.lock /tmp/myscript.sh