#!/bin/sh
LOCKFILE="/tmp/mutex-script.lock"
## noclobber prevents that an existing file gets overwritten when using the ">" operator
if /bin/sh -c "set -o noclobber; echo \"$$\" > \"$LOCKFILE\""; then
    echo "aquired $LOCKFILE"
    trap "rm -f \"$LOCKFILE\"; echo \"released $LOCKFILE\"; exit \$?" INT TERM EXIT KILL
else
    echo "could not aquire $LOCKFILE"
    exit 1
fi
echo "DO STUFF"
sleep 20
echo "DONE"

#!/bin/sh
LOCKDIR="/tmp/mutex-script.lock"
## "mkdir" is atomic
if mkdir "$LOCKDIR"; then
    echo "aquired $LOCKDIR"
    trap "rmdir \"$LOCKDIR\"; echo \"released $LOCKDIR\"; exit \$?" INT TERM EXIT KILL
else
    echo "could not aquire $LOCKDIR"
    exit 1
fi
echo "DO STUFF"
sleep 20
echo "DONE"