如何使Shell脚本只有一个实例在运行

LOCKFILE=/tmp/lock.txt
if [ -e ${LOCKFILE} ] && kill -0 `cat ${LOCKFILE}`; then
echo "already running"
exit
fi

# make sure the lockfile is removed when we exit and then claim it
trap "rm -f ${LOCKFILE}; exit" INT TERM EXIT
echo $$ > ${LOCKFILE}

#################
# your stuff here
#################

rm -f ${LOCKFILE}