Creating a Spring-boot Application, want to running it as a single instance. At the beginning, try to validate directly inside Java code. But thought it again, it seems quite complex and unnecessary, we can simply and directly control this by Shell Script while starting Application.

The basic idea is to find whether current application is running with Process Id.

The Process Id can be found in PID file. If there’s no PID file for this applicaiton means the application is not started. But there is a exception, if application is exit abnormally, PID file could still exist with no Process Id in it.

Based on these information, first idea is:

If we know the application’s name

we can find running process with either Process Id or Applicaiton’s name with ps command.

PIDFILE=/path/to/pid-file
APP_NAME='applications name'
if [ -e ${PIDFILE} ]; then
    PID=`cat ${PIDFILE}`
    PSINFO=`ps -f -p ${PID} | grep ${APP_NAME}`
    if [ ! -z "${PSINFO}" ]; then
        echo ${APP_NAME} is already running PID=${PID}
        exit 1
    fi
fi

Later in Stack Overflow, find a much elegent implementation. no need to know Application’s Name at all.

But If we dont know app’s name at all

PIDFILE=/path/to/pid-file
if [ -f "$PIDFILE" ] && kill -0 `cat $PIDFILE` 2>/dev/null; then
    echo still running
    exit 1
fi  
echo $$ > $PIDFILE

The most celever statement is: Kill -0

In Kill’s manual, you won’t find -0 at all:

     Some of the more commonly used signals:

     1       HUP (hang up)
     2       INT (interrupt)
     3       QUIT (quit)
     6       ABRT (abort)
     9       KILL (non-catchable, non-ignorable kill)
     14      ALRM (alarm clock)
     15      TERM (software termination signal)

Doesn’t matter, let’s try to kill -0 with a running process id:

kill -0 {a running process id}

you will see no output to screen, that means it return 0 .

But if we kill a non-running process id, it will return 1 with a exception error:

$ kill -0 312333 # un-exsited process id
kill: kill 312333 failed: no such process

Thus, with kill command, we would be able to judge whether App is running just based on PID file, what a Hack!!!! Salute you ~