Question

I am using the following to keep a single instance of a script running on my server. I have a cronjob to run this every minute.

How do I daemonize an arbitrary script in unix?

#!/bin/bash

if [[ $# < 1 ]]; then
    echo "Name of pid file not given."
    exit
fi

# Get the pid file's name.
PIDFILE=$1
shift

if [[ $# < 1 ]]; then
    echo "No command given."
    exit
fi

echo "Checking pid in file $PIDFILE."

#Check to see if process running.
PID=$(cat $PIDFILE 2>/dev/null)
if [[ $? = 0 ]]; then
    ps -p $PID >/dev/null 2>&1
    if [[ $? = 0 ]]; then
        echo "Command $1 already running."
        exit
    fi
fi

# Write our pid to file.
echo $$ >$PIDFILE

# Get command.
COMMAND=$1
shift

# Run command
$COMMAND "$*"

Now I found out that my script had hung for some reason and therefore it was stuck. I’d like a way to check if the $PIDFILE is “old” and if so, kill the process. I know that’s possible (check the timestamp on the file) but I don’t know the syntax or if this is even a good idea. Also, when this script is running, the CPU should be pretty heavily used. If it hangs (rare but it happened at least once so far), the CPU usage drops to 0%. It would be nice if I could check that the process is really hung/not active, but I don’t know if there’s an easy way to do that (and I don’t want to have many false positives where it gets killed but it’s running fine).

Answer 1

To answer the question in your title, which seems quite different from your problem, use timeout.

Now, for your problem, I don’t see where it could hang, unless you gave it a fifo queue for the pid file. Now, to run and respawn, you can just run this script once, on startup:

#!/bin/bash

while /bin/true; do
    "$@"
    wait
done

Which brings up another bug in the code you got from the other question: "$*" will pass all the arguments to the script as a single argument; without the quotes it’ll split arguments with white space. "$@" will pass them individually and handling white space properly.

Call with /path/to/script command [argument]....