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Full Discussion: How to check if process is running?
Special Forums UNIX and Linux Applications How to check if process is running? Post 302417072 by Corona688 on Wednesday 28th of April 2010 12:01:33 PM
Old 04-28-2010
A traditional way is for a process to create a PID file under /var/run/. I create it under /tmp/ just as an example that can run as any user. I also use a fifo as a lock file to prevent a race condition.

Code:
#!/bin/bash

function rm_pidfile
{
        rm -f /tmp/myscript.pid
}

function is_running
{
        # File does not exist, we can run
        [ -f /tmp/myscript.pid ] || return 1
        # Read PID from file
        read PID < /tmp/myscript.pid
        # File exists but process does not.
        [ -d /proc/${PID} ] || return 1
        return 0
}

# This guarantees other scripts will wait for each other
# before checking the pidfile.  otherwise one might create the PID
# file while we're not looking.
while ! mkfifo /tmp/myscript.lock 2> /dev/null
do
        sleep 1
done

# beyond this point, /tmp/myscript.lock exists and belongs to us.  Other
# instances of the script will loop.

if is_running
then
        echo "An instance(PID ${PID}) is already running"
        rm -f /tmp/myscript.lock
        exit 1
fi

# create PID file
echo "$$" > /tmp/myscript.pid

# make sure we delete it when the program quits
trap "rm_pidfile" EXIT

# Remove the lock file
rm -f /tmp/myscript.lock

# the rest of the code goes here

while true
do
        sleep 1
done

 

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LEARN ABOUT LINUX

lockfile-touch

lockfile-progs(1)						 Lockfile programs						 lockfile-progs(1)

NAME

       lockfile-progs - command-line programs to safely lock and unlock files and mailboxes (via liblockfile).

SYNOPSIS

       mail-lock [--use-pid] [--retry retry-count]
       mail-unlock
       mail-touchlock [--oneshot]

       lockfile-create [--use-pid] [--retry retry-count] [--lock-name] filename
       lockfile-remove [--lock-name] filename
       lockfile-touch [--oneshot] [--lock-name] filename
       lockfile-check [--use-pid] [--lock-name] filename

DESCRIPTION

       Lockfile-progs provides a set a programs that can be used to lock and unlock mailboxes and files safely (via liblockfile):

	   mail-lock - lock the current user's mailbox
	   mail-unlock - unlock the current user's mailbox
	   mail-touchlock - touch the lock on the current user's mailbox

	   lockfile-create - lock a given file
	   lockfile-remove - remove the lock on a given file
	   lockfile-touch - touch the lock on a given file
	   lockfile-check - check the lock on a given file

       By  default,  the filename argument refers to the name of the file to be locked, and the name of the lockfile will be filename .lock.  How-
       ever, if the --lock-name argument is specified, then filename will be taken as the name of the lockfile itself.

       Each of the mail locking commands attempts to lock /var/spool/mail/<user>, where <user> is the name associated with the effective user  ID,
       as determined by via geteuid(2).

       Once  a file is locked, the lock must be touched at least once every five minutes or the lock will be considered stale, and subsequent lock
       attempts will succeed.  Also see the --use-pid option and the lockfile_create(3) manpage.

       The lockfile-check command tests whether or not a valid lock already exists.

OPTIONS

       -q, --quiet
	   Suppress any output.  Success or failure will only be indicated by the exit status.

       -v, --verbose
	   Enable diagnostic output.

       -l, --lock-name
	   Do not append .lock to the filename.  This option applies to lockfile-create, lockfile-remove, lockfile-touch, or lockfile-check.

       -p, --use-pid
	   Write the current process id (PID) to the lockfile whenever a lockfile is created, and use that pid when checking  a  lock's  validity.
	   See	the lockfile_create(3) manpage for more information.  This option applies to lockfile-create, lockfile-remove, lockfile-touch, and
	   lockfile-check.

       -o, --oneshot
	   Touch the lock and exit immediately.  This option applies to lockfile-touch and mail-touchlock.  When not provided, these commands will
	   run forever, touching the lock once every minute until killed.

       -r retry-count, --retry retry-count
	   Try	to  lock filename retry-count times before giving up.  Each attempt will be delayed a bit longer than the last (in 5 second incre-
	   ments) until reaching a maximum delay of one minute between retries.  If retry-count is unspecified, the default is 9 which	will  give
	   up after 180 seconds (3 minutes) if all 9 lock attempts fail.

EXAMPLES

       Locking a file during a lengthy process:

	 lockfile-create /some/file
	 lockfile-touch /some/file &
	 # Save the PID of the lockfile-touch process
	 BADGER="$!"
	 do-something-important-with /some/file
	 kill "${BADGER}"
	 lockfile-remove /some/file

EXIT STATUS

       0
	   For lockfile-check this indicates that a valid lock exists, otherwise it just indicates successful program execution.

       Not 0
	   For	lockfile-check	a  non-zero  exit  status indicates that the specified lock does not exist or is not valid.  For other programs it
	   indicates that some problem was encountered.

SEE ALSO

       maillock(3)
       touchlock(3)
       mailunlock(3)
       lockfile_create(3)
       lockfile_remove(3)
       lockfile_touch(3)
       lockfile_check(3)

AUTHOR

       Written by Rob Browning <rlb@defaultvalue.org>

0.1.12								    2008-02-10							 lockfile-progs(1)
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