[ale] Only one process

[James Sumners]

This is a situation where I typically take advantage of screen. If you
tell screen to execute a process, then screen will exit when said
process quits. For example, `screen bash false` results in screen
starting and quitting immediately. Therefore, you can do the
following:

===== Begin script =====
#!/bin/bash

SESSION_NAME="myapp"

screen -list | grep ${SESSION_NAME} > /dev/null
case $? in
    [0]*)
        # screen is already running
        exit 1;;
    [1]*)
        # need to run this thing
        screen -dmS ${SESSION_NAME} bash -c /path/to/myapp
        ;;
esac
===== End Script =====

Then you can set a cron job to run this thing every once in a while.

On Fri, Jun 4, 2010 at 1:11 PM, Chris Fowler
<cfowler at outpostsentinel.com> wrote:
> I've been thinking of ways to guarantee that there is only one process
> of a script running.
>
> pid file
>
> process looks for pid file.  If it sees one it validates that there
> truly is a process at the pid.  If not, it creates new pid file and
> runs.  If so, it sends out error message and exits.
>
> The script is in perl so the what this would work is?
>
> if(-f $pid_file) {
>        # Get the pid info
>        my $pid = `cat $pid_file`;chomp $pid;
>
>        # look for process running
>        if(-f "/proc/$pid/cmdline") {
>          # We may want to look at cmdline and validate it is really
>          # this script and not just a reused pid #
>          die "Process running at $pid already!\n";
>        } else {
>          unlink "$pid_file";
>        }
>
> }
>
> # Ready to go!
>
>
> I'm just looking for what would be considered "The Right Way(tm)"
>
> Chris


-- 
James Sumners
http://james.roomfullofmirrors.com/

"All governments suffer a recurring problem: Power attracts
pathological personalities. It is not that power corrupts but that it
is magnetic to the corruptible. Such people have a tendency to become
drunk on violence, a condition to which they are quickly addicted."

Missionaria Protectiva, Text QIV (decto)
CH:D 59