Shell Programming and Scripting

That is not a pipe, that's a file redirection. | is a pipe.

And yes,

Code:

# Automatically append stdout to this file
exec 1>>/home/mcmaps/scripts/overviewer.log

That opens the file once in advance and attaches it to stdout, so you don't need to reopen-append the same file 17 times in a row later.

Anything you run inherits the same set of open files as the shell you're running. This is why they all print to the same screen. Change the shell's own "screen" (i.e. standard output, stdout, or file descriptor #1) into a file, and they'll all write to that file.

Wow... Thank you so much. I have a ton of reading to do now to benefit from all that you have taught me today.

Two comments
1.
In order to recover from a system crash the pidfile should reside in a file system that is emty or emptied at system boot.
2.
Collecting output from a background process can become garbled, especially if mixed with another stream like the foreground echo at the end.

The program logic doesn't just blindly check if the file exists, it checks if it exists and is valid.

It will not become garbled here due to printing before the process exists and after it quits and nowhere inbetween.

1.
a different process can run with that PID...
Maybe the ps $PID is safe enough on earth but not on a mars mission.

Hi everyone,

So the thread so far has left me confused. So I kept digging. If found some code and modified it to this:

Code:

if ps -u mcmaps | grep 'overviewer.py'
then
     echo Running
else
     echo Not Running
fi

So far, this works. The "overviewer.py" is only run from the mcmaps user. And when it is currently running, the script above reports "Running." When it's not running, it accurately states, "Not Running" (so far).

So, would this work? What are the dangers of this?

Here is what I would do for the final script (if people say it would be safe enough):

Code:

if ps -u mcmaps | grep 'overviewer.py'
then
     echo overviewer.py is already running.
else
     overviewer.py --verbose --quiet --processes 1 --config=/home/mcmaps/servers/bukkitpvt/overviewer.conf 
fi

The idea here is that I run the script as a cron job every 30 minutes. And most of the time, it completes in 10 minutes or less. But if there were a lot of changes for the overviewer.py to work on, the script could take 30+ minutes. In which case, I want the script to exit out and wait for the next scheduled cron job event to run rather than trying to run two instances over overviewer.py.

Am I on the right track? Is this way of doing it flawed?

That's another way to do it.
The ps command lists many processes, but with-u mcmaps the overhead is small.
There might be the problem that ps cuts the overviewer.py to say overview. Run the ps command on the command line to find out!