Howto find process that processes a bash pipe - linux

Question

I am running a while loop in bash to periodically delete all "old" files in a directory (keeping only the 100 newest files). This process is ran in the background:
((cd /tmp/test && while true; do ls -t | sed -e '1,100d' | xargs -I{} -d '\n' rm -R {}; sleep 1; done)&)
How do I find the process of this bash process in ps?
The objective is that part of a larger script I would like to automatically detect if this process is already running and if not, I would like to start it.
[edit / clarification] Saving the PID is not a solution, because the script can be executed multiple times. It is supposed to ensure that the machine is prepared for a following process. The intention is that the user / developer can just run it to make sure everything is setup. If parts of the conditions are already fulfilled, they will be skipped. Most of what needs to be run are idempotent commands. This is the only command I am struggling to make idempotent. It is an intermediate hack until we have a proper provisioning system in place.

Answer 1

One way would be to do something like in your script:
lock="/tmp/${name}.lock"
if [ -f "${lock}" ];then
echo "already running. pid: $(cat "${lock}")"
exit 1
fi
trap rm "${lock}" 2>/dev/null;exit" 0 1 2 15
echo $$ > "${lock}"
Instead of deleting the N oldest files I'd suggest deleting files older than a certain age, like N days, like so:
N=5 # delete everything older than N days
find /tmp/test -ctime +"${N}"d -depth 1 -exec rm -r {} +
This has a lot of benefits, like being able to handle files with spaces or other odd chars in the name. It also is more predictable because if your process creates, say, 10000 files in /tmp/test, then deleting 100 is going to leave you a mess, or, the other way, may delete new files you really wanted to keep. Notice also that if you have new files in an old directory your new files will be wiped out. -depth 1 keeps /tmp/test itself from being deleted.
If you are really wanting to go with deleting the oldest N files I'd do it within a script and call it something like thecleaner.py & and search ps for that.
#! /usr/bin/env python
# -*- coding: UTF8 -*-
import os
import shutil
import time
secs = 5 # number of seconds to pause during each loop
N = 100 # delete this many of the oldest files
top = "/tmp/test" # remove N oldest files in this directory
while True:
paths = sorted(os.listdir(top), key=os.path.getctime)
oldest = paths[:N]
for path in oldest:
print("removing: {}".format(path))
if os.path.isfile(path):
os.remove(path)
else:
shutil.rmtree(path) # delete entire directory tree
time.sleep(secs)
Otherwise I don't know of any way to reliably find the loop you've given.

Answer 2

ps aux should show the most processes and the line that actually ran them, so you can grep what you need. lsof might show some more, probably too much.
If you need to create something easy to grep, put it in a file, say ~/my.cmd, and run
bash --init-file my.cmd
or just make it runable and run my.cmd.
you could even use a setsid with these to detach from the terminal, so it runs in the background always. Then just
ps aux | grep "my.cmd"
should identify it if there are two hits (two hits - one for grep, the other the run). You can use | wc -l and check if it returns 2.
I suggest you check out crontab though, it seems better suited to what you want over all.