mjhjmtu

I have written a script to check how many instances of a process are running in an OpenWrt based system. If I run the following in my terminal

COUNT_PS=$(echo `ps -w | grep -v grep | grep upmpdcli | wc -l`)
root@SHAULA-720:~# echo $COUNT_PS

the result is

1

Below is the code of the shell script , if I run this script the result is 4 instead of 1

#!/bin/ash

#for debug 
ps -w | grep -v grep | grep upmpdcli



COUNT_PS=$(echo `ps -w | grep -v grep | grep upmpdcli | wc -l`)
echo we have $COUNT_PS instances for upmpdcli;
logger we have $COUNT_PS instances for upmpdcli;

if [[ $COUNT_PS == 1 ]]; then 
 logger "we have only one instance"


 #HERE PUT CODE TO START NEW PROCESS
elif [[ $COUNT_PS == 2 ]]; then
 logger "we have 2 instances lets kill all and start a single"
 kill -9 `pgrep upmpdcli`

elif [[ $COUNT_PS == 0 ]]; then
 logger "we have no instance lets wait for cron to start it"


else
 logger "we have $COUNT_PS instances"
fi

so if I run /etc/upmpd-check.sh the result is we have 4 instances for upmpdcli which is strange to me.

What am I missing here?

The main confusion comes from your ps | grep pipeline matching the name of your script, which includes the string upmpdcli.

With pgrep you would not have the same issue as pgrep will look at the command names only by default and would not mistake upmpdcli-check for upmpdcli.

Ideally, you would use

pgrep -x /usr/bin/upmpdcli

to get the PIDs for that process.

To kill that process, or those processes, use

pkill -x /usr/bin/upmpdcli

That is, do not use the PIDs had from pgrep (as these may not be up to date).

To kill only the oldest upmpdcli process, use pkill with -o, and use -n to kill only the newest. See the pkill manual.

Also note that

variable=$( echo `somecommand` )

is better written as

variable=$( some_command )

unless you are relying on the fact that the shell will do word splitting and filename expansions on the result of some_command (you are not).