adb: Find PID from the adb shell

Not sure if you can get the PID directly however you can try the following

set `ps |grep android.process.acore`
echo $2

This has the affect of setting the output of the ps command into variables $1, $2, $3 etc. The PID value is in $2

Android versions starting with 6.0 already include pidof utility:

usage: pidof [-s] [-o omitpid[,omitpid...]] [NAME]...

Print the PIDs of all processes with the given names.

-s      single shot, only return one pid.
-o      omit PID(s)