How to find the PID of an app without ADB?

If you are using Android 7 or above, you need to have access to ADB (either command-line client, or an app which used adb on localhost) or root access to see the PIDs of other processes.

If you are in Terminal emulator app or in Termux app, and you have root access, do this:

su

ps -A -o PID,USER,NAME

This would output text segregated in three columns: first column as PID, second column is the user which owns the process, and the third column showing the name of the process.

Example output (posting only a snippet of it here):

28574 system

 net.oneplus.odm

28622 u11_a149
  com.oneplus.gallery

28646 u0_a149

com.oneplus.gallery

28701 u11_a149
  com.oneplus.gallery:recyclebin

28727 u11_a149
  com.oneplus.gallery:contentdetector

28778 root

[kworker/1:2H]

28789 u0_a329

su

28794 root

sh

28872 u11_system
com.oneplus.opbackup

Instead of ps command, can also use pidof as suggested by Yahya Uddin here.

Note: PID and UID are altogether different things. If you are after UID, see How can I find app name by UID?