Since Android 7.0 the easiest way to find out the process ID by package name is to use pidof command:
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)
Just run it like this:
adb shell pidof my.app.package
In Android before 7.0 people used ps command and then parsed its output using either built-in filter by comm value (which for android apps is the last 15 characters of the package name) or grep command. The comm filter did not work if the last 15 characters of the name started with a digit and the grep was not included by default until Android 4.2. But even after the proper process line was found the PID value still needed to be extracted.
There were multiple ways to do that. Here is how finding the process and extracting PID could be done with a single sed command:
adb shell "ps | sed -n 's/^[^ ]* *\([0-9]*\).* my\.app\.package$/\1/p'"
Again the problem is that sed was was not included by default until Android 6.0.
But if you must use an older device you can always use the following Android version independent solution. It does not use any external commands – just Android shell built-ins:
adb shell "for p in /proc/[0-9]*; do [[ $(<$p/cmdline) = my.app.package ]] && echo ${p##*/}; done"
The most popular reason for looking for a PID is to use it in some other command like kill. Let’s say we have multiple instances of logcat running and we want to finish them all gracefully at once. Just replace the echo with kill -2 in the last command:
adb shell "for p in /proc/[0-9]*; do [[ $(<$p/cmdline) = logcat ]] && kill -2 ${p##*/}; done"
Replace " with ' if running the commands from Linux/OSX shell.