Hello Raj999,

Welcome to forum Raj, please use code tags as per forum rules for your commands/codes/Inputs used in your posts. Following is the link where you can go through from the forum rules.
https://www.unix.com/misc.php?do=cfrules

Now coming to your question.
1st of all I think there is no need to use for loop you can check with following command. Be aware of using command which you have shown us because I think you don't want to kill all processes and command which you showed us can kill all processes if used correctly, I am a bit confuse here I guess if you are looking for a particular id of a process for example any java process, any script's process you can do following then.

Code:

ps -auxww| grep -v grep | grep "java" | awk '{print "kill -9 " $2}'

If happy with above result use following then.

Code:

ps -auxww| grep -v grep | grep "java" | awk '{print "kill -9 " $2}' |sh

Now coming to your questions of using variables, here is some explaination for same.
i- ${PID}and $PIDseems to be same.
ii- `$PID` is having no meaning as either `PID` or $PID should do the trick to get value of variable PID.
iii- '$PID' this will simply print value $PID as echo will consider it as a simple text.


Please go through my post carefully, DONOT use commands blindly, read the post and try to understand things, get back to us if you have any questions on same.

Thanks,
R. Singh