how to get process file name from PID in MacOS?

One way of getting the process's binary location is to use lsof and grep for the first txt segment. For example, in the shell to see what binary the shell is, use the shell's PID in place of $$.

$ lsof -p $$ | grep txt
bash    78228 blair  txt    REG   14,2   1244928   6568359 /bin/bash
bash    78228 blair  txt    REG   14,2   1059792  23699809 /usr/lib/dyld
bash    78228 blair  txt    REG   14,2 136368128  81751398 /private/var/db/dyld/dyld_shared_cache_i386

You can see that the shell is using /bin/bash.

This technique works if the process was launched using an absolute or relative path. For example, going into one shell and running

$ sleep 1234567

and using ps in another shell only shows how it was launched:

$ ps auxww|grep '[s]leep'
blair    79462   0.0  0.0   600908    744 s011  S+   11:17PM   0:00.00 sleep 1234567

using lsof shows which binary it ran:

$ lsof -p 79462 | awk '$4 == "txt" { print $9 }'
/opt/local-development/bin/gsleep

I have MacPorts coreutils +with_default_names installed, which explains that I picked up gsleep and not /bin/sleep.

ps -ef with grep works for me. For a specific file name, simply pipe through grep thus:

MacBook:~ Me$ ps -ef | grep Safari | grep -v grep
  501 15733   301   0   0:25.76 ??         1:58.24 /Applications/Safari.app/Contents/MacOS/Safari -psn_0_4977855

(That final 'grep -v grep' simply stops you getting your own grep command in the output)

Example: you're after the associate process command name for PID 45109...

>    % ps awx | awk '$1 == 45109 { print $5 }'
>    /Applications/Safari.app/Contents/MacOS/Safari

ps -p <pid> -Ocommand

L1A1:~ a1155344$ ps -p1 -Ocommand
  PID   TT  STAT      TIME COMMAND
    1   ??  Ss     0:02.26 /sbin/launchd

try ps aux