How to Identify if the process in User Interface Process?

There is no way to determine based purely on the PID number what a specific process is. The reason for this: Process IDs are assigned (somewhat) sequentially from PID=1 on startup, and startup can be different for different systems. The process ID will also be reassigned if, for example, Finder or Dock crashes and has to be restarted.

If you can run a terminal command with a specific pid that you have, though, do this:

ps -p <pid> -o ucomm=

You'll get the filename of the process, which you can check against a list of ones you know are UI processes. For example, here is the output of certain ps commands on my system for my current login session:

> ps -p 110 -o ucomm=Dock> ps -p 112 -o ucomm=Finder

And the following command will give you a list of processes in order of process ID, with only the name:

> ps -ax -o pid=,ucomm=   1 launchd  10 kextd  11 DirectoryService     ...

EDIT: You may be able to do what you ask, though it is convoluted. This answer mentions:

The function CGWindowListCopyWindowInfo() from CGWindow.h will return an array of dictionaries, one for each window that matches the criteria you set, including ones in other applications. It only lets you filter by windows above a given window, windows below a given window and 'onscreen' windows, but the dictionary returned includes a process ID for the owning app which you can use to match up window to app.

If you can obtain all the CGWindows and their respective pids, then you will know the pids of all UI applications without needing to run ps at all.

Rahul has implemented the following code for this approach, which he requested I add to my answer:

CFArrayRef UiProcesses(){    CFArrayRef  orderedwindows = CGWindowListCopyWindowInfo(kCGWindowListOptionAll, kCGNullWindowID);    CFIndex count = CFArrayGetCount (orderedwindows);    CFMutableArrayRef uiProcess = CFArrayCreateMutable (kCFAllocatorDefault , count,  &kCFTypeArrayCallBacks);    for (CFIndex i = 0; i < count; i++)    {        if (orderedwindows)        {            CFDictionaryRef windowsdescription = (CFDictionaryRef)CFArrayGetValueAtIndex(orderedwindows, i);            CFNumberRef windowownerpid  = (CFNumberRef)CFDictionaryGetValue (windowsdescription, CFSTR("kCGWindowOwnerPID"));            CFArrayAppendValue (uiProcess, windowownerpid);        }    }    return uiProcess;}

Try the following.

#include <unistd.h>  if (isatty(STDIN_FILENO) || isatty(STDOUT_FILENO) || isatty(STDERR_FILENO))    // Process associated with a terminal  else    // No terminal - probably UI process

On the lines of darvidsOn, below is the answer to your question.

  CFArrayRef UiProcesses()    {        CFArrayRef  orderedwindows = CGWindowListCopyWindowInfo(kCGWindowListOptionAll, kCGNullWindowID);        CFIndex count = CFArrayGetCount (orderedwindows);        CFMutableArrayRef uiProcess = CFArrayCreateMutable (kCFAllocatorDefault , count,  &kCFTypeArrayCallBacks);        for (CFIndex i = 0; i < count; i++)        {            if (orderedwindows)            {                CFDictionaryRef windowsdescription = (CFDictionaryRef)CFArrayGetValueAtIndex(orderedwindows, i);                CFNumberRef windowownerpid  = (CFNumberRef)CFDictionaryGetValue (windowsdescription, CFSTR("kCGWindowOwnerPID"));                CFArrayAppendValue (uiProcess, windowownerpid);            }        }        return uiProcess;    }

Just compare the processid you have against the array items to get the desired result.