Question

I know ps command can see this , but given the pid , how can one know which user started that program ? I see a loginuid in /proc/[pid]/ , but that seems to be a complemental-code of the real uid , not sure.

Answer 1

How about status ?

cnicutar@lemon:/proc/self$ cat status
Name:   bash
State:  S (sleeping)
#....
Uid:    1000    1000    1000    1000
Gid:    1000    1000    1000    1000

There are 4 UIDs: Real, effective, saved set, and file system. It’s all described in proc(5).