Allows any user to run a executable program with the permissions of the owner of the program-2. to keep programs that are needed constantly in non-swappable ram-like files or directories, process also runs under the some ownership. by default process start under the ownership of executer. means who is going to execute the command, process start under the ownership or security context of that user or group. when suid or sgid bit is set the executable file, process starts under the security context of file owner then executer. #chmod u+s file : which sets suid bit on file #chmod u+g file : which sets sgid bit on file.