I know there's no original way to obtain process id in a lua file unless you install additional packages to support, But unfortunately I have no privilege to install additional packages.
I just want to write this process id like number to the log file for indication.
I have a multi-process lua engine program to write a single log file. So I want to know if there is another way to write an unique number like process id to the log file?
Thanks in advance!

Well, if you can read arbitrary files, then reading the first word (number) from /proc/self/stat will yield the process ID of the process reading the file in Linux.

You mean how to obtain an unique ID? In Linux, the best bet for a scripting language is to use the process ID, process group ID, or session ID. All three can be parsed from the /proc/self/stat file, although the executable name part (in parentheses, but may contain spaces) makes parsing the latter to a bit annoying. The latter two are the same for a family of related processes (forked from the same parent), so they might be useful if you don't want to identify the exact process but the application (group of processes).

See man 5 proc for details and description of the /proc/self/stat file (under /proc/[pid]/stat ).