How can I get the process with the biggest pid using
This doesn't use
ps, but parsing
ps is likely to be difficult (not to mention non-portable). This ought to be easier (and at least a bit more portable):
( cd /proc; printf "%s\n" *; ) | sort -n | tail -n 1
That looks for the highest numbered directory inside /proc, which works because on many Unix systems there's one /proc/### directory per pid that contains information about that process.
ps -Ao pid= | sort -rn | head -n 1
would be POSIX.
On Linux, process ids share the same namespace as the thread ids. There, you can do:
ps -LAo tid= | sort -rn | head -n 1
To get the highest thread or process id number.