Why doesn't ulimit -n work when called inside a script?

Question

This is very odd. I'm trying to set the ulimit to 60000 via my startup.sh:

#!/bin/bash

ulimit -n 60000
echo "Hello! File Descriptor set"

I can execute this with ./startup.sh (755 file permissions) and the echo line is printed, and no errors are shown. However, when I do ulimit -n it still shows 1024, what's going on?

What I also find fascinating is I can type ulimit -n 60000 in the terminal, and then do ulimit -n and it works perfectly. Why can't I set the file descriptor limit through a script?

Debian 8, 64-bit. OpenVZ Container

Answer 1

That's simply because the limit was set inside the shell script i.e. the shell (bash in this case) spawned by executing the script (kernel actually puts the script as an argument to the shell).

So, once the script execution is done, that shell is gone (and the limit) too. And you're back to your original shell.

To test, run the ulimit -n inside the script.

Answer 2

Processes can only set limits for themselves and their children; what you're trying to set with ulimit -n is a global limit, not a per-process one. In order to set global limits as you desire, you could create a Python script using setrlimit() and resource.RLIMIT_NOFILE from the resource module.