linux ulimit with java does not work properly

I run code on linux ubuntu 17.10

public class TestExec {
public static void main(String[] args) {
    try {
        Process p = Runtime.getRuntime().exec(new String[]{"/bin/sh", "-c", "ulimit", "-n"});
        BufferedReader in = new BufferedReader(
                            new InputStreamReader(p.getInputStream()));
        String line = null;
        while ((line = in.readLine()) != null) {
            System.out.println(line);
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}
}

this code returns “unlimited”

but whenever I run command from terminal I get 1024.

Why those numbers are different?

Answer

You get the same result if you run the same command from the command line:

$ "/bin/sh" "-c" "ulimit" "-n"
unlimited

This is because -c only looks at the argument immediately following it, which is ulimit. The -n is not part of this argument, and is instead instead assigned as a positional parameter ($0).

To run ulimit -n, the -n needs to be part of that argument:

$ "/bin/sh" "-c" "ulimit -n"
1024

In other words, you should be using:

new String[]{"/bin/sh", "-c", "ulimit -n"}