Add execute external program magic.

[frankfliu]

Add a magic that allow user to execute external program in jupyter notebook.

Example:
%system ls -la

[neomatrix369]

@frankfliu great work, and someone has used it to create their own artifact and used it in a notebook, see https://twitter.com/theNeomatrix369/status/1199963064679968769

[ThibTrip]

Hello! I was wondering why this has not been merged.

I tried the code in src/main/java/io/github/spencerpark/ijava/magics/ExecMagics.java from commit 42ea416. It works but the process hangs instead of terminating.

I tried the code below with jdk-14 in Windows 10 with IJava. It converts a jupyter notebook called "learning java.ipynb" in the current directory into a jshell script.

import java.nio.charset.StandardCharsets;
import java.util.List;
import java.util.Scanner;

public class ExecMagics {
    public void exec(List<String> args) throws Exception {
        ProcessBuilder pb = new ProcessBuilder();
        pb.command(args);
        pb.redirectErrorStream(true);
        Process process = pb.start();

        try (Scanner scanner = new Scanner(process.getInputStream(), StandardCharsets.UTF_8)) {
            while (scanner.hasNext()) {
                System.out.println(scanner.nextLine());
            }
        }
    }
}

ExecMagics ex = new ExecMagics();
List<String> cmd = new ArrayList<String>();
cmd.add("cmd");
cmd.add("/k");
cmd.add("jupyter-nbconvert --to script");
cmd.add("./learning_java.ipynb");
ex.exec(cmd);

[NbConvertApp] Converting notebook ./learning_java.ipynb to script
[NbConvertApp] Writing 8396 bytes to .\learning_java.jshell

Note: I am a total newbie in java 🙈

[ThibTrip]

I am an idiot... I was using the wrong command option. If I use cmd.add("/c") instead of cmd.add("/k") it works (see https://stackoverflow.com/a/515380/10551772)! Still, would be nice to see this merged 👍 !

[firasm]

Agreed with above - would be nice if this was merged in to master (main). are there any blockers?