feat: export an additional env var pointing to the location of jbang

[dsyer]

It's really hard (impossible?) to find the location of the script that launched the JVM from within jbang. It would be really useful to be able to do that so you can run jbang again with some logic to precompute the command line. This works really simply:

$ jbang --interactive
WARNING: Using incubator modules: jdk.incubator.vector, jdk.incubator.foreign
|  Welcome to JShell -- Version 17.0.7
|  For an introduction type: /help intro

jshell> System.getenv("JBANG")
$1 ==> "/home/dsyer/.sdkman/candidates/jbang/current/bin/jbang"

Edit: I don't know how to do this for Windows users. Anyone got an idea?

[maxandersen]

I'm not totally against exporting this variable assuming we find way it works on windows but Thus far we haven't exposed this as its not guaranteed to work.

i.e. the same app could have been run with pure java or native exc launched.

I would prefer some more internal name like JBANG_LAUNCH_SCRIPT or similar.

[dsyer]

I don't care about the name really. How about JBANG_LAUNCHER? I get that it's not guaranteed to work if the launcher isn't used, but at least it provides a way to launch jbang in the case that the launcher is used (which is the majority I suspect). I can't think of anything that would work completely generically.

[maxandersen]

okey, so I pushed code that should work on osx, linux and windows (need ci to verify the latter one).

I also checked what other env vars we actually expose and added tests for those ...BUT I can't actually spot where we use them.. @quintesse can we remove those?

[maxandersen]

@quintesse what is the use of this variable?

[quintesse]

The runtime scripts tells JBang (Java code) what shell (eg. *nix sh/bash/etc or CMD or Powershell) it was started from. There's just no good way to otherwise detect what kind of command line we should be generating.

[maxandersen]

@quintesse what is the use of this variable?

[quintesse]

This tells JBang (Java code) that we don't have a console/terminal connected to stdin. Java can't detect this by itself.

[dsyer]

I believe this is a pretty good approximation, in case you ever need it:

boolean isAnsi = System.console()!=null && 
  System.getProperty("os.name").toLowerCase(Locale.ENGLISH).contains("win");

[quintesse]

@dsyer that would only work on *nix systems and we need it to work for Windows too.

[maxandersen]

But where in code are we using that env var?

[quintesse]

See Util.haveConsole() which is used by ConsoleInput.get() which is used by Util.askInput() to detect if we can ask the user for input on the console or if we should use a GUI.

[maxandersen]

kk

[maxandersen]

I actually prefer if we call this _JBANG_LAUNCH_CMD but kept with JBANG_ as we use that above already.

Reason beeing we actually have a few JBANG_* env var used for input - so wanted _JBANG to signal its not input just temporarily available.

[dsyer]

I said I didn't care about the name, but I don't like starting env var names with non-alpha chars, so I vote for JBANG_LAUNCH_CMD.

[maxandersen]

@all-contributors please add @dsyer for code

[allcontributors]

@maxandersen

I've put up a pull request to add @dsyer! 🎉