Added support for jabba exec.

[ketan]

Refactored use command to improve usability by introducing a GetVars
function that returns the functions to be exported for use by exec.

Implemented an exec that simply passes the args as-is to the specified
binary via syscall.Exec.

Fixes #285

[ketan]

I've taken the liberty of bumping the version in the changelog, hope that's not going to be a problem.

Could you let me know when you release the new binary? Hope it's sooner than later :)

Thanks.

[shyiko]

@ketan thank you! This looks great.

Any chance you would be willing to do something about Windows (syscall.Exec returns EWINDOWS)? Ideally, exec should work on all target OSs. A message showing that it's not supporfted on Windows would work too I suppose.

[ketan]

Sorry, I did not see EWINDOWS, since I'm on a mac. I can take a look at implementing this via a an exec.Command which waits and exits with the exit code of the wrapped process. Not ideal, but is cross platform.

[ketan]

FYI: exec.Command is exactly what I was trying to avoid by replacing the current process with the process corresponding to the args that were passed in.

[shyiko]

@ketan I understand. I'm not asking to replace syscall.Exec. It should stay. All we need is runtime.GOOS == "windows" case :).

[ketan]

To be honest, my motivation of doing exec was running without a shell/powershell shim on CI systems across different platforms that we want to test with. So I'd much rather implement something that works.

[shyiko]

Up to you. I'm fine with either case.

[jkroepke]

Any news about this feature?

[nicerloop]

Not a followup of this feature per se, but you could have a look at https://github.com/nicerloop/jabba-wrapper