What does "shell out" or "shelling out" mean?

Question

As used in these examples, for instance:

shell out to bundle from inside a command invoked by bundle exec

or

shell out to a Ruby command that is not part of your current bundle,

http://bundler.io/man/bundle-exec.1.html

or

i'm shelling out to the heroku command in the rake task

https://github.com/sstephenson/rbenv/issues/400

Answer 1

It means executing a subprocess using backticks (as in `command`), the system call, or other similar methods. These execute the process in a sub-shell, hence the name.

You can find a lot more details in this answer: https://stackoverflow.com/a/18623297/29470

Answer 2

Spawning a pipeline of connected programs via an intermediate shell — a.k.a. “shelling out”

http://julialang.org/blog/2012/03/shelling-out-sucks/

And the related reddit comment thread: http://www.reddit.com/r/programming/comments/1bwbyf/shelling_out_sucks/

So, from what I can gather, I presume it means "going out from the context of the executing program, to the surrounding program, or execution environment", in broad terms. Usually you go out to the unix shell, hence the term shell out.