Expose an env variable that indicates if pre-commit is running

[lorenzwalthert]

In some circumstances, it could be helpful to know inside an executable / function / script if it was invoked from pre-commit or from some other process. I wrote a hook that just printed all env variables set but I could not find an env variable like PRECOMMIT_RUNNING or similar. Would you consider setting such an env variable during running pre-commit? I searched old issues and documentation but I could not find anything.

[asottile]

I'd be curious to hear the concrete usecase, it seems a bit odd to need to know this information

[lorenzwalthert]

The use case is hacky so I don't post this on GitHub to maintain good reputation 😄 (#todayInOpensource). Well, the thing is that R has a startup process that runs something like a .bash_rc and this may be slow, depending on the code people put in and it is in mostly not necessary for the hooks. The startup process is ran every time a hook is invoked that uses R by default. With the proposed solution above, they could run this code conditional on whether or not a hook invokes the startup process.

Other approach is to always exclude this .bash_rc-like file from startup. But since shebang does generally only take one argument, I can't set the option for excluding (--vanilla) in the shebang #!/usr/bin/env Rscript when using the script language, which is the language used for my hooks. Also, this would make it impossible for people to include it again for whatever reason they have, which I may want to allow.

I understand if you don't want to support this hack 😊. It's also only a minor problem.

[asottile]

The use case is hacky so I don't post this on GitHub to maintain good reputation 😄 (#todayInOpensource).

lol, I know that feel ;)

it should be easy to add something that sets PRE_COMMIT=1 or something -- probably would go in pre_commit/command/run.py if you'd like to send a patch