During old style jenkins jobs I can easily print current commit ID (e.g. GIT_COMMIT, SVN_REVISION) to the console.
But when I want to use it in Jenkinsfile (multi-branch pipeline)I tried

env.GIT_COMMIT

It gives me null/empty output.
Commit hash might be useful for informing some revision software about build that succeeded or failed (e.g. I want to use it to ping Atlassian Stash about my build).

I think this is one of the basic SCM information that should be available (just like current branch name in env.BRANCH_NAME).