So...yeah. There's no way to know with consistency what branch we're on without branch-api providing BRANCH_NAME. You can do something custom yourself to check for the branch name in a non-multibranch Jenkinsfile, though, like

when {
  expression {
    return sh(script: "git rev-parse --abbrev-ref HEAD", returnStdout: true) == "master"
  }
}

This should be made clearer in the documentation. There is no mention of the when-branch directive only working for multibranch pipeline

https://jenkins.io/doc/book/pipeline/syntax/#when

npicon Good catch - opened https://github.com/jenkins-infra/jenkins.io/pull/982 to update that.

abayer
I'd suggest that this is still a defect. The defect is in an underlying plugin, but it is still a defect and should be fixed.
Aren't there related bugs filed, that this could be marked as dependent on instead of resolving it?

bitwiseman I disagree - this isn't a bug anywhere, it's just that when branch doesn't work outside of multibranch. That's as designed, not a bug.

Code changed in jenkins
User: Andrew Bayer
Path:
content/doc/book/pipeline/syntax.adoc
http://jenkins-ci.org/commit/jenkins.io/37472aedc9de44ca1f3cbe98c48becfefe8f706f
Log:
JENKINS-43104 Clarify that when->branch only works on multibranch

Code changed in jenkins
User: R. Tyler Croy
Path:
content/doc/book/pipeline/syntax.adoc
http://jenkins-ci.org/commit/jenkins.io/69263ee53b6f54e1d31485149be76fe08388a212
Log:
Merge pull request #982 from abayer/when-branch-only-for-multibranch

JENKINS-43104 Clarify that when->branch only works on multibranch

Compare: https://github.com/jenkins-infra/jenkins.io/compare/2822a194249e...69263ee53b6f

FYI, trim() is required for the workaround in my case.

when {
  expression {
    return sh(script: "git rev-parse --abbrev-ref HEAD", returnStdout: true).trim() == "master"
  }
}

Another workaround is to use:

when {
  expression {
    return env.GIT_BRANCH == "origin/master"
  }
} 

Bulk closing resolved issues.