Why an each loop in a Jenkinsfile stops at first iteration

The accepted answer here states that it's a known bug, and uses a workaround that didn't work for me, so I'll offer an update with what I've found lately.

Despite the resolution of JENKINS-26481 (fairly recent, as of this writing) many people may be stuck with an older version of Jenkins where the fix is not available. For-loop iteration over a literal list might work sometimes but related issues like JENKINS-46749 and JENKINS-46747 seem to continue to bedevil many users. Also, depending on the exact context in your Jenkinsfile, possibly echo will work whereas sh fails, and things might fail silently or they might crash the build with serialization failures.

If you dislike surprises (skipped loops and silent failures) and if you want your Jenkinsfiles to be the most portable across multiple versions of Jenkins, the main idea seems to be that you should always use classic counters in your for-loops and ignore other groovy features.

This gist is the best reference I've seen and spells out many cases that you'd think should work the same but have surprisingly different behaviour. It's a good starting place to establish sanity checks and debug your setup, regardless of what kind of iteration you're looking at and regardless of whether you're trying to use @NonCPS, do your iteration directly inside node{}, or call a separate function.

Again, I take no credit for the work itself but I'm embedding the gist of iteration test cases below for posterity:

abcs = ['a', 'b', 'c'] node('master') { stage('Test 1: loop of echo statements') { echo_all(abcs) } stage('Test 2: loop of sh commands') { loop_of_sh(abcs) } stage('Test 3: loop with preceding SH') { loop_with_preceding_sh(abcs) } stage('Test 4: traditional for loop') { traditional_int_for_loop(abcs) } } @NonCPS // has to be NonCPS or the build breaks on the call to .each def echo_all(list) { list.each { item -> echo "Hello ${item}" } } // outputs all items as expected @NonCPS def loop_of_sh(list) { list.each { item -> sh "echo Hello ${item}" } } // outputs only the first item @NonCPS def loop_with_preceding_sh(list) { sh "echo Going to echo a list" list.each { item -> sh "echo Hello ${item}" } } // outputs only the "Going to echo a list" bit //No NonCPS required def traditional_int_for_loop(list) { sh "echo Going to echo a list" for (int i = 0; i < list.size(); i++) { sh "echo Hello ${list[i]}" } } // echoes everything as expected