Fix stderr leak in Docker ES process detection

[ebarlas]

Fixes a bug in the Docker packaging test infrastructure where bash error messages leak into process detection output, causing the wait loop to exit early with a false positive.

This bug is related to the following issues, but could also relate to previously muted docker tests:

• [CI] EnrollmentProcessTests test20DockerAutoFormCluster failing #140145
• [CI] DockerTests test072RunEsAsDifferentUserAndGroup failing #140127

The Problem

Test test072RunEsAsDifferentUserAndGroup was failing periodically because the certs directory wasn't found - but the real issue was that Elasticsearch never started at all.

The Clue

The test logs showed something suspicious:

[15:39:02,369] Ran: [...] exitCode = [0] stdout = [bash: line 1: /proc/95/cmdline: No such file or directory
0]

The script found a Java process (PID 95), but it subsequently failed to read /proc/95/cmdline. Bash printed an error, and the script returned 0 processes found.

But then the test immediately proceeded as if ES had started. Why?

The Bug

The wait loop checks:

if (psOutput.contains("1")) {
    isElasticsearchRunning = true;
    break;
}

The error message bash: line 1: ... contains "1". This matched the check, causing the loop to exit after 4 seconds instead of waiting 45.

Root Cause

The script uses input redirection:

cmdline=$(tr "\0" " " < /proc/$pid/cmdline 2>/dev/null)

When the file doesn't exist, bash opens it and prints the error. The 2>/dev/null only applies to tr, not to bash's redirection. With docker exec --tty, stderr mixes into the output.

The Fix

Use cat instead:

cmdline=$(cat /proc/$pid/cmdline 2>/dev/null | tr "\0" " ")

Now cat opens the file, so its 2>/dev/null properly suppresses the error.

[elasticsearchmachine]

Hi @ebarlas, I've created a changelog YAML for you.

[elasticsearchmachine]

Pinging @elastic/es-delivery (Team:Delivery)

[rjernst]

LGTM

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	9.3
✅	8.19

[ebarlas]

Missed the label for 9.2 backport. Doing that via tool.

[ebarlas]

💚 All backports created successfully

Status	Branch	Result
✅	9.2

Questions ?

Please refer to the Backport tool documentation

[ebarlas]

💚 All backports created successfully

Status	Branch	Result
✅	9.1

Questions ?

Please refer to the Backport tool documentation