Add a composable string-strip substitution

[Dylan-Gallagher]

Description

Add a composable StringStripSubstitution, exposed to XML and YAML frontends as $(string-strip ...).

This lets a newline-terminated command result be stripped before it is composed with another substitution:

<let name="hostname" value="$(string-strip $(command 'hostname'))"/>

Command itself is unchanged and continues to return raw stdout, including trailing newlines. string-strip is opt-in and also composes with launch configurations, environment variables, and other string-valued substitutions.

Fixes #995.

Is this user-facing behavior change?

Yes. Python launch descriptions can use StringStripSubstitution(value), and XML/YAML launch descriptions can use $(string-strip value). The substitution applies Python's str.strip() semantics, removing leading and trailing whitespace without escaping or otherwise transforming the value.

Testing

• 287 core launch functional tests passed.
• 29 launch_xml functional tests passed.
• 17 launch_yaml functional tests passed.
• The exact command -> strip -> eval XML composition was exercised successfully.
• Changed Python files pass Flake8; new files pass ament_copyright; compilation and diff checks pass.

Did you use Generative AI?

OpenAI Codex (GPT-5) assisted with the implementation, tests, and documentation in this change. I reviewed the complete diff, reproduced the reported failure, and ran the validation listed above.

Additional Information

Stripping resolves the trailing-newline failure reported in #995. It does not make arbitrary command output safe to interpolate as Python source; quotes and backslashes still require context-appropriate handling.

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[peci1]

Wow, you're reading my mind! I just needed this last week :-D

I looked at the PR and it looks good!

[ahcorde]

Pulls: #997
Gist: https://gist.githubusercontent.com/ahcorde/98a6d0846d0a02d94eaa07dea0e6afe3/raw/ae805cbbb23ba00ea93b998b037f21e2e5ca8714/ros2.repos
BUILD args: --packages-above-and-dependencies launch launch_xml launch_yaml
TEST args: --packages-above launch launch_xml launch_yaml
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/20189

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

[peci1]

Wow, you're reading my mind!

Nope, you're reading my issues, I figured :)

I've used your PR as a seed for my agent to produce IsEmpty and NotEmpty: #998 . It worked flawlessly =)