`ros2 param dump` silently overwrites existing file

Bug report

Required Info:

• Operating System:
  • Ubuntu 20.04
• Installation type:
  • Debian packages
• Version or commit hash:
  • ros-rolling-ros2param 0.13.0-1
• DDS implementation:
  • CycloneDDS
• Client library (if applicable):
  • rclpy

Steps to reproduce issue

In terminal 1:

ros2 run turtlesim turtlesim_node

In terminal 2:

echo "foo" > turtlesim.yaml  # turtlesim.yaml now contains "foo"
ros2 param dump /turtlesim  # this overwrites the existing turtlesim.yaml, which may be surprising

Expected behavior

When ros2 param dump is choosing a filename, it should either error out if a file with the name it chose already exists, or it should keep choosing names until it finds one that has no conflict.

Actual behavior

ros2 param dump silently ovewrites the existing file.

[hidmic]

Sometimes it is practical for it to just overwrite it. Perhaps making opt-in would be best.

[clalancette]

What if the default were to write to stdout, with an option to write to a named file? Then overwriting is no big deal because the user had to give a filename to write to.

That’s a pretty good idea, I like it. This is also more Unix-y; you could always do ros2 param dump /turtlesim > turtlesim.yaml.

The downside is that this breaks compatibility with earlier releases. That is, in Foxy, if you do ros2 param dump /turtlesim, by default it writes to a file. If you then pass --print, it prints to stdout. If we were to make this change, then in Galactic ros2 param dump /turtlesim would print to stdout, and passing --write <fname> (or whatever) would write it to a file.

I don’t know, how do people feel about a command-line break like that?