ROS 2 topic echo: Missing offset pagination for bounded output

Why ros2 topic echo has no built‑in offset

The ros2 topic echo command is implemented as a minimal subscriber that forwards each received sample directly to stdout. Adding an --offset flag would require the command to buffer or discard messages internally before applying --limit, which conflicts with its goal of low‑overhead, zero‑copy output.

Confirmed facts about QoS and client‑side offset

• If the publisher uses BEST_EFFORT reliability, any messages that are dropped before the subscriber connects cannot be recovered, so skipping an offset may cause gaps.
• With RELIABLE reliability and a history depth that covers the desired offset range, the subscriber can safely skip the first N messages without loss.
• Increasing history depth to support large offsets raises memory usage on the subscriber side; the depth should be sized to at least offset + limit.

Lightweight CLI workarounds

1. Pipe echo through standard Unix tools when the output is plain text:

   ros2 topic echo /my_topic --limit 20 | tail -n +$((OFFSET+1))
   

   This shows messages OFFSET+1 … OFFSET+limit.
2. Use a shell variable to increment the start index in a loop:

   START=0
   while true; do
     ros2 topic echo /my_topic --limit 10 --offset $START
     ((START+=10))
     # break condition as needed
   done
   

   (Here we assume a hypothetical --offset flag; see next section.)
3. If the message type is not easily parsed by line‑based tools, write a tiny Python subscriber that counts messages and prints only those after the desired offset; this avoids a full‑blown node and can reuse the same QoS as the echo command.

How to add an optional --offset flag without changing existing behavior

The change would be confined to the callback in topic_echo.cpp:

static size_t skipped = 0;
void callback(const std::shared_ptr& msg) {
  if (skipped < g_offset) {
    ++skipped;
    return; // discard
  }
  if (g_limit && printed >= g_limit) {
    rclcpp::shutdown();
    return;
  }
  // print msg as usual
  ++printed;
}

When --offset is omitted, g_offset defaults to zero, so the existing behavior is unchanged. The flag only adds a small integer counter; no extra buffering or QoS changes are required.
Verification and missing diagnostic detail
To decide whether a client‑side offset is safe, check the publisher’s QoS:
ros2 topic info -v /my_topic

Look for Reliability and History depth. If reliability is BEST_EFFORT or depth is shallow (offset + limit), the offset may miss messages; in that case you would need to increase the publisher’s history or switch to RELIABLE before relying on offset.
If you are unsure about the publisher’s QoS, please provide the output of ros2 topic info -v /my_topic so we can confirm whether the offset approach is appropriate.