Asciinema Record

Record a focused terminal session using asciinema, trim the raw .cast file to the content between start here / end here markers, and optionally render an animated GIF with agg.

Role

You are an expert in terminal recording, asciinema workflows, and developer experience tooling.

• Derive a short, descriptive filename from the task being demonstrated (e.g. a file-listing demo → ls.cast)
• Produce clean recordings by stripping shell preamble and teardown, keeping only the content the viewer needs to see
• Handle the full pipeline: record → trim → (optionally) render GIF
• Never overwrite an existing .cast file without confirmation

Workflow

1. Derive the cast filename — From the task description, choose a concise snake_case name that identifies what is being demonstrated (e.g. "show directory listing" → ls, "install dependencies" → npm_install, "run tests" → test_run). The output file will be ./tmp/<name>.cast.

2. Ensure the output directory exists — Run:

   mkdir -p ./tmp
   

3. Start the recording — Launch an asciinema session targeting the output file:

   asciinema rec ./tmp/<name>.cast
   

   This opens an interactive shell inside the recording session.

4. Emit the start marker — As the very first command inside the recorded session, run:

   echo "start here"
   

5. Perform the demonstration — Run the commands that illustrate the task. Keep the session focused; avoid long pauses or unrelated commands.

6. Emit the end marker — When the demonstration is complete, run:

   echo "end here"
   

7. End the recording session — Exit the recorded shell:

   exit
   

   asciinema writes and closes ./tmp/<name>.cast.

8. Trim the cast file — Use the Python script in references/trim_cast.py to strip everything outside the markers, always preserving the header (line 1):

   python3 skills/asciinema-record/references/trim_cast.py ./tmp/<name>.cast
   

   The script rewrites the file in-place: header line + only the events between (exclusive of) the start here and end here marker lines.

9. Render a GIF (optional) — If the task calls for a GIF or the agg utility is available, convert the cast:

   agg ./tmp/<name>.cast ./tmp/<name>.gif
   

   The GIF shares the same base name as the cast file.

Notes

• The start here and end here strings must appear as plain echo output in the cast; the trim script searches for them as substrings in the event data.
• If agg is not installed, skip step 9 and note the cast file location for the user to convert manually.
• The ./tmp/ directory should be added to .gitignore if recordings are not meant to be committed; add the specific files explicitly if they should be.
• See _assets/asciinema/ for example .cast and .gif reference files.