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asciicast file format (version 2)

asciicast v2 file is NDJSON (newline delimited JSON) file where:

  • first line contains header (initial terminal size, timestamp and other meta-data), encoded as JSON object,
  • all subsequent lines form an event stream, each line representing a separate event stream item, encoded as JSON array.

By making the event stream a first class concept in the v2 format we get the following benefits:

  • it enables live, incremental writing to a file during recording (with v1 format the final recording JSON can only be written as a whole after finishing the recording session),
  • it allows the players to start the playback as soon as they read the meta-data line (contrary to v1 format which requires reading the whole file),
  • whether you're recording to a file or streaming via UNIX pipe or WebSocket the data representation is the same.

Header

asciicast header is JSON-encoded object containing recording meta-data.

Example header:

{"version": 2, "width": 80, "height": 24, "timestamp": 1504467315, "title": "Vim tutorial #1", "env": {"TERM": "xterm-256color", "SHELL": "/bin/zsh"}}

Required header attributes:

version

Must be set to 2.

width

Initial terminal width (number of columns). Integer.

height

Initial terminal height (number of rows). Integer.

Optional header attributes:

timestamp

Unix timestamp of the beginning of the recording session. Integer.

duration

Duration of the whole recording in seconds (when it's known upfront). Float.

command

Command that was recorded, as given via -c option to asciinema rec. String.

title

Title of the asciicast, as given via -t option to asciinema rec. String.

env

Map of captured environment variables. Object (String -> String).

Example env:

"env": {
  "SHELL": "xterm-256color",
  "TERM": "/bin/bash"
}

Official asciinema recorder captures only SHELL and TERM by default. All implementations of asciicast-compatible terminal recorder should not capture any additional environment variables unless explicitly permitted by the user.

theme

Color theme of the recorded terminal. Object, with the following attributes:

  • fg - normal text color,
  • bg - normal background color,
  • palette - list of 8 or 16 colors, separated by colon character.

All colors are in the CSS #rrggbb format.

Example theme:

"theme": {
  "fg": "#d0d0d0",
  "bg": "#212121",
  "palette": "#151515:#ac4142:#7e8e50:#e5b567:#6c99bb:#9f4e85:#7dd6cf:#d0d0d0:#505050:#ac4142:#7e8e50:#e5b567:#6c99bb:#9f4e85:#7dd6cf:#f5f5f5"
}

A specific technique of obtaining the colors from a terminal (using xrdb, requesting them from a terminal via special escape sequences etc) doesn't matter as long as the recorder can save it in the above format.

Event stream

Each element of the event stream is a 3-tuple encoded as JSON array:

[time, event-type, event-data]

Where:

  • time (float) - indicates when this event happened, represented as the number of seconds since the beginning of the recording session,
  • event-type (string) - one of: "o", "i", "size",
  • event-data (any) - event specific data, described separately for each event type.

For example, let's look at the following line:

[1.001376, "o", "Hello world"]

It represents the event which:

  • happened 1.001376 sec after the start of the recording session,
  • is of type "o" (print to stdout, see below),
  • has data "Hello world".

Supported event types

This section describes the event types supported in asciicast v2 format.

The list is open to extension, and new event types may be added in both the current and future versions of the format. For example, we may add new event type for text overlay (subtitles display).

A tool which interprets the event stream (web/cli player, post-processor) should ignore (or pass through) event types it doesn't understand or doesn't care about.

"o" - data written to stdout

Event of type "o" represents printing new data to terminal's stdout.

event-data is a string containing the data that was printed to a terminal. It has to be valid, UTF-8 encoded JSON string as described in JSON RFC section 2.5, with all non-printable Unicode codepoints encoded as \uXXXX.

"i" - data read from stdin

Event of type "i" represents character(s) typed in (or pasted) by the user, or more specifically, data sent from terminal emulator to stdin of the recorded shell.

event-data is a string containing the captured character(s). Like with "o" event, it's UTF-8 encoded JSON string, with all non-printable Unicode codepoints encoded as \uXXXX.

Note: Official asciinema recorder doesn't capture stdin by default. All implementations of asciicast-compatible terminal recorder should not capture it either unless explicitly permitted by the user.

"size" - terminal resize (planned?)

TODO

not supported by current versions of the recorder and players

Complete asciicast v2 example

A very short asciicast v2 file looks like this:

{"version": 2, "width": 80, "height": 24, "timestamp": 1504467315, "title": "Demo", "env": {"TERM": "xterm-256color", "SHELL": "/bin/zsh"}}
[0.248848, "o", "\u001b[1;31mHello \u001b[32mWorld!\u001b[0m\n"]
[1.001376, "o", "This is overwritten\rThis is better."]
[2.143733, "o", " "]
[6.541828, "o", "Bye!"]