asciicast file format (version 2)

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

first line contains meta-data (duration, initial terminal size, etc), 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.

Meta-data

The following meta-data is required in asciicast v2:

version - set to 2,
width - initial terminal width (number of columns),
height - initial terminal height (number of rows).

The following meta-data is optional in asciicast v2:

duration - duration of the whole recording in seconds (when it's known upfront),
command - command that was recorded, as given via -c option to asciinema rec,
title - title of the asciicast, as given via -t option to asciinema rec,
env - map of environment variables useful for debugging playback problems.

Example meta-data line:

{ "version": 2, "width": 80, "height": 24, "command": "/bin/zsh", "title": null, "env": { "TERM": "xterm-256color", "SHELL": "/bin/zsh" } }

Event stream

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

[ time, event-type, event-data ]

Where:

time (number) - relative time (in seconds) since the previous event occured (or, for the first event in the stream, 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 a previous event in the stream,
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 event types it doesn't understand or doesn't care about. However, it should honor the amount of time specified by such stream item (by pausing for this amount of time when playing back) in order to keep the subsequent events on the timeline.

"o" - output, printing 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" - input, from keyboard (planned?)

TODO

not supported by current versions of the recorder and players

"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, "command": "/bin/zsh", "title": null, "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." ]
[ 0.143733, "o", " " ]
[ 0.541828, "o", "Bye!" ]

The final "Bye!" was printed to a terminal after 1.935785 sec (0.248848 + 1.001376 + 0.143733 + 0.541828).

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