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How it works
asciinema project is built of several complementary pieces:
- command-line based terminal session recorder,
asciinema, - website with an API at asciinema.org,
- javascript player
When you run asciinema rec in your terminal the recording starts, capturing
all output that is being printed to your terminal while you're issuing the
shell commands. When the recording finishes (by hitting Ctrl-D or
typing exit) then the captured output is uploaded to asciinema.org website
and prepared for playback on the web.
Here's a brief overview of how these parts work.
Recording
You probably know ssh, screen or script command. Actually, asciinema
was inspired by script (and scriptreplay) commands. What you may not know
is they all use the same UNIX system capability: a
pseudo-terminal.
A pseudo terminal is a pair of pseudo-devices, one of which, the slave, emulates a real text terminal device, the other of which, the master, provides the means by which a terminal emulator process controls the slave.
Here's how terminal emulator interfaces with a user and a shell:
The role of the terminal emulator process is to interact with the user; to feed text input to the master pseudo-device for use by the shell (which is connected to the slave pseudo-device) and to read text output from the master pseudo-device and show it to the user.
In other words, pseudo-terminals give programs the ability to act as a
middlemen between the user, the display and the shell. It allows for
transparent capture of user input (keyboard) and terminal output (display).
screen command utilizes it for capturing special keyboard shortcuts like
Ctrl-A and altering the output in order to display window
numbers/names and other messages.
asciinema recorder does its job by utilizing pseudo-terminal for capturing all the output that goes to a terminal and saving it in memory (together with timing information). The captured output includes all the text and invisible escape/control sequences in a raw, unaltered form. When the recording session finishes it uploads the output to asciinema.org. That's all about "recording" part.
For the implementation details check out recorder source code.
Playback
When asciinema.org accepts the upload of the captured output it saves it in a file. Now, as the output is a raw, unaltered stream of text and control sequences it can't be just played by incrementally printing text in proper intervals. It requires interpretation of ANSI escape code sequences in order to correctly display color changes, cursor movement and printing text at proper places on the screen.
Escape sequence interpretation was initially handled by asciinema's own VT100 terminal emulation layer written in Javascript but was later replaced with libtsm based interpreter. libtsm, "terminal-emulator state machine", is a wonderful, rock solid library created by David Herrmann that is meant to be used by terminal emulator authors and others in need of an escape sequence interpreter.
asciinema.org pre-processes the captured stream with libtsm based converter and saves the result in a JSON file that contains simple representation of screen changes for each animation frame (for each line that was changed on the screen there is a string to be printed and color attributes for it). The player loads the JSON data and simply renders each change at a right time.
The end result is a smooth animation with all text attributes (bold, underline, inverse, ...) and 256 colors perfectly rendered.
For the implementation details check out asciinema.org website source code and player source code.