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 <kbd>Ctrl-D</kbd> 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 <kbd>Ctrl-A</kbd> 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 (in asciicast format) to asciinema.org. That’s all about “recording” part.

For the implementation details check out recorder source code.

Playback

As the recording is a raw 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.

The player comes with its own terminal emulator based on Paul Williams’ parser for ANSI-compatible video terminals. It covers only the display part of the emulation as this is what the player is about (input is handled by your terminal+shell at the time of recording anyway) and its handling of escape sequences is fully compatible with most modern terminal emulators like xterm, Gnome Terminal, iTerm, mosh etc.

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.