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75 lines
3.5 KiB
Markdown
75 lines
3.5 KiB
Markdown
8 years ago
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# How it works
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asciinema project is built of several complementary pieces:
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* command-line based terminal session recorder, `asciinema`,
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* website with an API at asciinema.org,
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* javascript player
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When you run `asciinema rec` in your terminal the recording starts, capturing
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all output that is being printed to your terminal while you're issuing the
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shell commands. When the recording finishes (by hitting <kbd>Ctrl-D</kbd> or
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typing `exit`) then the captured output is uploaded to asciinema.org website
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and prepared for playback on the web.
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Here's a brief overview of how these parts work.
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## Recording
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You probably know `ssh`, `screen` or `script` command. Actually, asciinema
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was inspired by `script` (and `scriptreplay`) commands. What you may not know
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is they all use the same UNIX system capability: [a
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pseudo-terminal](http://en.wikipedia.org/wiki/Pseudo_terminal).
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> A pseudo terminal is a pair of pseudo-devices, one of which, the slave,
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> emulates a real text terminal device, the other of which, the master,
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> provides the means by which a terminal emulator process controls the slave.
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Here's how terminal emulator interfaces with a user and a shell:
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> The role of the terminal emulator process is to interact with the user; to
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> feed text input to the master pseudo-device for use by the shell (which is
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> connected to the slave pseudo-device) and to read text output from the
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> master pseudo-device and show it to the user.
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In other words, pseudo-terminals give programs the ability to act as a
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middlemen between the user, the display and the shell. It allows for
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transparent capture of user input (keyboard) and terminal output (display).
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`screen` command utilizes it for capturing special keyboard shortcuts
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like <kbd>Ctrl-A</kbd> and altering the output in order to display window
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numbers/names and other messages.
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asciinema recorder does its job by utilizing pseudo-terminal for capturing all
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the output that goes to a terminal and saving it in memory (together with timing
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information). The captured output includes all the text and invisible
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escape/control sequences in a raw, unaltered form. When the recording session
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finishes it uploads the output (in
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[asciicast format](https://github.com/asciinema/asciinema/blob/master/doc/asciicast-v1.md))
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to asciinema.org. That's all about "recording" part.
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For the implementation details check out [recorder source
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code](https://github.com/asciinema/asciinema).
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## Playback
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As the recording is a raw stream of text and control
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sequences it can't be just played by incrementally printing text in proper
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intervals. It requires interpretation of [ANSI escape code
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sequences](http://en.wikipedia.org/wiki/ANSI_escape_code) in order to
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correctly display color changes, cursor movement and printing text at proper
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places on the screen.
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The player comes with its own terminal emulator based on
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[Paul Williams' parser for ANSI-compatible video terminals](http://vt100.net/emu/dec_ansi_parser).
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It covers only the display part of the emulation as this is what the player is
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about (input is handled by your terminal+shell at the time of recording anyway)
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and its handling of escape sequences is fully compatible with most modern
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terminal emulators like xterm, Gnome Terminal, iTerm, mosh etc.
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The end result is a smooth animation with all text attributes (bold,
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underline, inverse, ...) and 256 colors perfectly rendered.
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For the implementation details check out [asciinema.org website source
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code](https://github.com/asciinema/asciinema.org) and [player source
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code](https://github.com/asciinema/asciinema-player).
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