CODEWITHSUNDEEP
node.jsterminalsignalsstdout
Alexander Mills
Alexander Mills

Reputation: 100020

Listen for command/ctrl + L signal in terminal

Traditionally, command/ctrl + (lowercase L) in a terminal will clear it.

I can clear the terminal using Node.js by writing this to stdout:

  process.stdout.write('\x1Bc');

but how can I listen for signals specifically a "command + l" key combination? (that is a lowercase L).

I think with Node.js the answer is probably related to: readline.emitKeypressEvents https://nodejs.org/api/readline.html#readline_readline_emitkeypressevents_stream_interface

Upvotes: 0

Views: 1136

Answers (1)

Gregoire Lodi
Gregoire Lodi

Reputation: 557

I dig up a bit and wrote a quick script as a proof of concept. That is an example of handling Ctrl-C, Ctrl-D and Ctrl-L signals AND having an event listener on new lines, even with raw mode enabled.

#!/usr/bin/env node

process.stdin.currentLine = '';
process.stdin.setRawMode(true);

process.stdin.on('data', (buf) => {
    const charAsAscii = buf.toString().charCodeAt(0);

    switch (charAsAscii) {
        case 0x03:
            console.log('You pressed Ctrl-C. Sending SIGINT.')
            process.kill(process.pid, 'SIGINT');
            break;

        case 0x04:
            console.log('You pressed Ctrl-D. Bye!')
            process.exit(0);
            break;

        case 0x0c:
            console.log('You pressed Ctrl-L. Clearing screen.')
            break;

        case 0x0d:
            process.stdin.emit('line', process.stdin.currentLine);
            process.stdin.currentLine = '';
            break;

        default:
            process.stdin.currentLine += String.fromCharCode(charAsAscii);
            break;
    }
});

process.stdin.on('line', line => console.log(`New line: ${line}`));

How does it work:

When in raw mode, input is always available character-by-character, not including modifiers. Additionally, all special processing of characters by the terminal is disabled, including echoing input characters. Note that CTRL+C will no longer cause a SIGINT when in this mode.

With that in mind, you can convert the buffer you get in the 'data' event, and check if it's a SIGINT (Ctrl-C), a 'clear' command (Ctrl-L), an EOF (Ctrl-D), ...

If it's a normal character, it simply adds the character to an internal buffer. When the return key is pressed, a line event is emitted with the internal buffer as argument. Then, the internal buffer is reset to an empty string.

Upvotes: 1

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