Listening to stdin in Swift

Question

Currently I am trying to listen to user input from the command line in my swift application.

I am aware of the readLine() method but it does not really fit my needs. I want to listen for data being inserted on the command line. Like when a user is pressing the ‘up key’ inside the terminal.

Something like what can be done in Node.js:

stdin.on( 'data', function( key ){ 
    if (key === '\u0003' ) {
        process.exit();   
    }   // write the key to stdout all normal like                 

    process.stdout.write( key ); 
});

I tried searching but I couldn’t find an equivalent to this in Swift. I thought maybe something with ‘Inputstream’ but didn’t a find a appropriate solution either.

If someone could give me some hints on how to do something like this in Swift I would highly appreciate it.

Sulthan · Accepted Answer · 2018-04-11T16:39:28.277

Normally standard input buffers everything until a newline is entered, that's why a typical standard input is read by lines:

while let line = readLine() {
   print(line)
}

(press CTRL+D to send EOF, that is end the input)

To really read every character separately, you need to enter raw mode and that means use the low level terminal functions:

// see https://stackoverflow.com/a/24335355/669586
func initStruct<S>() -> S {
    let struct_pointer = UnsafeMutablePointer<S>.allocate(capacity: 1)
    let struct_memory = struct_pointer.pointee
    struct_pointer.deallocate() 
    return struct_memory
}

func enableRawMode(fileHandle: FileHandle) -> termios {
    var raw: termios = initStruct()
    tcgetattr(fileHandle.fileDescriptor, &raw)

    let original = raw

    raw.c_lflag &= ~(UInt(ECHO | ICANON))
    tcsetattr(fileHandle.fileDescriptor, TCSAFLUSH, &raw);

    return original
}

func restoreRawMode(fileHandle: FileHandle, originalTerm: termios) {
    var term = originalTerm
    tcsetattr(fileHandle.fileDescriptor, TCSAFLUSH, &term);
}

let stdIn = FileHandle.standardInput
let originalTerm = enableRawMode(fileHandle: stdIn)

var char: UInt8 = 0
while read(stdIn.fileDescriptor, &char, 1) == 1 {
    if char == 0x04 { // detect EOF (Ctrl+D)
        break
    }
    print(char)
}

// It would be also nice to disable raw input when exiting the app.
restoreRawMode(fileHandle: stdIn, originalTerm: originalTerm)

Reference https://viewsourcecode.org/snaptoken/kilo/02.enteringRawMode.html

This is the way I wanted to read the stdin. Thanks for providing me this snippet. No I only need to parse the arrow key presses since they are producing 3 characters. Additionally your reference was very helpful! — grahan, Apr 12 '18 at 15:52
@grahan I have translated to Swift only the basics because I am no terminal expert. However, there won't be much difference between C and Swift on this level. — Sulthan, Apr 12 '18 at 16:27
No worries, your answer gave me the right direction on how to do it. I think I can achieve the rest. — grahan, Apr 12 '18 at 16:28

score 6 · Answer 2 · answered Apr 11 '18 at 14:04

6

You probably want FileHandle.standardInput.

Something like:

let file = FileHandle.standardInput

while true {
    let data = file.availableData
    print("\(String(bytes: data, encoding: .utf8))")
}

will echo out input the way I think you want it. Standard disclaimers about being careful with input and that this is probably a dangerous activity, sanitise your inputs and so on.

I'm not exactly sure how you'd go about matching specific control and arrow keys, but this is a start.

answered Apr 11 '18 at 14:04

caseynolan

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1
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11

Thanks for your answer. I tried it and it seems promising! But I only print when I hit the `Enter` key. So I assume that availableData will only return if it sees an end of line ? Is there something to print after every keystroke ? – grahan Apr 11 '18 at 15:18
1

@grahan That's not how standard input works. If you need that, you have to listen to the keyboard, not the standard input. This is usually implemented using the `readLine()` function though. – Sulthan Apr 11 '18 at 15:46

Listening to stdin in Swift

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