Get terminal output after a command swift

Question

I run some commands in terminal with this code:

system("the command here")

And after I want to know what is the result of running this command, e.g. if I run

system("git status")

I want to read the actual information about changes in my repo. Is there any way to do that in swift?

@MartinR, there's no information about the way to run multiple commands at once or run commands in one session — pomo_mondreganto
– pomo_mondreganto, Commented Apr 8, 2015 at 12:43
To execute a command and capture its output, you have to use NSTask. For multiple commands, create and execute multiple NSTasks. — Martin R
– Martin R, Commented Apr 8, 2015 at 12:50
you could always use popen if you’re really enthusiastic about using C functions :) — Airspeed Velocity
– Airspeed Velocity, Commented Apr 8, 2015 at 12:55

Community · Accepted Answer · 2017-05-23 12:00:29Z

38

NSTask is class to run another program as a subprocess. You can capture the program's output, error output, exit status and much more.

Expanding on my answer to xcode 6 swift system() command, here is a simple utility function to run a command synchronously, and return the output, error output and exit code (now updated for Swift 2):

func runCommand(cmd : String, args : String...) -> (output: [String], error: [String], exitCode: Int32) {

    var output : [String] = []
    var error : [String] = []

    let task = NSTask()
    task.launchPath = cmd
    task.arguments = args

    let outpipe = NSPipe()
    task.standardOutput = outpipe
    let errpipe = NSPipe()
    task.standardError = errpipe

    task.launch()

    let outdata = outpipe.fileHandleForReading.readDataToEndOfFile()
    if var string = String.fromCString(UnsafePointer(outdata.bytes)) {
        string = string.stringByTrimmingCharactersInSet(NSCharacterSet.newlineCharacterSet())
        output = string.componentsSeparatedByString("\n")
    }

    let errdata = errpipe.fileHandleForReading.readDataToEndOfFile()
    if var string = String.fromCString(UnsafePointer(errdata.bytes)) {
        string = string.stringByTrimmingCharactersInSet(NSCharacterSet.newlineCharacterSet())
        error = string.componentsSeparatedByString("\n")
    }

    task.waitUntilExit()
    let status = task.terminationStatus

    return (output, error, status)
}

Sample usage:

let (output, error, status) = runCommand("/usr/bin/git", args: "status")
print("program exited with status \(status)")
if output.count > 0 {
    print("program output:")
    print(output)
}
if error.count > 0 {
    print("error output:")
    print(error)
}

Or, if you are only interested in the output, but not in the error messages or exit code:

let output = runCommand("/usr/bin/git", args: "status").output

Output and error output are returned as an array of strings, one string for each line.

The first argument to runCommand() must be the full path to an executable, such as "/usr/bin/git". You can start the program using a shell (which is what system() also does):

let (output, error, status) = runCommand("/bin/sh", args: "-c", "git status")

The advantage is that the "git" executable is automatically found via the default search path. The disadvantage is that you have to quote/escape arguments correctly if they contain spaces or other characters which have a special meaning in the shell.

Update for Swift 3:

func runCommand(cmd : String, args : String...) -> (output: [String], error: [String], exitCode: Int32) {

    var output : [String] = []
    var error : [String] = []

    let task = Process()
    task.launchPath = cmd
    task.arguments = args

    let outpipe = Pipe()
    task.standardOutput = outpipe
    let errpipe = Pipe()
    task.standardError = errpipe

    task.launch()

    let outdata = outpipe.fileHandleForReading.readDataToEndOfFile()
    if var string = String(data: outdata, encoding: .utf8) {
        string = string.trimmingCharacters(in: .newlines)
        output = string.components(separatedBy: "\n")
    }

    let errdata = errpipe.fileHandleForReading.readDataToEndOfFile()
    if var string = String(data: errdata, encoding: .utf8) {
        string = string.trimmingCharacters(in: .newlines)
        error = string.components(separatedBy: "\n")
    }

    task.waitUntilExit()
    let status = task.terminationStatus

    return (output, error, status)
}

edited May 23, 2017 at 12:00

CommunityBot

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answered Apr 8, 2015 at 15:59

Martin R

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12 Comments

pomo_mondreganto Over a year ago

I still can't run multiple programs in one session. For example, if the output for status doesn't contain words "nothing" and "clear" (some changes were made) I need to run git add . and then git commit and git push origin master

Martin R Over a year ago

@NikitinRoman: What do you mean with "one session"? Can't you run runCommand("/usr/bin/git", "status") first and then runCommand("/usr/bin/git", "add", ".") and finally runCommand("/usr/bin/git", "push", "origin", "master") ?

pomo_mondreganto Over a year ago

I firstly I need to run cd Project and after it one by one all this commands, without changing directory

Martin R Over a year ago

@NikitinRoman: Now I see what you mean. NSTask has a currentDirectoryPath property, that can be set to the directory where the command should be executed. I'll update the code ...

Martin R Over a year ago

@NikitinRoman: You are welcome! – Note that "cd" is shell built-in. If you type "cd project" in the shell, no external command is executed, the shell just remembers the new working directory and uses it as current working directory for the next commands.

|

Community · Accepted Answer · 2017-05-23 12:17:23Z

3

system spawns a new process so you can’t capture its ouput. The equivalent that gives you a way to do this would be popen, which you could use like this:

import Darwin

let fp = popen("ping -c 4 localhost", "r")
var buf = Array<CChar>(count: 128, repeatedValue: 0)

while fgets(&buf, CInt(buf.count), fp) != nil,
      let str = String.fromCString(buf) {
    print(str)
}

fclose(fp)

However, don’t do it this way. Use NSTask as Martin describes.

edit: based on your request to run multiple commands in parallel, here is some probably-unwise code:

import Darwin

let commands = [
    "tail /etc/hosts",
    "ping -c 2 localhost",
]

let fps = commands.map { popen($0, "r") }

var buf = Array<CChar>(count: 128, repeatedValue: 0)

let results: [String] = fps.map { fp  in
    var result = ""
    while fgets(&buf, CInt(buf.count), fp) != nil,
          let str = String.fromCString(buf) {
        result += str
    }
    return result
}

fps.map { fclose($0) }

println("\n\n----\n\n".join(map(zip(commands,results)) { "\($0):\n\($1)" }))

(seriously, use NSTask)

edited May 23, 2017 at 12:17

CommunityBot

11 silver badge

answered Apr 8, 2015 at 13:06

Airspeed Velocity

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4 Comments

Martin R Over a year ago

The second argument to fgets() should be the actual buffer size :) – One reason that I avoid popen() if possible is that you have to quote/escape the arguments correctly if they contain spaces or any special she'll characters, that can get ugly.

Airspeed Velocity Over a year ago

D’oh! You’re right of course, I mostly post this because I’m amazed the C interop is so smooth.

pomo_mondreganto Over a year ago

is there any way to run multiple commands one by one and get output for each of them?

Sentry.co Over a year ago

@Nikitin Roman: Something like this: stackoverflow.com/questions/9400287/… An alternative is to call .sh file that has your multiple commands.

iUrii · Accepted Answer · 2022-09-06 14:05:20Z

Swift 5

Tu run your commands and get its outputs you can use next simple extention of Process class:

extension String : LocalizedError {
    public var errorDescription: String? { self }
}

extension Process {
    
    func run(_ executableURL: URL, arguments: [String]? = nil) throws -> String {
        self.executableURL = executableURL
        self.arguments = arguments
        
        let pipe = Pipe()
        standardOutput = pipe
        standardError = pipe
        
        try run()
        waitUntilExit()
        
        guard terminationStatus == EXIT_SUCCESS else {
            let error = String(data: pipe.fileHandleForReading.readDataToEndOfFile(), encoding: .utf8)
            throw (error?.trimmingCharacters(in: .newlines) ?? "")
        }
        
        let output = String(data: pipe.fileHandleForReading.readDataToEndOfFile(), encoding: .utf8)
        return output?.trimmingCharacters(in: .newlines) ?? ""
    }
}

How to use:

let process = Process()

do {
    let output = try process.run(URL(fileURLWithPath: "/bin/zsh"), arguments: ["-c", "echo 'hello'"])
    print("Output: \(output)")
}
catch {
    print(error)
}
print("Status: \(process.terminationStatus)")

Outputs:

Output: hello
Status: 0

ingconti · Accepted Answer · 2021-06-05 06:36:08Z

my 2 cents for swift 5.x , macOS with call back, invoked when done.

final func doTaskFor(cmd: String, arguments: [String], callback: CallBackWithStr = nil){


let task = Process()

let absolutePath = <add your specific path..> 
let fullCmd = absolutePath+cmd

#if DEBUG
// used to debug.
let debugstr :String = fullCmd + " " + arguments.oneLine()
print(debugstr)

#endif

task.executableURL = URL(fileURLWithPath: fullCmd)
task.arguments = arguments

// Create 2 Pipes and make the task
let outPipe = Pipe()
task.standardOutput = outPipe

let errPipe = Pipe()
task.standardError = errPipe

task.terminationHandler = { (process) in
    
    print("\ndidFinish: \(!process.isRunning)")
    
    // Get the data
    let outData = outPipe.fileHandleForReading.readDataToEndOfFile()
    let output = String(data: outData, encoding: .utf8)
    // print(output!)
    
    // Get the error
    let errData = errPipe.fileHandleForReading.readDataToEndOfFile()
    let err = String(data: errData, encoding: .utf8)
    // print(err!)
    
    // usually output is empty if error.
    
    callback?(output ?? "")
    
}

do {
    try task.run()
} catch {
    let msg = " \(error)"
    Log(msg: msg, safe: true)
    print(msg)
    
}

}

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