Kotlin-native execute command and get the output - terminal

Question

I'd like to know if there's a way in kotlin native to call a command via posix and receive it's terminal output. For example, I'd like to get the "git diff" command working without having to create a temporary file, write output to it and then read from that file.
On SO I've only found solutions requiring ProcessBuilder, which isn't available on kotlin-native, as it's a Java library.

Answer 1

I found a working piece of code I wanted to use, so I'm posting it here for future viewers!
fun executeCommand(command: String): String{
val fp: CPointer<FILE>? = popen(command, "r")
val buffer = ByteArray(4096)
val returnString = StringBuilder()
/* Open the command for reading. */
if (fp == NULL) {
printf("Failed to run command\n" )
exit(1)
}
/* Read the output a line at a time - output it. */
var scan = fgets(buffer.refTo(0), buffer.size, fp)
if(scan != null) {
while (scan != NULL) {
returnString.append(scan!!.toKString())
scan = fgets(buffer.refTo(0), buffer.size, fp)
}
}
/* close */
pclose(fp)
return returnString.trim().toString()
}

Answer 2

It's an improved version of exec command for Kotlin Native posted by mg-lolenstine, it throws an exception with command stderr instead of just returning exit(1) (which not always desirable behavior itself), also trim is now optional
import kotlinx.cinterop.*
import platform.posix.*
fun executeCommand(
command: String,
trim: Boolean = true,
redirectStderr: Boolean = true
): String {
val commandToExecute = if (redirectStderr) "$command 2>&1" else command
val fp = popen(commandToExecute, "r") ?: error("Failed to run command: $command")
val stdout = buildString {
val buffer = ByteArray(4096)
while (true) {
val input = fgets(buffer.refTo(0), buffer.size, fp) ?: break
append(input.toKString())
}
}
val status = pclose(fp)
if (status != 0) {
error("Command `$command` failed with status $status${if (redirectStderr) ": $stdout" else ""}")
}
return if (trim) stdout.trim() else stdout
}

Answer 3

Usually you can use the POSIX api and use fork and wait and some I/O related functions for your purpose
fun main() {
val childPid: pid_t = fork()
if (childPid == 0) {
val commands = listOf("git", "diff", "HEAD^1", "$projectDir/path/to/file", null)
val cwd = "$projectDir"
chdir(cwd)
memScoped {
execvp(commands[0], allocArrayOf(commands.map { it?.cstr?.ptr }))
}
} else {
wait(null)
}
}
Of course, this needs to deal with a lot of c-style code, so I also wrote a more practical library for this
repositories {
mavenCentral()
}
// add dependencies into your native target sourceSet
dependencies {
implementation("com.kgit2:kommand:0.1.4")
}
It is also very simple to use
fun main(args: Array<String>) {
val diffResult = Command("git")
.args("diff", "HEAD^1", "$projectDir/path/to/file")
.cwd("$projectDir")
.spawn()
.output()
}

Answer 4

playing around I used above's answers to create a working gradle kotlin native/jvm multiplatform multiproject that runs arbitrary local Processes/Commands:
here's my result:
https://github.com/hoffipublic/minimal_kotlin_multiplatform
import kotlinx.cinterop.refTo
import kotlinx.cinterop.toKString
import platform.posix.fgets
import platform.posix.pclose
import platform.posix.popen
actual object MppProcess : IMppProcess {
actual override fun executeCommand(
command: String,
redirectStderr: Boolean
): String? {
val commandToExecute = if (redirectStderr) "$command 2>&1" else command
val fp = popen(commandToExecute, "r") ?: error("Failed to run command: $command")
val stdout = buildString {
val buffer = ByteArray(4096)
while (true) {
val input = fgets(buffer.refTo(0), buffer.size, fp) ?: break
append(input.toKString())
}
}
val status = pclose(fp)
if (status != 0) {
error("Command `$command` failed with status $status${if (redirectStderr) ": $stdout" else ""}")
}
return stdout
}
}
on jvm
import java.util.concurrent.TimeUnit
actual object MppProcess : IMppProcess {
actual override fun executeCommand(
command: String,
redirectStderr: Boolean
): String? {
return runCatching {
ProcessBuilder(command.split(Regex("(?<!(\"|').{0,255}) | (?!.*\\1.*)")))
//.directory(workingDir)
.redirectOutput(ProcessBuilder.Redirect.PIPE)
.apply { if (redirectStderr) this.redirectError(ProcessBuilder.Redirect.PIPE) }
.start().apply { waitFor(60L, TimeUnit.SECONDS) }
.inputStream.bufferedReader().readText()
}.onFailure { it.printStackTrace() }.getOrNull()
}
}