Excecing Processes

In the previous example we looked at spawning external processes. We do this when we need an external process accessible to a running Go process. Sometimes we just want to completely replace the current Go process with another (perhaps non-Go) one. To do this we’ll use Go’s implementation of the classic exec function.

package main

import (
    "os"
    "os/exec"
    "syscall"
)

func main() {

    // For our example we’ll exec ls. Go requires
    // an absolute path to the binary we want to execute
    // so we’ll use exec.LookPath to find it (probably /bin/ls).
    binary, lookErr := exec.LookPath("ls")
    if lookErr != nil {
        panic(lookErr)
    }

    // Exec requires arguments in slice form 
    // (as opposed to one big string).
    // We’ll give ls a few common arguments.
    // Note that the first argument should be the program name.
    args := []string{"ls", "-a", "-l", "-h"}

    // Exec also needs a set of environment variables to use.
    // Here we just provide our current environment.
    env := os.Environ()

    // Here’s the actual syscall.Exec call.
    // If this call is successful, the execution 
    // of our process will end here and be replaced 
    // by the /bin/ls -a -l -h process.
    // If there is an error we’ll get a return value.
    execErr := syscall.Exec(binary, args, env)
    if execErr != nil {
        panic(execErr)
    }
}

When we run our program it is replaced by ls.

$ go run execing-processes.go
total 16
drwxr-xr-x  4 mark 136B Oct 3 16:29 .
drwxr-xr-x 91 mark 3.0K Oct 3 12:50 ..
-rw-r--r--  1 mark 1.3K Oct 3 16:28 execing-processes.go

Note that Go does not offer a classic Unix fork function. Usually this isn’t an issue though, since starting goroutines, spawning processes, and exec’ing processes covers most use cases for fork.