Os Exec, Command

Often a program needs to start other programs. Not everything can be done in a single Go program. So we can invoke external processes.

We can import the os/exec package. Then we create a Command with 1 or 2 arguments (we pass the process name, and its arguments in separately).

Example

To begin, we Start a Firefox process. You will need to adjust the path to the Firefox application yourself—on Linux this is firefox-bin.

package main

import (
    "fmt"
    "os/exec"
)

func main() {
    // Create command.
    // ... The application is not executed yet.
    cmd := exec.Command("/home/sam/Downloads/firefox/firefox-bin")
    fmt.Println("Starting command")
    // Run firefox-bin.
    // ... This starts a web browser instance.
    //     It does not wait for it to finish.
    cmd.Start()
    fmt.Println("DONE")
}
Starting command
DONE

Argument

Here we start an external process with an argument. We pass the argument string to the Command, after the process name itself.

package main

import (
    "fmt"
    "os/exec"
)

func main() {
    // New command with 1 argument.
    browser := "/home/sam/Downloads/firefox/firefox-bin"
    argument := "https://en.wikipedia.org/";
    cmd := exec.Command(browser, argument)

    // Run firefox-bin and load URL specified in argument.
    cmd.Start()
    fmt.Println("DONE")
}
DONE

Run

Unlike Start, the Run() func waits for the process to end before returning. Here we Run() a Python program. When the program ends, we read its exit value (which is 100 here).

package main

import (
    "fmt"
    "os/exec"
)

func main() {
    // Run this Python program from Go.
    cmd := exec.Command("python", "/home/sam/test.py")
    fmt.Println("Running")
    // Wait for the Python program to exit.
    err := cmd.Run()
    fmt.Println("Finished:", err)
}
Running
Finished: exit status 100
# This is a Python program.
exit(100)

LookPath, slice arguments

This is a more complex, and probably useful, example. Sometimes we need to use LookPath to get the actual program location to invoke it correctly.

package main

import (
    "fmt"
    "os/exec"
)

func main() {

    // Get path for cwebp program.
    exe, _ := exec.LookPath("cwebp")

    // Build up argument string with slice.
    arguments := []string{}
    arguments = append(arguments, "/Users/sam/todo.png")
    arguments = append(arguments, "-o")
    arguments = append(arguments, "/Users/sam/todo.webp")

    // Call exec.Command with arguments string.
    cmd := exec.Command(exe, arguments...)

    // Run program.
    err := cmd.Run()
    fmt.Println(err)
}
<nil>

With Command, we launch external processes like Firefox and Python (or any EXE or script). We must invoke Start() or Run() to execute the code.