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Go exec command

last modified October 13, 2020

Go exec command tutorial shows how to execute shell commands and programs in Golang.

The Run function starts the specified command and waits for it to complete, while the Start starts the specified command but does not wait for it to complete; we need to use Wait with Start.

Go os/exec

The os/exec package runs external commands. It wraps os.StartProcess to make it easier to remap stdin and stdout, connect I/O with pipes, and do other adjustments.

Go exec program

The Run starts the specified command and waits for it to complete.

runprg.go
package main

import (
    "log"
    "os/exec"
)

func main() {

    cmd := exec.Command("firefox")

    err := cmd.Run()

    if err != nil {
        log.Fatal(err)
    }
}

In the example, we execute the Firefox browser.

Go exec.Command

The Command returns the Cmd struct to execute the specified program with the given arguments. The first parameter is the program to be run; the other arguments are parameters to the program.

command.go
package main

import (
    "bytes"
    "fmt"
    "log"
    "os/exec"
    "strings"
)

func main() {
    cmd := exec.Command("tr", "a-z", "A-Z")

    cmd.Stdin = strings.NewReader("and old falcon")

    var out bytes.Buffer
    cmd.Stdout = &out

    err := cmd.Run()

    if err != nil {
        log.Fatal(err)
    }

    fmt.Printf("translated phrase: %q\n", out.String())
}

In the example, we transform the input via the tr command.

cmd := exec.Command("tr", "a-z", "A-Z")

The tr standard Linux command translates, squeezes, and/or deletes characters from standard input, writing to standard output. In our case, we transform lowercase letters to uppercase ones.

cmd.Stdin = strings.NewReader("and old falcon")

Through the Stdin field, we pass a string to the command as its input.

var out bytes.Buffer
cmd.Stdout = &out

The output of the program will be written to the bytes buffer.

$ go run command.go 
translated phrase: "AND OLD FALCON"

This is the output.

Go exec command with multiple args

We can pass multiple arguments to the exec.Command.

multiple_args.go
package main

import (
    "fmt"
    "os/exec"
)

func main() {

    prg := "echo"

    arg1 := "there"
    arg2 := "are three"
    arg3 := "falcons"

    cmd := exec.Command(prg, arg1, arg2, arg3)
    stdout, err := cmd.Output()

    if err != nil {
        fmt.Println(err.Error())
        return
    }

    fmt.Print(string(stdout))
}

The example runs the echo command with three arguments.

$ go run multiple_args.go 
there are three falcons

This is the output.

Go exec command capture output

The Output runs the command and returns its standard output.

capture_output.go
package main

import (
    "fmt"
    "log"
    "os/exec"
)

func main() {

    out, err := exec.Command("ls", "-l").Output()

    if err != nil {
        log.Fatal(err)
    }

    fmt.Println(string(out))
}

The example captures the output of the ls command and prints it.

Go cmd.StdinPipe

The pipe allows us to send the output of one command to another. The StdinPipe returns a pipe that will be connected to the command's standard input when the command starts.

stdinpipe.go
package main

import (
    "fmt"
    "io"
    "log"
    "os/exec"
)

func main() {
    
    cmd := exec.Command("cat")
    stdin, err := cmd.StdinPipe()
    if err != nil {
        log.Fatal(err)
    }

    go func() {
        defer stdin.Close()
        io.WriteString(stdin, "an old falcon")
    }()

    out, err := cmd.CombinedOutput()
    if err != nil {
        log.Fatal(err)
    }

    fmt.Printf("%s\n", out)
}

In the example, we write a string to the standard input inside a goroutine.

cmd := exec.Command("cat")

The cat command concatenates the given files to the standard output. When no file is given, or with -, the command reads standard input and prints it to standard output.

stdin, err := cmd.StdinPipe()

We get the standard input pipe of the cat command.

go func() {
    defer stdin.Close()
    io.WriteString(stdin, "an old falcon")
}()

Inside the goroutine, we write a string to the stdin pipe.

$ go run stdinpipe.go 
an old falcon

This is the output.

Go cmd.StdoutPipe

The StdoutPipe returns a pipe that will be connected to the command's standard output when the command starts.

stdoutpipe.go
package main

import (
    "fmt"
    "io/ioutil"
    "log"
    "os/exec"
    "strings"
)

func upper(data string) string {

    return strings.ToUpper(data)
}

func main() {
    cmd := exec.Command("echo", "an old falcon")

    stdout, err := cmd.StdoutPipe()

    if err != nil {
        log.Fatal(err)
    }

    if err := cmd.Start(); err != nil {
        log.Fatal(err)
    }

    data, err := ioutil.ReadAll(stdout)

    if err != nil {
        log.Fatal(err)
    }

    if err := cmd.Wait(); err != nil {
        log.Fatal(err)
    }

    fmt.Printf("%s\n", upper(string(data)))
}

The example reads the output of the echo command through the pipe and transforms it into uppercase letters.

cmd := exec.Command("echo", "an old falcon")

The command to run is the echo command with a single string argument.

stdout, err := cmd.StdoutPipe()

We get the standard output pipe.

if err := cmd.Start(); err != nil {
    log.Fatal(err)
}

The command is executed with the Start function; it does not wait wait for it to complete.

data, err := ioutil.ReadAll(stdout)

We read the data from the pipe.

if err := cmd.Wait(); err != nil {
    log.Fatal(err)
}

The Wait waits for the command to exit and waits for any copying to stdin or copying from stdout or stderr to complete. It closes the pipe after seeing the command exit.

$ go run stdoutpipe.go 
AN OLD FALCON

This is the output.

In this tutorial, we have executed external commands in Golang.

List all Go tutorials.