How to Redirect Input to exec.Command() in Go?

Redirecting Input with exec.Command()

Issuing commands with input redirection in Go can be a challenge. Consider the common command-line operation:

/sbin/iptables-restore < /etc/iptables.conf

This command tells iptables-restore to read its configuration from /etc/iptables.conf. How can we accomplish this with Go's exec.Command()?

Failed Attempts

Initial attempts to pass the input file's path as arguments or pipe the file name into stdin were unsuccessful.

// Fails
cmd := exec.Command("/sbin/iptables-restore", "<", "/etc/iptables.conf")

// Also fails
cmd := exec.Command("/sbin/iptables-restore", "< /etc/iptables.conf")

// Attempt to pipe file name into stdin
// Fails
cmd := exec.Command("/sbin/iptables-restore")
stdin, err := cmd.StdinPipe()
if err != nil {
    log.Fatal(err)
}

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

io.WriteString(stdin, "/etc/iptables.conf")

Solution: Reading and Writing File Contents

To successfully execute the command, we must first read the contents of /etc/iptables.conf and then write those contents to the StdinPipe().

package main

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

func main() {
    bytes, err := ioutil.ReadFile("/etc/iptables.conf")
    if err != nil {
        log.Fatal(err)
    }
    cmd := exec.Command("/sbin/iptables-restore")
    stdin, err := cmd.StdinPipe()
    if err != nil {
        log.Fatal(err)
    }
    err = cmd.Start()
    if err != nil {
        log.Fatal(err)
    }
    _, err = io.WriteString(stdin, string(bytes))
    if err != nil {
        log.Fatal(err)
    }
}

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