How to Execute a Command with Input Redirection Using Go's `exec.Command`?

Command Execution with Input Redirection in Go: A Comprehensive Guide

In Go, leveraging the exec.Command function provides a powerful way to execute commands. However, when it comes to executing commands with input redirection, understanding how to properly use exec.Command is crucial. This question explores how to run a simple Bash command that reads from a file using exec.Command.

Problem Statement

The objective is to execute the following command from Go:

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

This command reads the IPTables configuration from the specified file and refreshes the IPTables. However, translating this command directly into Go code using exec.Command proves challenging.

Attempted Solutions

The question outlines several unsuccessful attempts to execute the command with exec.Command. Two common approaches are:

1. Attempting to pass the redirection operator < as an argument:

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

2. Attempting to pipe the filename into the command's standard input:

   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")

A Comprehensive Solution

The key to successfully executing the command with input redirection lies in using a combination of ioutil.ReadFile and exec.Command. The following solution accomplishes this:

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)
    }
}

By first reading the contents of the IPTables configuration file and then writing it to the command's standard input, we effectively perform the input redirection operation.

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