How to Properly Redirect Input to `exec.Command` in Go?

exec.Command with Input Redirection

In Go, the exec.Command function enables the execution of external commands. To redirect input into the command via a pipe, the StdinPipe method must be used.

Consider the following task: running the command "/sbin/iptables-restore < /etc/iptables.conf". This command updates the IPTables based on a configuration file, but configuring the input redirection using exec.Command can be a challenge.

The first attempt, exec.Command("/sbin/iptables-restore", "<", "/etc/iptables.conf"), misinterprets the < as a command flag. Additionally, using exec.Command("/sbin/iptables-restore", "< /etc/iptables.conf") interprets < as an argument to IPTables and results in an error.

To resolve this, provide the input data explicitly through the stdin pipe:

package main

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

func main() {
    // Read the contents of the input file.
    bytes, err := ioutil.ReadFile("/etc/iptables.conf")
    if err != nil {
        log.Fatal(err)
    }

    // Create the command.
    cmd := exec.Command("/sbin/iptables-restore")

    // Get the stdin pipe.
    stdin, err := cmd.StdinPipe()
    if err != nil {
        log.Fatal(err)
    }

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

    // Write the input data to the stdin pipe.
    _, err = io.WriteString(stdin, string(bytes))
    if err != nil {
        log.Fatal(err)
    }

    // Ensure stdin is closed.
    err = stdin.Close()
    if err != nil {
        log.Fatal(err)
    }

    // Wait for the command to finish.
    err = cmd.Wait()
    if err != nil {
        log.Fatal(err)
    }
}

With this code, the IPTables configuration file is read and written into cmd.StdinPipe(), achieving the desired input redirection.

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