Preserve stdout after os/exec.Cmd.Wait()-CodePudding

I'm working with os/exec, sending input and receiving output as a command runs. I need to store the command's return code when it finishes, so I have a goroutine with err := cmd.Wait(), and I get any failure return code from the err. But Wait() seems to throw away the remaining stdout which I need also. So how do I preserve the remaining stdout of a os/exec.Cmd after Cmd.Wait()?

Example code, using the Unix bc calculator command:

package main

import (
    "fmt"
    "os/exec"
    "bufio"
    "io"
    "time"
)

func main() {
    cmd := exec.Command("sh", "-c", "bc")
    stdin, _ := cmd.StdinPipe()
    stdout, _ := cmd.StdoutPipe()
    scanner := bufio.NewScanner(stdout)
    cmd.Start()

    go func() {
        cmd.Wait()
        fmt.Println("finished")
    }()

    io.WriteString(stdin, "1   2\n")
    fmt.Println(scanner.Scan(), scanner.Text())
    io.WriteString(stdin, "3   4\n")
    fmt.Println(scanner.Scan(), scanner.Text())
    io.WriteString(stdin, "5   6\n")
    io.WriteString(stdin, "quit\n")  // cmd.Wait() runs
    time.Sleep(time.Second)
    // Prints false :(
    fmt.Println(scanner.Scan(), scanner.Text())
}

This prints: true 3 true 7 finished false

I'd like: true 3 true 7 finished true 11

I also tried setting cmd.Stdout to a bytes.Buffer like:

    var buf bytes.Buffer
    cmd.Stdout = &buf
    scanner := bufio.NewScanner(&buf)

But that was unreliable. It printed all false unless I added in delays with time.Sleep().

CodePudding user response：

Call cmd.Wait() after reading to the end of stdout.

Option 1: call cmd.Wait from the main goroutine after scanner.Scan() returns false.

cmd := exec.Command("sh", "-c", "bc")
stdin, _ := cmd.StdinPipe()
stdout, _ := cmd.StdoutPipe()
scanner := bufio.NewScanner(stdout)
cmd.Start()

io.WriteString(stdin, "1   2\n")
fmt.Println(scanner.Scan(), scanner.Text())
io.WriteString(stdin, "3   4\n")
fmt.Println(scanner.Scan(), scanner.Text())
io.WriteString(stdin, "5   6\n")
io.WriteString(stdin, "quit\n") // cmd.Wait() runs
fmt.Println(scanner.Scan(), scanner.Text())
fmt.Println(scanner.Scan(), scanner.Text()) // prints false
cmd.Wait()

Option 2: read from the waiting goroutine:

cmd := exec.Command("sh", "-c", "bc")
stdin, _ := cmd.StdinPipe()
stdout, _ := cmd.StdoutPipe()
scanner := bufio.NewScanner(stdout)
cmd.Start()

var wg sync.WaitGroup
wg.Add(1)
go func() {
    for scanner.Scan() {
        fmt.Println(scanner.Text())
    }
    cmd.Wait()
    defer wg.Done()
}()

io.WriteString(stdin, "1   2\n")
io.WriteString(stdin, "3   4\n")
io.WriteString(stdin, "5   6\n")
io.WriteString(stdin, "quit\n") // cmd.Wait() runs
wg.Wait()