如何在交互式shell中执行多个命令

Question

My application works with all kind of shell commands, provided from the console (curl, date, ping, whatever). Now I'd like to cover the case with interactive shell commands (like mongo shell), using os/exec.

• e.g. as a first step, connect to mongodb: mongo --quiet --host=localhost blog

• then perform arbitrary number of commands, getting the result on every step db.getCollection('posts').find({status:'INACTIVE'})

• and then exit

I tried the following, but it allows me to perform only one command per mongo connection:

func main() {

    cmd := exec.Command("sh", "-c", "mongo --quiet --host=localhost blog")

    cmd.Stdout = os.Stdout
    cmd.Stderr = os.Stderr

    stdin, _ := cmd.StdinPipe()

    go func() {
        defer stdin.Close()
        io.WriteString(stdin, "db.getCollection('posts').find({status:'INACTIVE'}).itcount()")
        // fails, if I'll do one more here
    }()

    cmd.Run()
    cmd.Wait()
}

Is there a way to run multiple commands, getting stdout result per executed command?

Answer 1

As Flimzy noted, you should absolutely be using a mongo driver to work with mongo, not trying to interact with it via shell exec.

However, to answer the root question, of course you can execute multiple commands - there's no reason you can't. Every time you write to the process' stdin, it's like you're at a terminal typing into it. There's no secret limitation on that, other than processes which specifically detect if they're connected to a TTY.

Your code has several issues, though - you should definitely review the os/exec package documentation. You're calling cmd.Run, which:

starts the specified command and waits for it to complete.

And then calling cmd.Wait, which... also waits for the command to complete. You're writing to the stdin pipe in a goroutine, even though this is a very serialized process: you want to write to the pipe to execute a command, get the result, write another command, get another result... concurrency only muddles matters and should not be used here. And you're not sending newlines to tell Mongo you're done writing a command (just like you'd do in the shell - Mongo won't just start executing as soon as you enter the closing paren, you have to hit enter).

What you would want to do to interact with a process via stdin/stdout (again, noting that this is absolutely not the way to interact with a database, but could be valid for other external commands):

cmd := exec.Command("sh", "-c", "mongo --quiet --host=localhost blog")

cmd.Stdout = os.Stdout
cmd.Stderr = os.Stderr

stdin, _ := cmd.StdinPipe()

// Start command but don't wait for it to exit (yet) so we can interact with it
cmd.Start()

// Newlines, like hitting enter in a terminal, tell Mongo you're done writing a command
io.WriteString(stdin, "db.getCollection('posts').find({status:'INACTIVE'}).itcount()
")
io.WriteString(stdin, "db.getCollection('posts').find({status:'ACTIVE'}).itcount()
")

// Quit tells it you're done interacting with it, otherwise it won't exit
io.WriteString(stdin, "quit()
")

stdin.Close()

// Lastly, wait for the process to exit
cmd.Wait()