I have a for-loop that calls a function runCommand()
which runs a remote command on a switch and prints the output. The function is called in a goroutine on each iteration and I am using a sync.Waitgroup
to synchronize the goroutines. Now, I need a way to capture the output and any errors of my runCommand()
function into a channel. I have read many articles and watched a lot of videos on using channels with goroutines, but this is the first time I have ever written a concurrent application and I can't seem to wrap my head around the idea.
Basically, my program takes in a list of hostnames from the command line then asynchronously connects to each host, runs a configuration command on it, and prints the output. It is ok for my program to continue configuring the remaining hosts if one has an error.
How would I idiomatically send the output or error(s) of each call to runCommand()
to a channel then receive the output or error(s) for printing?
Here is my code:
package main
import (
"fmt"
"golang.org/x/crypto/ssh"
"os"
"time"
"sync"
)
func main() {
hosts := os.Args[1:]
clientConf := configureClient("user", "password")
var wg sync.WaitGroup
for _, host := range hosts {
wg.Add(1)
go runCommand(host, &clientConf, &wg)
}
wg.Wait()
fmt.Println("Configuration complete!")
}
// Run a remote command
func runCommand(host string, config *ssh.ClientConfig, wg *sync.WaitGroup) {
defer wg.Done()
// Connect to the client
client, err := ssh.Dial("tcp", host+":22", config)
if err != nil {
fmt.Println(err)
return
}
defer client.Close()
// Create a session
session, err := client.NewSession()
if err != nil {
fmt.Println(err)
return
}
defer session.Close()
// Get the session output
output, err := session.Output("show lldp ne")
if err != nil {
fmt.Println(err)
return
}
fmt.Print(string(output))
fmt.Printf("Connection to %s closed.
", host)
}
// Set up client configuration
func configureClient(user, password string) ssh.ClientConfig {
var sshConf ssh.Config
sshConf.SetDefaults()
// Append supported ciphers
sshConf.Ciphers = append(sshConf.Ciphers, "aes128-cbc", "aes256-cbc", "3des-cbc", "des-cbc", "aes192-cbc")
// Create client config
clientConf := &ssh.ClientConfig{
Config: sshConf,
User: user,
Auth: []ssh.AuthMethod{ssh.Password(password)},
HostKeyCallback: ssh.InsecureIgnoreHostKey(),
Timeout: time.Second * 5,
}
return *clientConf
}
EDIT: I got rid of the Waitgroup, as suggested, and now I need to keep track of which output belongs to which host by printing the hostname before printing its output and printing a Connection to <host> closed.
message when the gorouttine completes. For example:
$ go run main.go host1[,host2[,...]]
Connecting to <host1>
[Output]
...
[Error]
Connection to <host1> closed.
Connecting to <host2>
...
Connection to <host2> closed.
Configuration complete!
I know the above won't necessarily process host1
and host2
in order, But I need to print the correct host value for the connecting and closing messages before and after the output/error(s), respectively. I tried defering printing the closing message in the runCommand()
function, but the message is printed out before the output/error(s). And printing the closing message in the for-loop after each goroutine call doesn't work as expected either.
Updated code:
package main
import (
"fmt"
"golang.org/x/crypto/ssh"
"os"
"time"
)
type CmdResult struct {
Host string
Output string
Err error
}
func main() {
start := time.Now()
hosts := os.Args[1:]
clientConf := configureClient("user", "password")
results := make(chan CmdResult)
for _, host := range hosts {
go runCommand(host, &clientConf, results)
}
for i := 0; i < len(hosts); i++ {
output := <- results
fmt.Println(output.Host)
if output.Output != "" {
fmt.Printf("%s
", output.Output)
}
if output.Err != nil {
fmt.Printf("Error: %v
", output.Err)
}
}
fmt.Printf("Configuration complete! [%s]
", time.Since(start).String())
}
// Run a remote command
func runCommand(host string, config *ssh.ClientConfig, ch chan CmdResult) {
// This is printing before the output/error(s).
// Does the same when moved to the bottom of this function.
defer fmt.Printf("Connection to %s closed.
", host)
// Connect to the client
client, err := ssh.Dial("tcp", host+":22", config)
if err != nil {
ch <- CmdResult{host, "", err}
return
}
defer client.Close()
// Create a session
session, err := client.NewSession()
if err != nil {
ch <- CmdResult{host, "", err}
return
}
defer session.Close()
// Get the session output
output, err := session.Output("show lldp ne")
if err != nil {
ch <- CmdResult{host, "", err}
return
}
ch <- CmdResult{host, string(output), nil}
}
// Set up client configuration
func configureClient(user, password string) ssh.ClientConfig {
var sshConf ssh.Config
sshConf.SetDefaults()
// Append supported ciphers
sshConf.Ciphers = append(sshConf.Ciphers, "aes128-cbc", "aes256-cbc", "3des-cbc", "des-cbc", "aes192-cbc")
// Create client config
clientConf := &ssh.ClientConfig{
Config: sshConf,
User: user,
Auth: []ssh.AuthMethod{ssh.Password(password)},
HostKeyCallback: ssh.InsecureIgnoreHostKey(),
Timeout: time.Second * 5,
}
return *clientConf
}