Linux Expect Expect_out（buffer）不包含任何内容

Question

I've been trying to capture the result of grep, logging into a remote machine, using ssl in Expect command. I read "except_out(buffer)" variable to contain the output of the spawned process, but it seemed empty... A pointer'd be greatly appreciated!

#!/bin/bash

username=hoge
password=hoge
hostname=machine20

prompt="\[$username@$hostname ~\]$"

expect -c "
set timeout -1
spawn ssh -l $username $hostname
expect {
   \"$username@$hostname's password:\" {
  send \"$password\n\"
  } \"Are you sure you want to continue connecting (yes/no)?\" {
  send \"yes\n\"
  expect \"$username@$hostname's password:\"
  send \"$password\n\"
  }
}

expect \"$prompt\"
sleep 2

expect \"$prompt\"
send \"ps axuw | grep java | grep -vc grep\n\"

expect -re -indices \"(.*)\"
send \"echo result : $expect_out(buffer)\"

expect version : 5.43.0

Answer 1

That code is a real mess. In particular, you've got interactions between bash and expect/tcl which are causing you trouble because when bash sees $var for a variable it doesn't know, it replaces it with the empty string.

While you could update things by changing how you do quoting, it's actually better to rewrite things to actually use a direct expect/tcl script, like this:

#!/usr/bin/env expect

set username "hoge"
set password "hoge"
set hostname "machine20"

set prompt "\[$username@$hostname ~\]$"

set timeout -1
spawn ssh -l $username $hostname
expect {
    "$username@$hostname's password:" {
        send "$password\r"
    }
    "Are you sure you want to continue connecting (yes/no)?" {
        send "yes\r"
        exp_continue
    }
}

# These next two lines look suspicious, BTW...
expect "$prompt"
sleep 2

expect "$prompt"
send "ps axuw | grep java | grep -vc grep\r"

expect -re -indices "(.*)"
send "echo result : $expect_out(buffer)"

However, I'd actually configure the remote host to use RSA keys for logins (indeed, I'd configure the remote host to only use them as they're much more attack-resistant than passwords and easier to manage too) and then just do this (with a local grep so it doesn't need to be filtered):

ssh $username@$host ps axuw | grep java