Bash – condition && command + exit

Expect's model of interaction does not include a separation of stdout and stderr streams. When you spawn a process, you create a new pseudo-TTY which behaves almost identically to what a user would see in their terminal, and a user cannot typically distinguish between the two. Try it:

$ echo "This is stdout"
This is stdout
$ echo "This is stderr" 1>&2
This is stderr

Without doing something to label the difference, Expect cannot distinguish them (just like a user on the command line). So Expect may be the wrong tool for what you need to do.

Now, without knowing exactly why you need this, it's hard to come up with an alternative method for you. For what you need, it looks like you can accomplish this with a single ssh command:

ssh -q -o StrictHostKeyChecking=no $username@$hostname "ls -lrt; df -h" 1>>successlog 2>>errorlog

For another scripting language with built-in support for spawning a child process and getting its stdout and stderr separately, try Python and its subprocess module.

import shlex
from subprocess import Popen, PIPE
cmd = Popen(shlex.split('ssh -q -o StrictHostKeyChecking=no $username@$hostname "ls -lrt; df -h"'), stdout=PIPE, stderr=PIPE)
cmd.wait() # Wait for the command to complete
success_output = cmd.stdout
error_output = cmd.stderr

But if you really, really want to do this in Expect, I think the best approach may be to check the exit value of the last process and to write to your error log file when it is non-zero. Here's an example of something like that:

spawn ssh -q -o StrictHostKeyChecking=no $username@$hostname
# Receive password prompt and send the password here...
expect {
    -ex $prompt { send "ls -lrt\r" }
    timeout {
        send_user "Everything is terrible forever.\n"
        exit 1
    }
}
expect {
    -ex $prompt {
        set output $expect_out(buffer)
        send {echo RETVAL::$?::}
        send "\r"
    }
    timeout {
        send_user "Everything is terrible forever.\n"
        exit 1
    }
}
expect {
    -ex {RETVAL::0::} {
        set fp [open successlog a]
        puts $fp $output
        close $fp
    }
    -re {RETVAL::[1-9][0-9]*::} {
        set fp [open errorlog a]
        puts $fp $output
        close $fp
    }
    timeout {
        send_user "Everything is terrible forever.\n"
        exit 1
    }
}

You have to send a signal to the main shell:

# start of the main script:
MAIN_SHELL_PID=$$

[...]

myfunction() {
    ... || { echo "error" >&2 ; kill -HUP "$MAIN_SHELL_PID"; }
}