Expect中interact与return联用含义及SSH复用登录脚本优化问题

1. Fixing the SSH Multiplexing Password Prompt Hang #

Your current script hangs when SSH multiplexing is active because it unconditionally waits for a Password* prompt. To handle both password-required and password-less scenarios smoothly, use a multi-pattern expect block that covers both cases:

#!/usr/bin/expect
set cmd [lindex $argv 0]
set relay_num [lindex $argv 1]
set timeout -1

spawn bash -c "$cmd"

# Handle two possible scenarios: password prompt or direct option menu
expect {
    "Password*" {
        send "mypassword\r"
        exp_continue  # Keep waiting for the Option prompt after sending password
    }
    "Option*" {
        # Do nothing—we're ready to send the relay number next
    }
}

# Trigger return to script control when the Option menu appears
interact -o -nobuffer -re "Option" return

send -- "$relay_num\r"
interact  # Hand control back to you after selecting the machine

The exp_continue command ensures that after sending the password, Expect keeps watching for the Option prompt instead of exiting the expect block early.

2. What interact -o Means #

The -o flag stands for output (referring to the spawned process's output to your terminal). By default, interact only forwards your keyboard input to the remote process and displays its output to you. Adding -o lets Expect capture that process output, so you can use pattern matching (like -re "Option") to trigger actions. Without -o, the script would never detect the Option prompt and the return trigger would never fire.

3. interact ... return Explained #

Normally, interact gives full control to you—the user—to type directly into the SSH session. When you pair it with -re "Option" return, you're telling Expect:

"Let the session run normally until the output matches the 'Option' pattern. Once that happens, stop the interactive mode and pass control back to the script so it can run the next commands."

In your use case, this lets the script automatically detect when the machine selection menu appears, then take over to send your pre-specified relay number.

4. send vs send -- #

The key difference is how they handle strings starting with a dash (-):

• send tries to interpret any string starting with - as a command option. For example, send "-1\r" would throw an error because Expect thinks -1 is a valid flag (which it's not).
• send -- explicitly tells Expect: "Treat everything after this as a literal string to send, even if it starts with a dash."

It's a safety habit to use send -- for dynamic content (like your $relay_num) to avoid unexpected parsing errors, even if your current values don't start with a dash.

内容的提问来源于stack exchange，提问作者Rain