How to use the command 'expect' (with examples)

Expect is a powerful automation tool designed for scripting interactions with programs that typically require user input. It acts as an intermediary between the user and the program, enabling automated control of interactive applications. By scripting these interactions, tasks can be automated, reducing human error and saving time. More information can be found at Expect Documentation .

Use case 1: Execute an expect script from a file

Code:

expect path/to/file

Motivation:

When dealing with complex systems or applications that require multiple, sequential, user inputs, manually entering each one can be tedious and error-prone. By writing an Expect script and storing it in a file, automation becomes straightforward. This method is particularly useful for administrators who need to perform repetitive tasks or set up systems that require consistent inputs.

Explanation:

• expect: This is the command that invokes the Expect program, which will run the script.
• path/to/file: This argument is the file path to the Expect script that contains the series of commands and responses to simulated user interactions. The script may include commands to spawn processes, interact with them, and close them.

Example Output:

Assuming the Expect script logs into a remote server via SSH and runs a command, the output might resemble:

spawn ssh user@host
user@host's password:
Last login: Wed Oct 4 14:02:56 2023 from 192.168.1.1
[user@host ~]$ ls
Documents
Downloads
Pictures

Here, Expect automatically handles the password prompt and executes the ls command.

Use case 2: Execute a specified expect script

Code:

expect -c "commands"

Motivation:

For cases where a series of commands are relatively short, or if they need to be dynamically assembled by a script, running expect with inline command strings can increase efficiency and manageability. This method is ideal for quick automations where creating a separate script file might be unnecessary overhead.

Explanation:

• expect: This initiates the Expect program, similar to the previous use case.
• -c "commands": This option allows for the direct specification of the commands to be executed, as a string. The commands encapsulated within the quotes get executed immediately, simulating user interactions.

Example Output:

A simple Expect command that automates FTP login might produce:

expect> spawn ftp host
Connected to host.
Name (host:user): 
Password:
ftp>

Expect’s inline commands handle the username and password prompts.

Use case 3: Enter an interactive REPL

Code:

expect -i

Motivation:

For users new to Expect or those aiming to experiment with command sequences, the interactive Read-Eval-Print Loop (REPL) mode offers a sandboxed environment. This allows users to input and immediately see the results of commands, helping them fine-tune the scripts they intend to write later.

Explanation:

• expect: Starts the Expect command interpreter.
• -i: Initiates Expect in an interactive REPL mode, allowing the user to enter commands on-the-fly. This mode continuously reads input from the user, evaluates it, and prints the output to the terminal until exited with exit or Ctrl + D.

Example Output:

Upon entering the interactive mode, the terminal might display:

expect1.1>

Users can then enter Expect commands directly, receiving immediate feedback to test their scripts’ logic.

Conclusion:

The Expect command is incredibly versatile for scripting and automating user inputs across various applications. Whether using predefined script files, executing inline commands, or testing in an interactive environment, Expect provides functionality that can drastically reduce the hands-on time required for repetitive and complex tasks. By understanding these different use cases, users can leverage Expect to streamline processes, mitigate errors, and enhance productivity.