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

The “expect” command is a script executor that interacts with other programs that require user input. It is commonly used for automating tasks and testing interactive applications. The “expect” command reads scripts line by line, expecting certain patterns or strings from the output of the program being interacted with. Once it matches a pattern, it takes an action defined in the script. This makes it useful for automating tasks that require user input, such as logging into remote servers or providing input to command-line applications.

Use case 1: Execute an expect script from a file

Code:

expect path/to/file

Motivation: Executing an expect script from a file is useful when you have a pre-written script that you want to run using the “expect” command. This allows you to automate complex tasks that require user input without having to manually enter commands each time.

Explanation:

• “expect”: The command itself that executes the expect script.
• “path/to/file”: Specifies the path to the file that contains the expect script.

Example output:

This is the output of the expect script.

Use case 2: Execute a specified expect script

Code:

expect -c "commands"

Motivation: Executing a specified expect script allows you to run a script directly from the command line, without the need to create a separate file. This is useful for performing quick, one-off automation tasks.

Explanation:

• “expect”: The command itself that executes the expect script.
• “-c”: Specifies that the next argument is a command or a script.
• “commands”: The commands or expect script to be executed.

Example output:

This is the output of the specified expect script.

Use case 3: Enter an interactive REPL

Code:

expect -i

Motivation: Entering an interactive REPL (Read-Eval-Print Loop) is useful when you want to manually interact with a program or test different scenarios. It allows you to send input to the program and view the output in real-time.

Explanation:

• “expect”: The command itself that starts the interactive REPL.
• “-i”: Specifies that an interactive REPL should be started.

Example output:

This is the interactive REPL prompt.
Enter your input here: Hello
Output from the program: World

Conclusion:

The “expect” command is a powerful tool for automating tasks and testing interactive applications. With the ability to execute scripts from files, run specified scripts directly from the command line, and enter an interactive REPL, it provides flexibility and convenience for automating tasks that require user input. Whether you need to automate server logins, test command-line applications, or simply interact with programs in an automated manner, the “expect” command is a valuable tool to have in your arsenal.