What Is Expect Scripting Language?


Larry Thompson

Expect scripting language is a powerful tool that allows automation and interaction with various applications and systems. It is primarily used for automating tasks that require user input or response to specific events. With Expect, you can script interactions with command-line applications, network protocols, and even GUI-based programs.

What is Expect Scripting Language?

Expect is an extension to the Tcl scripting language and was created by Don Libes in the late 1980s. It provides additional features that make it easy to automate interactive tasks by simulating user input or capturing output from programs.

How Does Expect Work?

Expect works by spawning a child process (usually a command-line application) and then controlling its behavior based on predefined patterns or responses. It uses regular expressions to match patterns in the output and responds accordingly.

Key Features of Expect

  • Automating Command-Line Applications: Expect allows you to automate command-line applications by providing specific inputs and capturing the expected outputs.
  • Interacting with Network Protocols: With Expect, you can automate interactions with network protocols such as SSH, Telnet, FTP, etc.
  • Simulating User Input: Expect makes it possible to simulate user input for applications that require manual interaction.
  • Capturing Output: You can capture program output using expect_out(buffer) variable and use it for further processing.
  • Error Handling: Expect provides error handling mechanisms to handle unexpected situations or errors during automation.

Getting Started with Expect

To get started with Expect, you need to have Tcl installed on your system. Once Tcl is installed, you can start writing expect scripts with a .exp extension.

Example Expect Script:

Let’s take a simple example to illustrate the usage of Expect. Consider a scenario where you need to automate the login process for an SSH server. Here’s an expect script that achieves this:

#!/usr/bin/expect -f

set timeout 10

spawn ssh user@hostname

expect {
    "password:" {
        send "your_password\r"


In the above script, we first spawn an SSH session using the `spawn` command. We then use the `expect` command to wait for the pattern “password:” in the output. Once matched, we use the `send` command to provide the password followed by a carriage return (`\r`).

Finally, we use the `interact` command to allow interaction with the spawned process after automation.


Expect scripting language is a versatile tool for automating interactive tasks and controlling applications through scripts. It provides a wide range of features that make it easy to automate command-line applications, network protocols, and GUI-based programs.

By incorporating Expect into your automation workflows, you can save time and effort by eliminating manual interactions and streamlining repetitive tasks. So give Expect a try and explore its capabilities in automating your own use cases!

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