Automating Programs with Input Redirection and Process Substitution in Bash

In the world of scripting and automation, Bash remains a popular choice for its flexibility and power. When working with interactive programs that require multiple user prompts, input redirection and process substitution become invaluable tools. These techniques allow you to automate tasks that would otherwise require manual input, making your scripts more efficient and reliable.

In this blog post, we will explore how to use input redirection and process substitution in Bash to automate programs that require multiple prompts to be entered. We’ll start with a brief explanation of these concepts and then provide practical examples to demonstrate their application.

Understanding Input Redirection

Input redirection in Bash allows you to redirect the standard input stream (stdin) of a program from a file or another source. Instead of manually typing input, you can feed your script a predefined set of responses. This is extremely useful when dealing with programs that require multiple user interactions.

The syntax for input redirection is as follows:

command < input_file

• command is the program you want to run.
• < is the input redirection operator.
• input_file is the file containing the input responses.

Process Substitution

Process substitution is another powerful feature in Bash that allows you to treat the output of a command as a file. This is especially handy for automating interactive programs because you can use the output of one command as input to another without needing temporary files.

The syntax for process substitution is as follows:

command < <(another_command)

• command is the program you want to run.
• < is the input redirection operator.
• <( and ) enclose the command whose output you want to use as input.

Practical Examples

Now, let’s dive into some practical examples to demonstrate how to use input redirection and process substitution for automation.

Example 1: Automating a Simple Calculator

Suppose you want to automate a simple calculator program that takes user input for two numbers and an operator. Here’s how you can do it with input redirection:

#!/bin/bash echo "3 + 5" > input.txt ./calculator < input.txt

In this example, the script first creates an input.txt file containing the input for the calculator program. Then, it runs the calculator program and redirects the contents of input.txt to the program’s stdin. The calculator will perform the calculation without any manual input.

Example 2: Automating a Password Change Script

Imagine you have a script that changes your password and requires you to enter the old password and the new password. Using process substitution, you can automate this process:

#!/bin/bash  passwd $(whoami) < <(echo "old_password" && echo "new_password")

In this example, the script redirects the input of the passwd command to a substituted process, the substituted process sends the the old password and the new password as separate values to the redirected stdin effectively passing the values needed to both prompts required by the passwd command

Conclusion

Input redirection and process substitution in Bash are powerful techniques for automating programs that require multiple user prompts. These methods can save you time and ensure that your scripts execute consistently, even in interactive scenarios. By mastering these tools, you can streamline your automation tasks and make your scripts more efficient and reliable. So, the next time you encounter a program with multiple prompts, consider leveraging input redirection and process substitution to simplify the automation process.