2. stdin (Standard In)

In our previous lesson, we learned about redirecting the standard output (stdout) stream. Similarly, we can also manage the standard input (stdin) stream. By default, a program receives its stdin from the keyboard, but we can also use files or the output from other processes as an input source.

Understanding stdin and stdout

Every command-line process in Linux operates with at least two fundamental data streams: standard input (stdin) and standard output (stdout). A program reads data from stdin and writes its results to stdout. Understanding how to control both stdin and stdout is crucial for effective command-line work.

How to Redirect stdin

Just as we use > for stdout redirection, we use the < operator to redirect stdin. This powerful feature allows you to tell a command to read its input from a file instead of waiting for you to type it on the keyboard. This is a core concept of input redirection.

Practical Example with cat stdin

Let's revisit the peanuts.txt file from the previous lesson, which contains the text "Hello World". Consider the following command:

cat < peanuts.txt > banana.txt

Here’s a breakdown of what happens:

1. The < peanuts.txt part tells the shell to redirect stdin for the cat command, making it read from peanuts.txt instead of the keyboard.
2. The cat command processes its input. In this case, using cat stdin means it reads the content of peanuts.txt.
3. The > banana.txt part redirects the standard output of cat to a new file named banana.txt.

Ultimately, the content of peanuts.txt is copied to banana.txt. This example effectively demonstrates how to manage both stdin and stdout in a single, efficient command.