STDIN STDOUT STDERR in Linux

Taking input commands, processing them and providing results are the main functions of a Linux operating system. It uses various components to perform these tasks. It uses data streams to transfer data between these components.

A data stream is a data flow between a source, a processing unit, and a destination. A source and a destination can be any device, program or process that sends and receives data. A processing unit sits between the source and the destination. It can be a command, script, program, process, or device. It receives data from the source, processes it, and sends the processed data to the destination. If any error occurs when processing the incoming data, based on the configuration, it sends the error to either the same or separate error-handling destination.

A processing unit uses three different types of data streams. These streams are the input data stream, the output data stream, and the error data stream. The input data stream connects the source to the processing unit. The output and error data streams connect the processing unit to the output and error-handling destinations.

The following image shows how data flows in data streams.

Standards streams (STDIN, STDOUT, STDERR)

Linux defines the input data stream, output data stream, and error data stream as the STDIN (Standard Input), STDOUT (Standard Output), and STDERR (Standard Error). Linux uses data streams as communication channels. When we open a file, execute a command, or start a process, Linux automatically attaches these channels (STDIN, STDOUT, and STDERR) to it.

STDIN (Standard Input)

STDIN works between the source and the processing unit. The source provides the data. The processing unit receives and processes the data.

Let's take an example to understand how STDIN works.

A user accesses a shell prompt, types a command using the keyboard, and hits the Enter key. The shell receives the command and executes it. In this process, the keyword is the source, and the shell is the processing unit. The keyboard places the typed characters (command) in the STDIN or input data stream. The other end of the default input data steam connects to the shell. The shell receives the typed characters and processes them as a command. The default device for STDIN or input data stream is the keyboard. You can use it or any other source, such as a file or the output of a command, to place data on STDIN.

STDOUT (Standard Output)

The processing unit processes the command and loads the command's output in STDOUT. The device or application connected to the other end of the output data stream receives the output and processes it based on its configuration. For example, if a monitor is connected to the other end of the data stream, it displays the output on the screen. Or, if a printer is connected to the other end of the data stream, it prints the output. The monitor is the default output device. Like STDIN, you can connect any device or application to the other end of the output data stream to receive the output or processed data.

The following image shows the data flows between STDIN and STDOUT.

STDERR (Standard Error)

STDERR or error data stream handles errors. If the processing unit finds an error when processing the received data, it loads that error in the error data stream. The device connected to the other end of the error stream handles the error. The monitor is the default STDERR device. The monitor displays the error on the screen. The following image shows how data flows between two ends of the STDERR stream.

Standard data streams are very flexible. You can manipulate them as you want. You can redirect the output data stream on any device, program, or process or store it as a file. In the same way, you can also use a file, device, or even the output of a process as the input stream. I/O redirections manipulate data streams. To learn I/O redirections, you can check the following tutorial.

Input Output Redirection in Linux

Conclusion

Understanding the standard data streams (STDIN, STDOUT, and STDERR) is crucial for effectively interacting with the Linux operating system. These streams facilitate data flow between the user, commands, and devices, enabling seamless communication and processing. Users can input commands, receive outputs, and handle errors using these communication channels efficiently.