stdin, stdout, and stderr are three data streams created when you launch a Linux command. You can use them to tell if your scripts are being piped or redirected. We show you how.
Streams Join Two Points
As soon as you start to learn about Linux and Unix-like operating systems, you’ll come across the terms stdin, stdout, and stederr. These are three standard streams that are established when a Linux command is executed. In computing, a stream is something that can transfer data. In the case of these streams, that data is text.
Data streams, like water streams, have two ends. They have a source and an outflow. Whichever Linux command you’re using provides one end of each stream. The other end is determined by the shell that launched the command. That end will be connected to the terminal window, connected to a pipe, or redirected to a file or other command, according to the command line that launched the command.
The Linux Standard Streams
In Linux, stdin is the standard input stream. This accepts text as its input. Text output from the command to the shell is delivered via the stdout (standard out) stream. Error messages from the command are sent through the stderr (standard error) stream.
So you can see that there are two output streams, stdout and stderr, and one input stream, stdin. Because error messages and normal output each have their own conduit to carry them to the terminal window, they can be handled independently of one another.
Streams Are Handled Like Files
Streams in Linux—like almost everything else—are treated as though they were files. You can read text from a file, and you can write text into a file. Both of these actions involve a stream of data. So the concept of handling a stream of data as a file isn’t that much of a stretch.
Each file associated with a process is allocated a unique number to identify it. This is known as the file descriptor. Whenever an action is required to be performed on a file, the file descriptor is used to identify the file.
These values are always used for stdin, stdout, and stderr:
- 0: stdin
- 1: stdout
- 2: stderr
Reacting to Pipes and Redirects
To ease someone’s introduction to a subject, a common technique is to teach a simplified version of the topic. For example, with grammar, we are told that the rule is “I before E, except after C.” But actually, there are more exceptions to this rule than there are cases that obey it.
