Confused about stdin, stdout and stderr?

user_393459 0
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I am rather confused with the purpose of these three files. If my understanding is correct, stdin is the file in which a program writes into its requests to run a task in the process, stdout is the file into which the kernel writes its output and the process requesting it accesses the information from, and stderr is the file into which all the exceptions are entered. On opening these files to check whether these actually do occur, I found nothing seem to suggest so!

What I would want to know is what exactly is the purpose of these files, absolutely dumbed down answer with very little tech jargon!

stderr stdin stdout linux
11 Answers

For authoritative information about these files, check out the man pages, run the command on your terminal. 

$ man stdout

But for a simple answer, each file is for:

stdout for a stream out

stdin for a stream input

stderr for printing errors or log messages.

Each unix program has each one of those streams.

user_93233880

A file with associated buffering is called a stream and is declared to be a pointer to a defined type FILE. The fopen() function creates certain descriptive data for a stream and returns a pointer to designate the stream in all further transactions. Normally there are three open streams with constant pointers declared in the header and associated with the standard open files. At program startup three streams are predefined and need not be opened explicitly: standard input (for reading conventional input), standard output (for writing conventional output), and standard error (for writing diagnostic output). When opened the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device

https://www.mkssoftware.com/docs/man5/stdio.5.asp

user_83194740

Here is a lengthy article on stdin, stdout and stderr:

To summarize:

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

Ironically I found this question on stack overflow and the article above because I was searching for information on abnormal / non-standard streams. So my search continues.

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