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Connected input and output streams for computer programs
standard I/O file descriptors
In computer programming, standard streams are preconnected input and output communication channels between a computer program and its environment when it begins execution. The three input/output (I/O) connections are called standard input (stdin), standard output (stdout) and standard error (stderr). Originally I/O happened via a physically connected system console (input via keyboard, output via monitor), but standard streams abstract this. When a command is executed via an interactive shell, the streams are typically connected to the text terminal on which the shell is running, but can be changed with redirection or a pipeline. More generally, a child process inherits the standard streams of its parent process.

Users generally know standard streams as input and output channels that handle data coming from an input device, or that write data from the application. The data may be text with any encoding, or binary data. When a program is run as a daemon, its standard error stream is redirected into a log file, typically for error analysis purposes. Streams may be used to chain applications, meaning that the output stream of one program can be redirected to be the input stream to another application. In many operating systems this is expressed by listing the application names, separated by the vertical bar character, for this reason often called the pipeline character. A well known example is the use of a pagination application, such as more, providing the user control over the display of the output stream on the display.

In most operating systems predating Unix, programs had to explicitly connect to the appropriate input and output devices. OS specific intricacies caused this to be a tedious programming task. On many systems it was necessary to obtain control of environment settings, access a local file table, determine the intended data set, and handle hardware correctly in the case of a punch card reader, magnetic tape drive, disk drive, line printer, card punch, or interactive terminal. One of Unix's several groundbreaking advances was abstract devices, which removed the need for a program to know or care what kind of devices it was communicating with. Older operating systems forced upon the programmer a record structure and frequently non orthogonal data semantics and device control. Unix eliminated this complexity with the concept of a data stream: an ordered sequence of data bytes which can be read until the end of file. A program may also write bytes as desired and need not, and cannot easily declare their count or grouping. Another Unix breakthrough was to automatically associate input and output to terminal keyboard and terminal display, respectively, by default — the program (and programmer) did absolutely nothing to establish input and output for a typical input process output program (unless it chose a different paradigm). In contrast, previous operating systems usually required some—often complex—job control language to establish connections, or the equivalent burden had to be orchestrated by the program. Since Unix provided standard streams, the Unix C runtime environment was obliged to support it as well. As a result, most C runtime environments (and C's descendants), regardless of the operating system, provide equivalent functionality.

Standard input is a stream from which a program reads its input data. The program requests data transfers by use of the read operation. Not all programs require stream input. For example, the dir and ls programs (which display file names contained in a directory) may take command line arguments, but perform their operations without any stream data input. Unless redirected, standard input is inherited from the parent process. In the case of an interactive shell, that is usually associated with the input device of a terminal (or pseudo terminal) which is ultimately linked to a user's keyboard. On POSIX systems, the file descriptor for standard input is 0 (zero); the POSIX <unistd. h> definition is STDIN FILENO; the corresponding C <stdio. h> abstraction is provided via the FILE* stdin global variable. Similarly, the global C++ std::cin variable of type <iostream> provides an abstraction via C++ streams. Similar abstractions exist in the standard I/O libraries of practically every programming language.

Standard output is a stream to which a program writes its output data. The program requests data transfer with the write operation. Not all programs generate output. For example, the file rename command (variously called mv, move, or ren) is silent on success. Unless redirected, standard output is inherited from the parent process. In the case of an interactive shell, that is usually the text terminal which initiated the program. The file descriptor for standard output is 1 (one); the POSIX <unistd. h> definition is STDOUT FILENO; the corresponding C <stdio. h> variable is FILE* stdout; similarly, the C++ <iostream> variable is std::cout.

ALGOL 60 was criticized for having no standard file access.

In the C programming language, the standard input, output, and error streams are attached to the existing Unix file descriptors 0, 1 and 2 respectively. In a POSIX environment the <unistd. h> definitions STDIN FILENO, STDOUT FILENO or STDERR FILENO should be used instead rather than magic numbers. File pointers stdin, stdout, and stderr are also provided. Ken Thompson (designer and implementer of the original Unix operating system) modified sort in Version 5 Unix to accept " " as representing standard input, which spread to other utilities and became a part of the operating system as a special file in Version 8. Diagnostics were part of standard output through Version 6, after which Dennis M. Ritchie created the concept of standard error.

Graphical user interfaces (GUIs) do not always make use of the standard streams; they do when GUIs are wrappers of underlying scripts and/or console programs, for instance the Synaptic package manager GUI, which wraps apt commands in Debian and/or Ubuntu. GUIs created with scripting tools like Zenity and KDialog by KDE project make use of stdin, stdout, and stderr, and are based on simple scripts rather than a complete GUI programmed and compiled in C/C++ using Qt, GTK, or other equivalent proprietary widget framework. The Services menu, as implemented on NeXTSTEP and Mac OS X, is also analogous to standard streams. On these operating systems, graphical applications can provide functionality through a system wide menu that operates on the current selection in the GUI, no matter in what application. Some GUI programs, primarily on Unix, still write debug information to standard error. Others (such as many Unix media players) may read files from standard input. Popular Windows programs that open a separate console window in addition to their GUI windows are the emulators pSX and DOSBox. GTK server can use stdin as a communication interface with an interpreted program to realize a GUI. The Common Lisp Interface Manager paradigm "presents" GUI elements sent to an extended output stream.