![]() ![]() The file descriptor for standard input is 0 (zero) the POSIX definition is STDIN_FILENO the corresponding C variable is FILE* stdin similarly, the C++ variable is std::cin. In the case of an interactive shell, that is usually associated with the keyboard. Unless redirected, standard input is inherited from the parent process. 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. The program requests data transfers by use of the read operation. Standard input is a stream from which a program reads its input data. As a result, most C runtime environments (and C's descendants), regardless of the operating system, provide equivalent functionality. Since Unix provided standard streams, the Unix C runtime environment was obliged to support it as well. 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. A program may also write bytes as desired and need not, and cannot easily declare their count or grouping.Īnother 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). 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. Older operating systems forced upon the programmer a record structure and frequently non-orthogonal data semantics and device control. 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. 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. OS-specific intricacies caused this to be a tedious programming task. In most operating systems predating Unix, programs had to explicitly connect to the appropriate input and output devices. 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 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. 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 modern systems, the standard error stream of a program is redirected into a log file, typically for error analysis purposes. The data may be text with any encoding, or binary data. 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. These special characters make it easy to customize the output of the echo command.The standard streams for input, output, and error Using the -e option allows you to use escape characters. Here are some ways you can use the echo command in Linux: Changing the Output Format
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