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→Standard File Descriptors
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When running commands from the command line, all three of these file descriptors is are connected to the terminal, so the input to process comes from the terminal (keyboard), and normal output as well as error output goes to the terminal (screen).
The meaning of these file descriptors may vary in some contexts; for example, when a program is executed as a CGI script by a webserver, the CGI standard dictates that stdin receives POSTed data from the browser, and stdout is used to send the resulting data (typically HTML) from the script. Stderr is usually sent to a logfile.