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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.
The command ls -l is executed as a process, the output (stdout) of which is piped to the input (stdin) of the process for grep key; and likewise for the process for less. Each process takes input from the previous process and produces output for the next process via standard streams.
File descriptors for a single process, file table and inode table. Note that multiple file descriptors can refer to the same file table entry (e.g., as a result of the dup system call [3]: 104 ) and that multiple file table entries can in turn refer to the same inode (if it has been opened multiple times; the table is still simplified because it represents inodes by file names, even though an ...
This is because the redirection form opens the file as the stdin file descriptor which command can fully access, while the cat form simply provides the data as a stream of bytes. Another common case where cat is unnecessary is where a command defaults to operating on stdin, but will read from a file, if the filename is given as an argument ...
This syntax is because here documents are formally stream literals, and the content of the here document is often redirected to stdin (standard input) of the preceding command or current shell script/executable.
The C programming language provides many standard library functions for file input and output.These functions make up the bulk of the C standard library header <stdio.h>. [1] The functionality descends from a "portable I/O package" written by Mike Lesk at Bell Labs in the early 1970s, [2] and officially became part of the Unix operating system in Version 7.
Unix/Linux systems may offer other tools support using progress indicators from scripts or as standalone-commands, such as the program "pv". [52] These are not integrated features of the shells, however. PowerShell has a built-in command and API functions (to be used when authoring commands) for writing/updating a progress bar.
Typically, the syntax of these characters is as follows, using < to redirect input, and > to redirect output. command > file1 executes command, placing the output in file1, as opposed to displaying it at the terminal, which is the usual destination for standard output.