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In computing, redirection is a form of interprocess communication, and is a function common to most command-line interpreters, including the various Unix shells that can redirect standard streams to user-specified locations. The concept of redirection is quite old, dating back to the earliest operating systems (OS).
Unix shells use dup2 for input/output redirection. Along with pipe(), it is a tool on which Unix pipes rely. The following example uses pipe() and dup() in order to connect two separate processes (program1 and program2) using Unix pipes:
Example usage of tee: The output of ls -l is redirected to tee which copies them to the file file.txt and to the pager less. The name tee comes from this scheme - it looks like the capital letter T The tee command is normally used to split the output of a program so that it can be both displayed and saved in a file.
Process substitution can also be used to capture output that would normally go to a file, and redirect it to the input of a process. The Bash syntax for writing to a process is >(command). Here is an example using the tee, wc and gzip commands that counts the lines in a file with wc -l and compresses it with gzip in one pass:
A Unix shell is a command-line interpreter or shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command language and a scripting language , and is used by the operating system to control the execution of the system using shell scripts .
Console redirection Stream redirection Configurability Startup/shutdown scripts Batch scripts Logging Available as statically linked, independent single file executable Thompson shell: UNIX: sh 1971 — UNIX: UNIX — Yes Text-based CLI: No No — Yes — — — — — — Bourne shell 1977 version 7th Ed. UNIX: sh 1977 Yes [1] 7th Ed. UNIX ...
Version 1 AT&T UNIX dd: Filesystem Mandatory Convert and copy a file Version 5 AT&T UNIX delta: SCCS Optional (XSI) Make a delta (change) to an SCCS file PWB UNIX df: Filesystem Mandatory Report free disk space Version 1 AT&T UNIX diff: Text processing Mandatory Compare two files; see also cmp Version 5 AT&T UNIX dirname: Filesystem Mandatory
An important aspect of this, setting Unix pipes apart from other pipe implementations, is the concept of buffering: for example a sending program may produce 5000 bytes per second, and a receiving program may only be able to accept 100 bytes per second, but no data is lost. Instead, the output of the sending program is held in the buffer.