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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).
Every pipe is placed in the root directory of the named pipe filesystem (NPFS), mounted under the special path \\.\pipe\ (that is, a pipe named "foo" would have a full path name of \\.\pipe\foo). Anonymous pipes used in pipelining are actually named pipes with a random name. They are very rarely seen by users, but there are notable exceptions.
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.
Pipes Command substitution Process substitution Subshells TCP/UDP connections as streams Keystroke stacking; Bourne shell: bytes concurrent Yes No Yes No N/A [nb 19] POSIX shell bytes concurrent Yes No Yes No N/A [nb 19] bash (v4.0) bytes concurrent Yes Yes (if system supports /dev/fd/ n or named pipes) Yes Yes (client only) N/A [nb 19] csh ...
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:
The command can be used to capture intermediate output before the data is altered by another command or program. The tee command reads standard input, then writes its content to standard output. It simultaneously copies the data into the specified file(s) or variables. The syntax differs depending on the command's implementation.
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A Persistent URL is an address on the World Wide Web that causes a redirection to another Web resource. If a Web resource changes location (and hence URL), a PURL pointing to it can be updated. A user of a PURL always uses the same Web address, even though the resource in question may have moved.