In Unix-like computer operating systems, a pipeline is a sequence of processes chained together by their standard streams, so that the output of each process (stdout) feeds directly as input (stdin) to the next one.
The concept of pipelines was championed by Douglas McIlroy at Unix's ancestral home of Bell Labs, during the development of Unix, shaping its toolbox philosophy. It is named by analogy to a physical pipeline.
The standard shell syntax for pipelines is to list multiple commands, separated by vertical bars ("pipes" in common unix verbiage). For example, to list files in the current directory (ls), retain only the lines of ls output containing the string "key" (grep), and view the result in a scrolling page (less), a user types the following into the command line of a terminal:
"ls -l" produces 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. Each "|" tells the shell to connect the standard output of the command on the left to the standard input of the command on the right by an inter-process communication mechanism called an (anonymous) pipe, implemented in the operating system. Pipes are unidirectional; data flows through the pipeline from left to right.
All widely used Unix shells have a special syntax construct for the creation of pipelines. In all usage one writes the commands in sequence, separated by the ASCII vertical bar character "
|" (which, for this reason, is often called "pipe character"). The shell starts the processes and arranges for the necessary connections between their standard streams (including some amount of buffer storage).
By default, the standard error streams ("stderr") of the processes in a pipeline are not passed on through the pipe; instead, they are merged and directed to the console. However, many shells have additional syntax for changing this behavior. In the csh shell, for instance, using "
|&" instead of "
|" signifies that the standard error stream should also be merged with the standard output and fed to the next process. The Bourne Shell can also merge standard error, using
2>&1, as well as redirect it to a different file.
In the most commonly used simple pipelines the shell connects a series of sub-processes via pipes, and executes external commands within each sub-process. Thus the shell itself is doing no direct processing of the data flowing through the pipeline.
However, it's possible for the shell to perform processing directly, using a so-called mill, or pipemill, (since a
while command is used to "mill" over the results from the initial command). This construct generally looks something like:
Such pipemill may not perform as intended if the body of the loop includes commands, such as
ssh, that read from
stdin: on the loop's first iteration, such a program (let's call it the drain) will read the remaining output from
command, and the loop will then terminate (with results depending on the specifics of the drain). There are a couple of possible ways to avoid this behavior. First, some drains support an option to disable reading from
ssh -n). Alternatively, if the drain does not need to read any input from
stdin to do something useful, it can be given
< /dev/null as input.
Pipelines can be created under program control. The Unix
pipe() system call asks the operating system to construct a new anonymous pipe object. This results in two new, opened file descriptors in the process: the read-only end of the pipe, and the write-only end. The pipe ends appear to be normal, anonymous file descriptors, except that they have no ability to seek.
To avoid deadlock and exploit parallelism, the Unix process with one or more new pipes will then, generally, call
fork() to create new processes. Each process will then close the end(s) of the pipe that it will not be using before producing or consuming any data. Alternatively, a process might create a new thread and use the pipe to communicate between them.
Named pipes may also be created using
mknod() and then presented as the input or output file to programs as they are invoked. They allow multi-path pipes to be created, and are especially effective when combined with standard error redirection, or with
In most Unix-like systems, all processes of a pipeline are started at the same time, with their streams appropriately connected, and managed by the scheduler together with all other processes running on the machine. 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. When the receiving program is ready to read data, then next program in the pipeline reads from the buffer. In Linux, the size of the buffer is 65536 bytes (64KB). An open source third-party filter called bfr is available to provide larger buffers if required.
Tools like netcat and socat can connect pipes to TCP/IP sockets.
The pipeline concept was invented by Douglas McIlroy and first described in the man pages of Version 3 Unix. McIlroy noticed that much of the time command shells passed the output file from one program as input to another.
His ideas were implemented in 1973 when ("in one feverish night", wrote McIlroy) Ken Thompson added the
pipe() system call and pipes to the shell and several utilities in Version 3 Unix. "The next day", McIlroy continued, "saw an unforgettable orgy of one-liners as everybody joined in the excitement of plumbing." McIlroy also credits Thompson with the
| notation, which greatly simplified the description of pipe syntax in Version 4. The idea was eventually ported to other operating systems, such as DOS, OS/2, Microsoft Windows, and BeOS, often with the same notation.
Although developed independently, Unix pipes are related to, and were preceded by, the 'communication files' developed by Ken Lochner in the 1960s for the Dartmouth Time Sharing System.
In Tony Hoare's communicating sequential processes (CSP) McIlroy's pipes are further developed.
The robot in the icon for Apple's Automator, which also uses a pipeline concept to chain repetitive commands together, holds a pipe in homage to the original Unix concept.
This feature of Unix was borrowed by other operating systems, such as Taos and MS-DOS, and eventually became the pipes and filters design pattern of software engineering.