In
computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, hardware and softw ...
, redirection is a form of
interprocess communication
In computer science, interprocess communication (IPC) is the sharing of data between running processes in a computer system. Mechanisms for IPC may be provided by an operating system. Applications which use IPC are often categorized as clients ...
, and is a function common to most
command-line interpreter
A command-line interface (CLI) is a means of interacting with software via commands each formatted as a line of text. Command-line interfaces emerged in the mid-1960s, on computer terminals, as an interactive and more user-friendly alternativ ...
s, including the various
Unix shell
A Unix shell is a Command-line_interface#Command-line_interpreter, command-line interpreter or shell (computing), shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command languag ...
s that can redirect
standard streams
In computer programming, standard streams are preconnected input and output communication channels between a computer program and its environment when it begins execution. The three input/output (I/O) connections are called standard input (stdin), ...
to user-specified locations. The concept of redirection is quite old, dating back to the earliest operating systems (OS). A discussion of the design goals for redirection can be found already in the 1971 description of the
input-output subsystem of the
Multics
Multics ("MULTiplexed Information and Computing Service") is an influential early time-sharing operating system based on the concept of a single-level memory.Dennis M. Ritchie, "The Evolution of the Unix Time-sharing System", Communications of t ...
OS. However, prior to the introduction of
UNIX
Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
OS with its "
pipes
Pipe(s), PIPE(S) or piping may refer to:
Objects
* Pipe (fluid conveyance), a hollow cylinder following certain dimension rules
** Piping, the use of pipes in industry
* Smoking pipe
** Tobacco pipe
* Half-pipe and quarter pipe, semi-circu ...
", redirection in operating systems was hard or even impossible to do.
In
Unix-like
A Unix-like (sometimes referred to as UN*X, *nix or *NIX) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Uni ...
operating systems, programs do redirection with the
system call
In computing, a system call (syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, accessing a hard disk drive ...
, or its less-flexible but higher-level
stdio analogues, and .
Redirecting standard input and standard output
Redirection is usually implemented by placing certain
characters between
commands.
Basic
Typically, the
syntax
In linguistics, syntax ( ) is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure (constituenc ...
of these characters is as follows, using
<
to redirect input, and
>
to redirect output.
command > file1 executes , placing the output in , as opposed to displaying it at the terminal, which is the usual destination for standard output. This will
clobber any existing data in .
Using
command < file1 executes , with as the source of input, as opposed to the
keyboard, which is the usual source for standard input.
command < infile > outfile combines the two capabilities: reads from and writes to
Variants
To append output to the end of the file, rather than clobbering it, the
>>
operator is used:
command1 >> file1.
To read from a stream literal (an inline file, passed to the standard input), one can use a
here document, using the
<<
operator:
$ tr a-z A-Z << END_TEXT
> one two three
> uno dos tres
> END_TEXT
ONE TWO THREE
UNO DOS TRES
To read from a string, one can use a
here string, using the
<<<
operator:
tr a-z A-Z <<< "one two three", or:
$ NUMBERS="one two three"
$ tr a-z A-Z <<< "$NUMBERS"
ONE TWO THREE
Piping
Programs can be run together such that one program reads the output from another with no need for an explicit intermediate file.
command1 , command2 executes , using its output as the input for (commonly called
piping
Within industry, piping is a system of pipes used to convey fluids (liquids and gases) from one location to another. The engineering discipline of piping design studies the efficient transport of fluid.
Industrial process piping (and accomp ...
, with the "
,
" character being known as the "pipe").
The two programs performing the commands may run in parallel with the only storage space being working buffers (Linux allows up to 64K for each buffer) plus whatever work space each command's processing requires. For example, a "sort" command is unable to produce any output until all input records have been read, as the very last record received just might turn out to be first in sorted order. Dr. Alexia Massalin's experimental operating system,
Synthesis, would adjust the priority of each task as they ran according to the fullness of their input and output buffers.
This produces the same end result as using two redirects and a temporary file, as in:
$ command1 > tempfile
$ command2 < tempfile
$ rm tempfile
But here, does not start executing until has finished, and a sufficiently large scratch file is required to hold the intermediate results as well as whatever work space each task required. As an example, although DOS allows the "pipe" syntax, it employs this second approach. Thus, suppose some long-running program "Worker" produces various messages as it works, and that a second program, TimeStamp copies each record from ''stdin'' to ''stdout'', prefixed by the system's date and time when the record is received. A sequence such as
Worker , TimeStamp > LogFile.txt would produce timestamps only when Worker had finished, merely showing how swiftly its output file could be read and written.
A good example for command piping is combining
echo
with another command to achieve something interactive in a non-interactive shell, e.g.
echo -e 'user\npass' , ftp localhost. This runs the
ftp
The File Transfer Protocol (FTP) is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network. FTP is built on a client–server model architecture using separate control and dat ...
client with input , press , then .
In casual use, the initial step of a pipeline is often
cat
or
echo
, reading from a file or string. This can often be replaced by input indirection or a
here string, and use of cat and piping rather than input redirection is known as
useless use of cat. For example, the following commands:
$ cat infile , command
$ echo $string , command
$ echo -e 'user\npass' , ftp localhost
can be replaced by:
$ command < infile
$ command <<< $string
$ ftp localhost <<< $'user\npass'
As
echo
is often a shell-internal command, its use is not as criticized as cat, which is an external command.
Redirecting to and from the standard file handles
In
Unix shell
A Unix shell is a Command-line_interface#Command-line_interpreter, command-line interpreter or shell (computing), shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command languag ...
s derived from the original
Bourne shell
The Bourne shell (sh) is a shell command-line interpreter for computer operating systems. It first appeared on Version 7 Unix, as its default shell. Unix-like systems continue to have /bin/sh—which will be the Bourne shell, or a symbolic lin ...
, the first two actions can be further modified by placing a number (the
file descriptor
In Unix and Unix-like computer operating systems, a file descriptor (FD, less frequently fildes) is a process-unique identifier (handle) for a file or other input/output resource, such as a pipe or network socket.
File descriptors typically h ...
) immediately before the
character; this will affect which stream is used for the redirection. The Unix standard I/O streams are:
For example, executes , directing the
standard error stream to .
In shells derived from
csh (the
C shell
The C shell (csh or the improved version, tcsh) is a Unix shell created by Bill Joy while he was a graduate student at University of California, Berkeley in the late 1970s. It has been widely distributed, beginning with the 2BSD release of the ...
), the syntax instead appends the (ampersand) character to the redirect characters, thus achieving a similar result. The reason for this is to distinguish between a file named '1' and stdout, i.e. vs . In the first case, stderr is redirected to a file named '' and in the second, stderr is redirected to stdout.
Another useful capability is to redirect one standard file handle to another. The most popular variation is to merge
standard error into
standard output Standard may refer to:
Symbols
* Colours, standards and guidons, kinds of military signs
* Standard (emblem), a type of a large symbol or emblem used for identification
Norms, conventions or requirements
* Standard (metrology), an object t ...
so error messages can be processed together with (or alternately to) the usual output. For example, will try to find all files named . Executed without redirection, it will output hits to
stdout and errors (e.g. for lack of privilege to traverse protected directories) to
stderr. If standard output is directed to file , error messages appear on the console. To see both hits and error messages in file , merge
stderr (handle 2) into
stdout (handle 1) using .
If the merged output is to be piped into another program, the file merge sequence must precede the pipe symbol, thus,
A simplified but non-POSIX conforming form of the command, is (not available in Bourne Shell prior to version 4, final release, or in the standard shell
Debian Almquist shell used in Debian/Ubuntu): or .
It is possible to use
2>&1
before "
>
" but the result is commonly misunderstood.
The rule is that any redirection sets the handle to the output stream independently.
So "
2>&1
" sets handle
2
to whatever handle
1
points to, which at that point usually is ''stdout''.
Then "
>
" redirects handle
1
to something else, e.g. a file, but it does not change handle
2
, which still points to ''stdout''.
In the following example, standard output is written to ''file'', but errors are redirected from stderr to stdout, i.e. sent to the screen: .
To write both errors and standard output to ''file'', the order should be reversed. Standard output would first be redirected to the file, then stderr would additionally be redirected to the stdout handle that has already been changed to point at the file: .
Chained pipelines
The redirection and piping tokens can be chained together to create complex commands. For example,
sort infile , uniq -c , sort -n > outfile sorts the lines of in lexicographical order, writes unique lines prefixed by the number of occurrences, sorts the resultant output numerically, and places the final output in .
This type of construction is used very commonly in
shell script
A shell script is a computer program designed to be run by a Unix shell, a command-line interpreter. The various dialects of shell scripts are considered to be command languages. Typical operations performed by shell scripts include file manipu ...
s and
batch file
A batch file is a Scripting language, script file in DOS, OS/2 and Microsoft Windows. It consists of a series of Command (computing), commands to be executed by the command-line interpreter, stored in a plain text file. A batch file may contain a ...
s.
Redirect to multiple outputs
The standard command can redirect output from a command to several destinations:
ls -lrt , tee xyz. This directs the file list output to both standard output and the file .
See also
*
Here-document, a way of specifying text for input in command-line shells
*
Shell shoveling
*
Command substitution
*
Process substitution
*
Console redirection
References
Sources
*
*
External links
*
Redirection Definitionby The Linux Information Project (LINFO)
i
The Linux Documentation ProjectRedirection in WindowsCreating a Child Process with Redirected Input and Outputin Windows
{{DEFAULTSORT:Redirection (Computing)
Articles with example code
DOS technology
Unix software
Unix
Windows administration