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.
In Unix-like operating systems, programs do redirection with the system call, 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 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, 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 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 shells derived from the original Bourne shell, the first two actions can be further modified by placing a number (the file descriptor) 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 .