In computing, redirection 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 dup2(2) system call, or its less-flexible but higher-level stdio analogues, freopen(3) and popen(3).
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Redirection is usually implemented by placing certain characters between commands. Typically, the syntax of these characters is as follows:
command1 > file1
executes command1, placing the output in file1. This will clobber any existing data in file1.
To append output to the end of the file, use the >> operator:
command1 >> file1
command1 < file1
executes command1, using file1 as the source of input (as opposed to the keyboard).
command1 < infile > outfile
combines the two capabilities: command1 reads from infile and writes to outfile
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 command1, using its output as the input for command2 (commonly called piping, with the "|" character being known as "pipe").
This is similar to using two redirects and a temporary file:
command1 > tempfile command2 < tempfile rm tempfile
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 user, press return, then pass.
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:
| Handle | Name | Description |
|---|---|---|
| 0 | stdin | Standard input |
| 1 | stdout | Standard output |
| 2 | stderr | Standard error |
For example:
command1 2> file1
executes command1, directing the standard error stream to file1.
In shells derived from csh (the C shell), 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. 'cat file 2>1' vs 'cat file 2>&1'. In the first case, stderr is redirected to a file named '1' 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 so error messages can be processed together with (or alternately to) the usual output. Example:
find / -name .profile > results 2>&1
will try to find all files named .profile. 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 results, error messages appear on the console. To see both hits and error messages in file results, merge stderr (handle 2) into stdout (handle 1) using 2>&1 .
If the merged output is to be piped into another program, the file merge sequence 2>&1 must precede the pipe symbol, thus:
find / -name .profile 2>&1 | less
A simplified but non-POSIX conforming form of the command:
command > file 2>&1
is (not available in Bourne Shell prior to version 4, final release, or in the standard shell Debian_Almquist_shell used in Debian/Ubuntu):
command &>file
or:
command >&file
NOTE: It is possible 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., still pointing 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.
command 2>&1 > file
The redirection and piping tokens can be chained together to create complex commands. For example:
ls | grep '\.sh' | sort > shlist
lists the contents of the current directory, where this output is filtered to only contain lines which contain .sh, sort this resultant output lexicographically, and place the final output in shlist. This type of construction is used very commonly in shell scripts and batch files.
The standard command tee 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 xyz.