View source for Redirection (Linux)

'''Redirection''' is a function common to most [[Linux]] [[shell]]s which allow standard streams to be redirected to user-specified locations.

== Redirecting standard input and standard output ==

Redirection is usually implemented by placing certain characters (or tokens) between [[:Category:Linux Command Line Tools|commands]]. Typically, the syntax of these characters is as follows:

 command1 > file1  
executes <tt>command1</tt>, placing the output in <tt>file1</tt>.

 command1 < file1 
executes <tt>command1</tt>, using <tt>file1</tt> as the source of input (as opposed to the keyboard).

 command1 < infile > outfile
combines the two capabilities: <tt>command1</tt> reads from <tt>infile</tt> and writes to <tt>outfile</tt>

== 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 <tt>command1</tt>, using its output as the input for <tt>command2</tt> (commonly called [[Pipeline (Linux)|piping]], since the "|" character is known as a "pipe").

A good example for command piping is combining <tt>echo</tt> with another command to achieve something interactive in a non-interactive shell, e.g.    
 
 echo -e "user\npass" | ftp localhost
This would run the ftp client and enter <tt>user</tt>, press <tt>return</tt>, then <tt>pass</tt>.

== Redirecting to and from the standard file handles ==

In [[shell]]s 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 Linux standard I/O streams are:
{|
! handle !! name !! description
|-
! 0
| ([[Stdin|stdin]]) || standard input
|-
! 1
| ([[Stdout|stdout]]) || standard output
|-
! 2
| ([[Stderr|stderr]]) || standard error
|}

For example:
 command1 2> file1

executes <tt>command1</tt>, directing the [[standard error]] stream to <tt>file1</tt> (useful since standard error outputs to the terminal by default and is unaffected by redirection unless so specified).

In shells derived from csh (the [[C shell]]), the syntax instead appends the <tt>&</tt> character to the redirect characters, thus achieving a similar result.

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 files/directories named <tt>CVS</tt> in the <tt>/root</tt> directory. Executed without redirection, it will output hits to [[Stdout|stdout]] and errors (e.g. for lack of privilege to read certain directories) to [[Stderr]]. If we redirect standard output to file <tt>results</tt>, the error messages will continue to spam the console. To see both hits and error messages in file <tt>results</tt>, we merge [[Stderr|stderr]] (handle 1) into [[Stdout|stdout]] using '''<tt>2>&1</tt>''' .

If the merged output is to be piped into another program, the file merge sequence '''<tt>2>&1</tt>''' must precede the pipe symbol, thus:

 find / -name .profile 2>&1 | less

== Chained pipelines ==

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 <tt>.sh</tt>, sort this resultant output alphabetically, and place the final output in <tt>shlist</tt>.

This type of construction is used very commonly in Linux [[shell script]]s.

==Common standard I/O redirections==
''Note: These have been tested on a [[bash]] shell.''

<div style="float:left; margin:3px 0;">
{| align="center" style="border: 1px solid #999; background-color:#FFFFFF"
|-
! colspan="4" bgcolor="#EFEFEF" | '''Common Standard I/O Redirections'''
|-align="center" bgcolor="#1188ee"
!Function
!csh
!sh
|- align="left"
|Send stdout to file || <code>prg > file</code> || <code>prg > file</code>
|--bgcolor="#eeeeee"
|Send stderr to file || <code>prg 2> file</code> || 
|- align="left"
|Send stdout and stderr to file || <code>prg >& file</code> || <code>prg > file 2>&1</code>
|--bgcolor="#eeeeee"
|Take stdin from file || <code>prg < file</code> || <code>prg < file</code>
|- align="left"
|Send stdout to end of file || <code>prg >> file</code> || <code>prg >> file</code>
|--bgcolor="#eeeeee"
|Send stderr to end of file || <code>prg 2>> file</code> || 
|- align="left"
|Send stdout and stderr to end of file || <code>prg >>& file</code> || <code>prg >> file 2>&1</code>
|--bgcolor="#eeeeee"
|Read stdin from keyboard until c || <code>prg <<c</code> || <code>prg <<c</code>
|- align="left"
|Pipe stdout to prg2 || <code>prg <nowiki>|</nowiki> prg2</code> || <code>prg <nowiki>|</nowiki> prg2</code>
|--bgcolor="#eeeeee"
|Pipe stdout and stderr to prg2 || <code>prg <nowiki>|</nowiki>& prg2</code> || <code>prg 2>&1 <nowiki>|</nowiki> prg2</code>
|}
</div>
<br clear="all"/>

==External links==
*[http://www.linuxdevcenter.com/pub/a/linux/lpt/13_01.html Using Standard Input and Output]
*[http://www.ivarch.com/programs/pv.shtml pv - Pipe Viewer]
[[Category:Linux Command Line Tools]]