====== BASH - Redirection ======

In Linux/Unix, everything is a file.  Regular file, Directories, and even Devices are files.

Every File has an associated number called File Descriptor (FD).

  * Your screen has a File Descriptor.  When a program is executed the output is sent to File Descriptor of the screen, and you see program output on your monitor.
  * Your printer has a File Descriptor.  If the output is sent to File Descriptor of the printer, the program output would be printed.

----

===== Standard File Descriptors =====

Whenever you execute a program/command at the terminal, 3 files are always open, viz., standard input, standard output, standard error.  

These files are always present whenever a program is run.


^File^File Descriptor^
|Standard Input STDIN|0|
|Standard Output STDOUT|1|
|Standard Error STDERR|2|


The keyboard is the standard input device while your screen is the standard output device.

However, files can have their input and output redirected, using:

  * ">" is the output redirection operator.
  * ">>" appends output to an existing file
  * "<" is the input redirection operator
  * ">&" re-directs output of one file to another.
  * Errors can be re-directed using its corresponding File Descriptor 2.

----

===== Output Redirection =====

The '>' symbol is used for output (STDOUT) redirection. 

<code bash>
ls -al > listings
</code>

Here the output of command ls -al is re-directed to file "listings" instead of your screen.

<WRAP alert>
**NOTE:** Use the correct file name while redirecting command output to a file.

If there is an existing file with the same name, the redirected command will delete the contents of that file and then it may be overwritten.
</WRAP>

If you do not want a file to be overwritten but want to add more content to an existing file, then you should use '>>' operator.

----

===== Redirect standard output to devices =====

You can redirect standard output, to not just files, but also devices!

<code bash>
cat music.mp3 > /dev/audio
</code>

The **cat** command reads the file music.mp3 and sends the output to /dev/audio which is the audio device.

If the sound configurations in your PC are correct, this command will play the file music.mp3.

----

===== Input redirection =====

The '<' symbol is used for input (STDIN) redirection.

<code bash>
Mail -s "Subject" to-address < Filename
</code>

This would attach the file with the email, and it would be sent to the recipient.

----

===== Error Redirection =====

Error re-direction is one of the very popular features of Unix/Linux.

Frequent UNIX users will reckon that many commands give you massive amounts of errors.

  * For instance, while searching for files, one typically gets permission denied errors. These errors usually do not help the person searching for a particular file.
  * While executing shell scripts, you often do NOT want error messages cluttering up the normal program output.

The solution is to re-direct the error messages to a file.

==== Example 1 ====

<code bash>
myprogram 2>errorsfile
</code>

Executes a program names myprogram.

The file descriptor for standard error is 2.

Using "2>" we re-direct the error output to a file named "errorfile"

Thus, program output is not cluttered with errors.

----

==== Example 2 ====

<code bash>
find . -name 'my*' 2>error.log
</code>

Using the "find" command, we are searching the "." current directory for a file with "name" starting with "my".

----

=== Example 3 ===

A more complex example,

Server Administrators frequently, list directories and store both error and standard output into a file, which can be processed later.

<code bash>
ls Documents ABC> dirlist 2>&1
</code>

Here,

  *  which writes the output from one file to the input of another file. 2>&1 means that STDERR redirects to the target of STDOUT (which is the file dirlist)
  *  We are redirecting error output to standard output which in turn is being re-directed to file dirlist. Hence, both the output is written to file dirlist