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--- bash:redirection [2019/12/14 19:27] – created peter
+++ bash:redirection [2021/01/13 21:51] (current) – [BASH - Redirection] peter
@@ Line 1: / Line 1: @@
 ====== 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 =====
@@ Line 20: / Line 54: @@
 ----
+===== 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