BASH/C3/More-on-Redirection/English

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Title of script: More on redirection

Author: Lavitha Pereira

Keywords: Video tutorial, Error and output redirection, Append output

Visual Cue	Narration
Display Slide 1	Dear friends, Welcome to the spoken tutorial on More on redirection
Display Slide 2	In this tutorial, we will learn Redirection of both standard error and output Appending redirected output With the help of some examples
Display Slide 3Prerequisites	To follow this tutorial you should have knowledge of Shell Scripting in BASH. If not, for relevant tutorials please visit our website which is as shown,(http://www.spoken-tutorial.org)
Display Slide 4 System requirements	For this tutorial I am using Ubuntu Linux 12.04 Operating System and GNU BASH version 4.2 Please note, GNU Bash version 4 or above is recommended to practice this tutorial.
	In an earlier tutorial, we learned about standard output and standard errors.
Display Slide 5 Redirection of both stderr and stdout	* Both stdout as well as the stderr can be redirected to the same file. This can be done in multiple methods. We will cover two of the most important methods of redirections in this tutorial.
Display Slide 6 Redirection of both stderr and stdout Syntax: command &>filename	The first method to redirect both the standard output and error is by using &> ampersand followed by greater-than sign The syntax is Command space ampersand greater than space filename
	Let me open a file named redirect.sh. I have typed some code in this file.
Highlight #!/usr/bin/env bash	This is the shebang line.
ls /usr /user &> out_file	ls lists the directory content of 2 directories namely '/usr' and '/user' Note that '/user' directory does not exist. Hence the command 'ls' will throw an error. And ampersand followed by greater than will redirect stdout and stderr to out_file.txt
Press Ctrl+S	Now Save the file.
Switch to terminal	Let us run the file redirect.sh. Open the terminal using CTRL+ALT+T keys simultaneously on your keyboard
Type chmod +x redirect.sh>>Press Enter dot slash redirect.sh	And type:chmod space plus x space redirect dot sh Press Enter. Type dot slash redirect.sh Press Enter.
Terminal cat out_file.txt	We can see the output by opening out_file.txt Type cat space out_file.txt
Output [Highlight] ls: cannot access /user: No such file or directory /usr: arm-linux-gnueabi arm-linux-gnueabihf bin games i686-linux-gnu include lib local sbin share src	We can see both the error and output. The error for directory /user is recorded in this file. It says that there was no '/user' directory found. The directory content for /usr is displayed. Please note that the content for '/usr' directory may vary on your system.
Type rm out_file.txt	Now let us delete this file. So, on the terminal type rm out_file.txt
*	Come back to our slides.
Display Slide 7 Redirection of both stderr and stdout Syntax: command > filename 2>&1 command >/dev/null 2>&1	Another method is to use 2>&1 2 greater than ampersand 1 after the filename The syntax iscommand space greater than filename space 2 greater than ampersand 1We can also redirect to slash dev slash null (/dev/null) file.
	Let us learn a little more about slash dev slash null (/dev/null) file.
Display slide 8 /dev/null	* It is a special kind of file It is a null file or a place where we can dump anything. It includes the output and error messages It is also called as bit bucket.
	Let us now come back to our code in gedit.
	Let us redirect both standard output and error to the null file.
ls /usr /user &> out_file.txt	I will copy this line of code and paste it below over here.
And replace the out_file.txt with /dev/null 2>&1 So, we have ls /usr /user >/dev/null 2>&1	I want both the output and error messages to be discarded. So I will change this part of the copied code.> (greater than) means truncate or write. /dev/null is the null file 2>&1 (2 greater than ampersand 1) Number “2” will redirect standard error to standard output, denoted by number “1”.
Press Ctrl+S	Now click on Save. Save the code.
Switch to terminal	Let us run the file redirect.sh. Go to the terminal.
dot slash redirect.sh	Recall the previous command with the up-arrow key. dot slash redirect.sh and press Enter.
Terminal	We can see the output by typing cat out_file.txt
Display Slide 9 Appen redirected output Synatx: command >> filename	Come back to our slides. We can capture as well as append standard output or error to a file. The output or the error will be appended at the end of the file. If the file does not exist, it will create a new file. The syntax is:command space greater than greater than space followed by filename
	Let us understand this using an example. Let me open the file redirect.sh
date >> out_file.txt	Now, here let's type date space greater than greater than space out_file.txt
Switch to the terminal and type: date	The 'date' command will simply display the system date as output. We can check this command on the terminal by typing 'date'. Come back to the terminal. Type: date You can see that the system date i.e. the current date is displayed.
Highlight the date command code line	Now come back to our code file. The output of date command will be appended to the out_file.txt file. We are using this file to capture standard output and error of 'ls' command.
	Click on Save Switch to the terminal.
dot slash redirect.sh	Now press the uparrow key. Recall the previous command ./redirect.sh and press Enter.
cat out_file.txt	Let us check the output by opening out_file.txt Type cat space out underscore out_file.txt
[Highligh] Wed Jul 2 12:06:32 IST 2014	Observe that the output of 'date' command is appended to the end of the file.
	This brings us to the end of this tutorial.
Display Slide 14 Summary	Summary Let us summarise. In this tutorial we learnt Redirection of both standard error and output And to append the redirected output
Display slide 15 Assignment	As an assignment,# Create 'X_file.txt' file with some content. Redirect the content of both 'out_file.txt' and 'X_file.txt' to a new file.
Display Slide 16 http://spoken-tutorial.org /What\_is\_a\_Spoken\_Tutorial About the Spoken Tutorial Project	Watch the video available at the link shown below. It summarises the Spoken Tutorial project. If you do not have good bandwidth, you can download and watch it.
Display Slide 17 Spoken Tutorial Workshops	The Spoken Tutorial Project Team Conducts workshops using spoken tutorials Gives certificates to those who pass an online test For more details, please write to contact@spoken-tutorial.org
Display Slide 18 Acknowledgement	Spoken Tutorial Project is a part of the Talk to a Teacher project. It is supported by the National Mission on Education through ICT, MHRD, Government of India. More information on this Mission is available at: http://spoken-tutorial.org/NMEICT-Intro
Display Slide 19	The script has been contributed by FOSSEE and Spoken-Tutorial teams. This is Ashwini from IIT Bombay. Thank you for joining.

Contributors and Content Editors

Ashwini, Nancyvarkey