BASH/C2/String-and-File-attributes/English

Title of script: BASH comparison

Author: Ashwini Patil

Keywords: video tutorial, ==, !=, string compare, file attributes.

Visual Cue	Narration
Display Slide 1	Dear friends, Welcome to the spoken tutorial on BASH comparison.
Display Slide 2	In this tutorial, we will learn String comparison File attributes comparison with few examples
Display Slide 3	For this tutorial I am using Ubuntu Linux 12.04 Operating System and GNU BASH version 4.2.24 Please note, GNU bash version 4 or above is recommended to practice this tutorial.
Display Slide 4	There are two ways to compare a string in Bash. 1) Using == operator To compare two equal strings. 2) Using != operator To compare two not equal strings. Let us look at an example.
	I have a simple program here that checks the user ID. Open a file in your editor and save it as strcompare.sh Now, type the code as shown here, in your strcompare.sh file
Highlight #!/bin/bash	This is shebang line.
whoami	whoami command gives the username of the current user.
Highlight if [ "$(whoami)" != 'root' ]; then	The if statement checks the output of the variable whoami against the string “root”. We have used not-equal to operator here to compare strings.
Highlight echo "You have no permission to run $0 as non-root user."	If the current user is not the root user, then it will echo this statement - “You have no permission to run strcompare.sh as non-root user.” Here $0 is the zero'th argument which is a filename itself.
Highlight else echo "Welcome root!. "	If the user is the root user, it will echo - “Welcome root!”
Highlight exit 0;	Then we have the exit statement for the program. And here with fi, we end the if statement.
	Save your file and exit the editor.
	Let us switch back to our slides to know more about exit statement.
Display Slide	Every program returns an exit status. A successful command returns a 0 (zero). An erroneous command returns a non-zero value. It can be interpreted as an error code. We can customize the return value of the exit statement by stating the return value.
	Now, let's execute the program.
Press Ctrl+Alt+T on the Terminal:	Open the terminal by pressing Ctrl+Alt+T keys simultaneously on your keyboard.
Type: whoami	Now first let's check the current user of the system. Type whoami and press Enter. This will output the name of the current user.
Type: chmod +x strcompare.sh	Now let's make our file executable. Type chmod +x strcompare.sh Press Enter
Type ./strcompare.sh	Then type ./strcompare.sh Press Enter
Highlight The output	The output is displayed as: You have no permission to run ./strcompare.sh as non-root user.
On the terminal Type: sudo su Type: ./strcompare.sh	Now let's execute the same program as root user. Type: sudo ./strcompare.sh and press Enter
Highlight The output	The output is displayed as: Welcome root!.
FILE ATTRIBUTES	Now let's learn about the file attributes comparison.
	I already have a working example of the code.
	In this program we will check whether a given file exists or not.
Highlight file1=“/home/ttt/fileattrib.sh”	file1 is the variable in which we save the path of the file.
Highlight if [ -f $file1 ]; then	Then -f command checks whether the file exists or not. And whether it is a normal file.
Highlight echo "File exists and is a normal file"	If the condition is true, it will echo "File exists and is a normal file"
else echo "File does not exist" fi	If the condition is false, it will echo "File does not exist"
On the terminal	Let us switch to the terminal and execute the program.
Type: chmod +x fileattrib.sh	Let us first make the file executable. Type chmod +x fileattrib.sh and press Enter.
Type: ./fileattrib.sh	Type ./fileattrib.sh and press Enter.
Highlight Output	The output is: File exists and is a normal file
	Now we will check whether the file is empty or not.
Type on the terminal gedit empty.sh &	Before executing our program, I will create an empty file. Type gedit empty.sh & and save this file using Ctrl+S.
Replace if [ -f $file1 ]; then with if [ -s $file1 ]; then	Come back to our program. Let us replace the -f attribute with -s.
Replace file1= “/home/ttt/fileattrib.sh” with file1= “/home/ttt/empty.sh”	And also replace the filename with empty.sh .
Replace echo "File exists and is a normal with echo "File exists and is not empty" AND "File does not exist" with echo “File is empty”	Now replace the first echo statement with: echo “File exists and is not empty” And the second echo statement with: echo “File is empty”
On the terminal	Click on Save.
Type: ./fileattrib.sh	Now on the terminal, let's execute the program. Type ./fileattrib.sh and press Enter.
Highlight Output	The output is File is empty.
	Now let us see another file attribute which will check the write premission of any file.
Replace if [ -s $file1 ]; then with if [ -w $file1 ]; then	In our program, let us replace the -s command with -w command.
Replace file exists and is not empty" with echo “User has write permission to this file” AND echo "file is empty" with echo “User don't have write permission to this file”	Now replace the first echo statement with: echo “User has write permission to this file” And second echo statement with: echo “User doesn't have write permission to this file”
	I will use a different file for this example. I will select a file which is don't have read or write permission.
Replace the filepath with “/etc/mysql/debian.cnf” Save the file	Let me change the filepath to “/etc/mysql/debian.cnf” and save the changes.
Switch to the terminal	Let us execute our program now.
On the terminal Type: ./fileattrib.sh	Come back to the terminal and type: ./fileattrib.sh
Highlight Output	We see that the output as: User doesn't have write permission to this file.
	Let us see another example based on file attributes. In this example we will check whether file1 is newer than file2.
Switch to fileattrib2.sh Point cursor to the filename fileattrib2.sh	Let us see the program. Note that the filename is fileattrib2.sh I have already typed the code. So, let's go through it now.
Highlight file1= “/home/ttt/empty1.sh” file2= “/home/ttt/empty2.sh”	Here we have two variables file1 and file2. The two files have already been created and are empty.
Open empty1.sh	First, we will edit empty1.sh
Type: #!/bin/bash echo “Hiii”	I will just add an echo statement in it. Type: echo “Hiii”
On the terminal Type: chmod +x fileattrib2.sh	Let us execute the program. Switch to the temrinal and type - chmod +x fileattrib2.sh
On the terminal Type: ./fileattrib2.sh	Now type ./fileattrib2.sh
Highlight Output	We see the output as: file1 is newer than file2 file2 is older than file1
Open empty2.sh Type: #!/bin/bash echo “Hello”	Now let's edit empty2.sh Here also I will add an echo statement.
On the terminal type: ./fileattrib2.sh	Let's execute come back to our terminal and type ./fileattrib2.sh
Highlight Output	The output is displayed as: file2 is newer than file1 file1 is older than file2
Display Slide 8 Summary	Let us summarise. In this tutorial we learnt, String comparison file attributes == != -f -s -w -nt -ot
Display Slide 9 Assignment	As an assignment Explore some more attributes. Ex: -r -x -o
Display Slide 10 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 11 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 12 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
	The script has been contributed by Fossee and spoken-tutorial teams. This is Ashwini Patil from IIT Bombay Thank You for joining.

Contributors and Content Editors

Ashwini, Nancyvarkey