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Introduction to Linux Shell Scripts
One of the great things about Linux is all of the tools that are available at the tip of your fingertips. The command prompt, or shell, offers a way to quickly and easily send files to other computers, analyze a computers network, rip audio CDs, and countless other tasks.
By writing your own shell script, you can execute commands and preform simple programing logic. Using a shell script can make it much easier to use your favorite commands and allow you to accept and use input from the user, as well as helping to make preforming repetitive tasks, like network administration, much simpler.
While Windows includes a command line interface, it is not as powerful or useful as the Linux Shell. However, if you have Windows, you could use Bat Files to play around with many of the examples in this tutorial.
Preparing Your Enviroment
Below is an example of a simple shell script that prints Hello World!
First, save the following text in a file named foo.sh on your computers desktop.
#!/bin/bash #The Echo Command is used to Print Words and Sentences echo Hello World! #exit the shell script with a code of 0 exit 0
Line 1 tells the computer that it is a shell script.
Line 3 is a comment. The "#" sign tells the computer it is a comment and is ignored. Comments are added for the benefit of humans who might be reading the code. Make sure to check the comments in the rest of the examples to see how the program works.
Line 4 prints text using the echo command. You can enter any shell command in the shell script to run it or initiate it.
Line 7 Exits the script with return code of 0. You do not technically need to really use the exit command here, because the script automatically exits when it reaches the end of the file.
Executing Our First Shell Script
Once you have saved the file on your desktop, we are almost ready to execute it, however first we have to change the files permissions. This is because by default, normal files can not be executed as shell scripts.
Open up the shell and Type:
chmod +x foo.sh
The chmod command is used to change a files permissions, in this case allowing foo.sh to be executed as a script.
Next, execute the shell script by typing:
The "./" part of the command tells the shell to execute the script and you should see:
Accepting User Input
Next, lets read some user input from the command line. This allows your script to use and process information provided by the user in real time. We will read the user input and store it in a variable.
Open your text editor and change foo.sh so it only contains the following text.
#!/bin/bash #Ask the User for their Name echo Enter Your Name #Use the read command to read user input and store it in a variable called "name" read name #print the stored variable "name" echo "Hello $name" #exit the shell script with a code of 0 exit 0
Line 7 uses the "read" command to read the user input and store in the "name" variable.
Line 10 then prints the "name" variable. Note that on line the $ sign is used to print the variable "name".
Execute the command and when prompted type your name, you should see something similar to:
Enter Your Name
Creating a Simple User Interface
To give you a slightly more practical use for a shell script. The next example makes a simple user interface for copying files. This script works with an existing command to make it arguably easier to use.
This shell script prints the files in the directory and asks the user for the name of the file to copy and for the new name. Then, the file is copied using the "cp" command.
#!/bin/bash echo Files in this directory #Uses the "ls" command to list the files that are in the current directory. ls echo Enter the file name to copy #Stores the name of the file to copy as "oldName" read oldName echo Enter the New Name #Stores the new File Name as "newName" read newName echo "We are making a copy of $oldName and calling it $newName" #Uses the "cp" command to copy the files cp $oldName $newName #exit the shell script exit 0
This shell scripts builds upon the fundamentals that have been explained in this tutorial.
Lines 12 and 17 read and store variables from the user input. These variables are used to store the name of the file to be copied and the new name for the file.*
Line 23 then uses the "cp" command to copy the files using the information that the user entered.
*It is important that neither of the two examples so have validated the user input. This means that if the user just pressed enter, without entering any data, the shell script would not behave as expected. Normally, you would check to ensure that the user entered something and that it can be used by the shell script.
While the above examples offer an insight into shell scripts. This barely scratches the surface of the power and usefulness of Linux shell scripts. It is possible to use conditional operators, for loops, and many other programming language constructs, which will be discussed in future tutorials.