Shell scripting - Basic (PART -1)

Shell Scripting
Chitrakshi Jaiswal

• Basics ………..…3
• First script ______________________________ ………….…4
• Variables and Comments ………….…5
• System defined Variables __________________ ………….…6
• User defined variables ____________________ ………….…7
• Use of Backslash character ________________ ………….…9
• Backtick Symbol _________________________ ………….…11
• Read User Input ………….…12
• Use of “-p” (Read as it is) __________________ ………….…13
• For multiple Variables ____________________ ………….…14
• Hide input _____________________________ ………….…15
2
PART-1

Basics
Q. What is shell script and how to check shells of your system ?
Ans. A shell script is a text file that contains a sequence of commands for
a Linux-based operating system. It's called a shell script because it
combines into a "script" in a single file a sequence of commands that
would otherwise have to be presented to the system from a keyboard
one at a time.
To check shell (operating system's command interpreter ) use following
command “cat /etc/shells “
3

First Script…
Let’s start our first script with “echo” command……..
Task : To print “my world”
Step 1: vi test1.sh (“test1.sh” is my script name)
Step 2: Write following commands in your file
#! /bin/bash (For Interpreter -Specify shell to execute program)
echo “my world” (Here, “my world” is desired o/p; save and exit this
file)
*Always give execution permission to your SCRIPTS
• chmod +x test1.sh
Output: Now, Run your script
sh test1.sh ---------->
4

Variables and comments
• Comments : Use # to make anything commented. Generally it uses to
explain about your commands or to give details about your script.
• Variables : Which store some data in itself. Here, we have 2 types of
variables.
a) System defined
b) User defined
5

a. System defined Variables are created and maintained by your Linux
system, they are pre-defined in your system.
• Generally these variables are defined in CAPITAL LETTERS.
• We can tap into these environment variables from within your scripts by
using the environment variable’s name preceded by a dollar sign. This is
demonstrated in the following script 
6

b. User defined Variables are case sensitive, so the variable Var1 is different from
the variable var1.
• Values are assigned to user variables using an equal sign. (There is no space
b/w var and def. var)
• Here are a few examples of assigning values to user variables:
i. var1=10
ii. var2=-57
iii. var3=testing
iv. var4=“still more testing”
• The shell script automatically determines the data type used for the variable
value.
7

Example 1:
(Just like system variables, user variables can be referenced using the
dollar sign)
Output : (Each time the variable is referenced, it produces the value
currently assigned to it.)
8

Example 2: (Look at what happens in this example)
Output :
• That is obviously not what was intended. Whenever the script sees a dollar
sign within quotes, it assumes you’re referencing a variable. In this example
the script attempted to display the variable $1 (which was not defined),
and then the number 5. To display an actual dollar sign, you must
precede it with a backslash character.
9

• The backslash allowed the shell script to interpret the dollar sign as
an actual dollar sign, and not a variable.
Example 3:
Output :
10

Example 4: (Use of Backtick symbol (`) in shell variables)
The shell runs the command within the backticks and assigns the output to
the variable testing
Output :
Note : In bash you can also use the alternative $(…) syntax in place of
backtick (`),which has the advantage of being re-entrant.
11

Read User Input
• read command is used for getting user input.
Example 1:
Output : (Here, cj is the user input)
12

When we want input at same place  Use “-p”
Example 2:
Output : (Compare this with Example1)
13

For Multiple variables/User’s inputs 
Example 3 :
Output :
14

When we don’t want to show input  Use “-s”
Example 4 :
Here, I have used an extra echo
which makes output like this. (w/o echo
o/p ‘ll be like this)
Output : (Here, Input for Password is not visible)
15

PART-2
To be continued……….
16
• Array
• Input - Output redirection
• Pipes
• Control structures (if)
• Loops

Shell scripting - Basic (PART -1)

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