ShellScript – Continued (P4)

Let’s continue to explore the next part of the shell script!

Arithmetic (continued)

Using double parentheses (( )) , the syntax:

For example:

• Line 3 – This is the basic format, we can arrange it easily without quotes.
• Line 6 – It works the same way if we write without spaces.
• Line 9 – We can write including variables without the $ sign in front.
• Line 12 – Variables can be included with the $ sign if desired.
• Line 15 – Here the value of variable b is increased by 1. When we do this, we don’t need the $ sign before the parentheses.
• Line 18 – Here the value of variable b is increased by 3. It is a shorter spelling of b = b + 3 .

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The length of a variable, the syntax:

For example:

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IF statement

We will learn more about if

• The if allow us to make decisions in our Bash scripts. It allows us to decide whether or not to run the code based on the conditions we can set.

Basic IF statements

• A basic if statement will check: if a specific condition returns true then it will perform a certain set of actions.If it will not perform those actions, the syntax is as follows:

• Any statement between then and fi will be executed if the condition returns true.
• For example:

• Line 3 – here, will check if the first input variable is greater than 50
• Line 5 – If the input variable is greater than 50, it will echo to the text screen “Amazingggg.”

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Operator

Operator	Describe
! condition	If condition is true then returns false and vice versa
-n string	string length is greater than zero
-z string	string length is zero (ie empty)
string1 = string2	string1 is equal to string2
string1! = string2	The two strings are not the same
integer1 -eq integer2	integer1 is equal to integer2
integer1 -gt integer2	integer1 is greater than integer2
integer1 -lt integer2	integer1 is smaller than integer2
-d file	The file exists and is a directory
-e file	file exists
-r file	The file exists and is readable
-s file	The file exists and its size is bigger than 0
-w file	The file exists and is writable

• Note:
• = slightly different from -eq , for example [005 = 5] will return false, and [005 -eq 5] will return true.
• When we mention the above file will be interpreted as a path . A path – the path can be absolute or relative and can refer to a file or a directory.
• We can use the command test to test cases we mention, for example:

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Nested if statement

• We can have multiple if in our script. For example:

• Line 3 – Executes when the first parameter passed is greater than 50
• Line 6 – If we want to test an expression we can use the opening and closing of the parentheses as above
• Line 8 – Will run if expression line 6 returns true

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If else statement

• In cases where we want to perform certain actions if the condition in [] returns true and others if false, we can write in the form:

• For example:

If elif else statement

• It is possible to add multiple conditions that can lead to different results, by using if elif else . Syntax:

• For example:

• You can have as many elif branches as you want. And the end will use the else option.

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Boolean operator

• This operator includes: and - && and or- ||
• For example:

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Repeat the Case statement

• Basically, if and case pretty similar. However, we often use the case in mul-ti choice + simple expressions, and if will use for less-case problems and complex expressions.
• Syntax of using case :

• For example:

• Line 3: This line starts executing case commands
• Line 4: If the first passed variable $1 is yellow will execute the command inside it. The ) sign indicates the end of the case.
• Line 6: We determine the end of this set of statements with a minus ;;
• Line 13: The * symbol represents any character, we can consider this to be the default if the variable passed in does not fall into any case.
• Line 16: esac indicates that we are at the end of the case statement. Any other statements after it will be executed normally.

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Above I have learned more about Arithmetic and If Else . I would like to finish part 4 here.

Thank you for reading this article!