Functions, Conditional Statements and Loops in Bash
Let’s continue leaning about bash scripting. In this article we will deep dive in Functions, Conditional Statements and Loops in Bash.
You can checkout Basics of Bash before deep diving into scripting part.
Function in bash script
To organize and reuse code with more efficiency and readability of the script, we can use functions in bash script.
Here the function name is details, and function body consists of three echo commands. Every time a function call is made by using the function name, three echo commands are executed. Important point is that the function definition must precede function call, otherwise the script will return function not found error.
Output of above script is as follows:
Conditional Statements
To implement decision making based on certain conditions within a shell script, we can use conditional statements in bash script.
Two integer variables ‘a’ and ‘b’ are initialized with values 10 and 20 respectively. After that, we begin an if conditional block and compare both variables and if the comparison evaluation yields true, then echo command will print the result as the value within the variable $a is less than the value in variable ‘$b’. ‘fi’ keyword is used to close ‘if’ conditional block.
In the situation when the variable ‘$a’ greater than ‘$b’ , echo command will print the result as the value within the variable ‘$a’ is greater than the value in variable ‘$b’
Output of above script is as follows:
Bash Loops
To iterate given tasks number of times, we can use loops in bash script. Let’s take a look at various loops.
For loop:
There are 4 shell reserved keywords: ‘for’, ‘in’, ‘do’ and ‘done’
The above code can be read as:
“FOR each item in list [1, 2, 3], assign each item temporarily into a variable ‘i’ after which DO echo ‘$i’ in order to print the item and keep printing until all items in the list are DONE.”
Output of above script is as follows:
While Loop:
The enclosed code within the while loop will get executed only while the ‘cnt’ variable is less than 3. During each loop iteration, the variable ‘cnt’ is incremented by one. Once the variable ‘cnt’ is equal to 3, the condition defined in while loop becomes false and while loop execution is terminated.
Output of above script is as follows:
Positional parameters
Positional parameters are nothing but command line arguments which can be accessible within a script as $1, $2 … $N variables. During the script execution, any additional items supplied after the program name are considered arguments and are available during the script execution.
Here in above code snippet, $1 and $2 represents first and second command line argument. $* represents all the command line arguments. In below snippet, we are passing 3 command line arguments, namely ‘bash’, ‘scripting’, ‘rocks’.
$1 holds ‘bash’, $2 holds ‘scripting’ and $* holds all the three arguments.
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