We will build a simple calculator program using BASH scripting language and at the same time reinforce all the concepts already taught. In a quick summary, we’ve explored the fundamental topics on variables, decisions, control statements, and arguments. Then there were minor subtopics like using comments in bash programs and operator types. Make sure to refer to those previous chapters in case you need to relearn lost information.
Algorithm or procedure
There are two ways to use this calculator program based on the input methods:
Arguments – Use the program onceread command – Use the program infinitely
Both of them will have an exit option that will give the user the flexibility to exit the program if they want to. For arguments input, we have to make sure that a maximum of 2 arguments is acceptable because, in mathematics, a simple addition, subtraction, division, or multiplication operation requires only two operands. Otherwise, we will print a message informing the user about the program’s limitations. For read command input, we will use a while loop to implement an infinite control flow, so the program exits only when the user indicates it. The user will be asked to enter operand1 and operand2 values, and the result will be computed based on their choice.
Coding
#!/bin/bash
Two arguments are passed as inputs.
Will calculate based on the user choice
And exit the calculator program straightaway
if [[ $# -eq 2 ]] ; then # get operand1 and operand2 values from the arguments operand1=$1 operand2=$2 # Offer choices echo 1. Addition echo 2. Subtraction echo 3. Multiplication echo 4. Division echo 5. Exit echo Enter your choice: read choice # start computing if [[ $choice -eq 1 ]] ; then echo —————————————- echo Addition of $operand1 and $operand2 is $((operand1+operand2)) echo —————————————- echo elif [[ $choice -eq 2 ]] ; then echo —————————————- echo Subtraction of $operand1 and $operand2 is $((operand1-operand2)) echo —————————————- echo elif [[ $choice -eq 3 ]] ; then echo —————————————- echo Multiplication of $operand1 and $operand2 is $((operand1*operand2)) echo —————————————- echo elif [[ $choice -eq 4 ]] ; then echo —————————————- echo Division of $operand1 and $operand2 is $((operand1/operand2)) echo —————————————- echo elif [[ $choice -eq 5 ]] ; then exit else echo —————————————- echo Invalid choice… echo —————————————- echo fi
This portion will run the calculator program infinitely unless
the exit option 5 is given as input by the user
elif [[ $# -eq 0 ]] ; then while true do # Offer choices echo 1. Addition echo 2. Subtraction echo 3. Multiplication echo 4. Division echo 5. Exit echo Enter your choice: read choice # get operands and start computing based on the user’s choice if [[ $choice -eq 1 ]] ; then echo Enter operand1 value: read operand1 echo Enter operand2 value: read operand2 echo —————————————- echo Addition of $operand1 and $operand2 is $((operand1+operand2)) echo —————————————- echo elif [[ $choice -eq 2 ]] ; then echo Enter operand1 value: read operand1 echo Enter operand2 value: read operand2 echo —————————————- echo Subtraction of $operand1 and $operand2 is $((operand1-operand2)) echo —————————————- echo elif [[ $choice -eq 3 ]] ; then echo Enter operand1 value: read operand1 echo Enter operand2 value: read operand2 echo —————————————- echo Multiplication of $operand1 and $operand2 is $((operand1*operand2)) echo —————————————- echo elif [[ $choice -eq 4 ]] ; then echo Enter operand1 value: read operand1 echo Enter operand2 value: read operand2 echo —————————————- echo Division of $operand1 and $operand2 is $((operand1/operand2)) echo —————————————- echo elif [[ $choice -eq 5 ]] ; then exit else echo —————————————- echo Invalid choice.. Please try again echo —————————————- echo fi done else echo —————————————- echo You either passed too many parameters or too less echo than the optimum requirement. echo echo This program accepts a maximum of 2 arguments or no echo argument at all in order to run successfully. echo —————————————- fi
Some clarification
In the above calculator program, you might notice two new things: infinite while loop and the keyword exit.
infinite while loop
There are many ways to implement an infinite loop using either for, while, or until loops. In our program, we’ve implemented it using a while loop.
while true do statements done
Since the condition is always true, the loop executes infinitely. Of course, we did mention an exit option so the user can terminate the program if they wanted to. Right? We have the keyword…
exit
Once the bash interpreter encounters this keyword, it will directly terminate the program in execution. We’ve implemented an option, i.e., 5, to terminate the program, so the user is not stuck with the calculator program forever.
while true do … elif [[ $choice -eq 5 ]] ; then exit done
Final thoughts
The code is quite lengthy, but lucky you. You are getting a glimpse of the software project in real life. There’s much more to learn, but this series is intended to cover only the fundamentals, so you get the best benefits after learning this scripting language within a short span of time. Now you can implement many cool programs and share them with your pals, push yourself further by researching BASH scripting, get comfortable involved in learning other major programming languages like C and C++, etc. Of course, the syntax will vary and might get you puzzled at first, but you’ll notice that the concepts already taught here are the same for almost every programming language out there.
Conclusion
BASH is a scripting language and the default shell interpreter for almost all major Linux distributions. So you have high scalability in case you wish to deploy cool Linux bash programs across multiple distributions. Moreover, now you can understand and also customize most software configuration files with the concepts learned here. They are all scripts 🙂
