Using Variables
Variables are not only typical to shell scripting, we'll see them a lot in the modules to come; they are like special containers for holding values. The values they hold can be just about anything you can think of and they are identified by the name we give them.
Variables allow us to easily mutate (change, update) a value.
Before going on to dealing with variables in Shell scripting, let's examine how variables are named, and how they work.
A closer look at variables
Consider this JavaScript code:
let name = "Adeoye Adefemi";
let age = 34;
let gender = "male";
// introduce me
introduce_me(name, age, gender);
// update my age, and name
age = 45;
name = "Opeeolluwa";
//introduce me again using the new age and name
introduce_me(name,age,gender);
// introduce me again
age = 60;
name = "Adeoye Adefemi Opeoluwa";
introduce_me(name, age, gender);
function introduce_me(name, age, gender) {
console.log(`My name is ${name}, I am a ${age} years old ${gender}. `);
}
When you are done, click on the play icon on the top right corner to see the code in action.
The code, when run yields:
My name is Adeoye Adefemi, I am a 34 years old male.
My name is Opeeolluwa, I am a 45 years old male.
My name is Adeoye Adefemi Opeoluwa, I am a 60 years old male.
The key lesson here is that, the terms age, name, and gender made it possible to quickly update the values and to use them in multiple places. I encourage you to edit the code, use your name in place of mine, then run the code again.
These three things; age, gender and name as used here are called variables.
At first age stored the value 34, then I updated this value to 45, and finally 60.
Naming Variables
As a rule in many programming languages, or toolchain like the Shell scripting we are studying, it is often encouraged that:
-
A variable be descriptive
-
A variable starts with a letter followed by a letter, more alphabets, or underscore.
Here are a few examples:
my_name = "Adeoye"
my_country = "Nigeria"
title_of_a_king = "King Jaja of Opobo"
The first one, talks about my name, it's very descriptive, the next is my country, note that the variable my_country is very descriptive, and the same can be said about the last example.
With this out of the way, let's address variable naming conventions.
Variable naming conventions
As of today, there are 5 conventional methods of naming variables, they are as follows:
-
snake case
-
upper snake case
-
camel case
-
pascal case
-
kebab case
Snake case
In snake case, you separate your variables with underscore. For example,
my_name, my_friends_name.
Upper snake case
Like the snake case, you separate your variables with underscore, however these words must be capitalized. For example, MY_NAME, MY_FRIENDS_NAME. This is often used for variables that would not change, otherwise knows as constants.
Camel case
In camel case, you capitalize the first letter of every word except the first. This gives myName , myFriendsName.
Pascal case
Very similar to camel case, just that you capitalize the first letter of each word like this MyName, MyFriendsName.
Kebab case
The kebab case is very similar to the snake case, just that hyphens are used in place of underscores. For example, we have my-name, my-friends-name.
Variables in shell scripting
Finally, here we are, variables in Shell scripting. Consider a minimal reproduction of the JavaScript code, I'll create a file var and add the following content.
#!/bin/bash
name="adeoye"
age=4
echo $name
echo $age
On line 3 and 4 we declared, the variables name and age. Then on line 6 and 7 we print these values using the echo command.
Let's proeed to make this executable and run it.
chmod u+x var
./var
Notice how when we needed to access the variable values we had to prepend it with a dollar sign $. This is a very important concept when working with variables in Shell scripting.
The executed script output should be similar to this:
adeoye
4