Variables in BASH - Learn BASH

Welcome to the second chapter of BASH scripting series in which we will discuss variables in bash. Did you know that every programming language has a way to let programmers declare variables in their programs? But what exactly are they and why do we need variables in programming? Please read on below to learn more about variables.

What is a variable?

​In computer programming, a variable is a named storage location which contains a value.

Literally, a variable is like an empty bowl that can hold something. That “something” is usually:

• an integer number like 123
• floating point number like 123.1
• a character like ‘z’
• or string of characters as in “Hello!”

Variables in BASH

A bash variable is declared by starting with dollar symbol “$” and then typing out its identifier name. Below is a sample variable named “variable” – $variable But if you happen to declare a variable and assign some value to it at the same time, you’ll have to omit the “$” symbol as in: variable=123 Prefixing the “$” symbol to such (defined) variables would result in an error when you try to run your program.

It might seem confusing at first but you’ll get used to it after going through some example scripts afterward.  

Rules to follow

BASH has some naming conventions or rules to declare a variable:

• A variable name cannot contain any special characters other than underscore “_”
  Example-
  $@variable or $vari%ble is incorrect
  $_variable or $var_able is correct
• A variable name can contain number(s) but cannot start with it
  Example-
  $1variable is incorrect
  $var1able is correct

Apart from the above two somewhat-strict rules, in case if you are working on environment variables then there is a standard practice of using all uppercase letters as the variable identifier name. Don’t worry if you didn’t get that part. You’ll soon do as we progress on with the following chapters to come.

Purpose of variable

OK, so we’ve gone through some of the variable concepts in BASH but why do we really need them. What exactly is their purpose for holding values?

Variables to the computer are like stored memories in a huge memory storehouse (ie., RAM), a way to retain information from it just like we humans have a way of memorizing stuff and retaining ’em by citing.

Variables store data either temporarily or permanently. A variable definition such as variable=123 is an example of a temporary-stored variable ie., we can change that value 123 to another if we want. However, there will be some cases where values for certain variables remain fixed such as the value of PI in mathematics. Those variables are declared as constants and their value cannot be changed again once already set.  

Examples

Below are three example scripts you can try it out on your computer. So turn up your favorite text editor program, type in the sample scripts and save it in the bash directory we created in the Introduction chapter. Then launch your terminal program and change to the bash directory using the cd command. We will use the terminal to execute the example scripts.​

After you’re done typing and saving a script, use the below command to execute your script on the terminal program –

bash script_name.sh   

Simply store

This program will simply store a number value ie., 123 to the variable named “number” and then print it out on the screen.

#!/bin/bash
number=123
echo $number

echo is a command that displays whatever string follows after it as output. And since we don’t want to print “number” as output on the console screen but its value.

We prefixed it with “$” symbol to tell echo that number is a variable and not a string.

Compute and store

This program is somewhat realistic because most of the real-life program works this way.

Compute or execute the right-hand side and then store the result to the left-hand side.

#!/bin/bash
readonly x=2
readonly y=2
let sum=$x+$y
echo $sum

In the above program, we have two new commands used: readonly and let. 

readonly is a way to tell the computer that variables x and y are constants. So their values 2 are permanently stored and cannot be changed.

let is used to compute a mathematical expression on the right-hand side (in our case it’ll compute the sum of variables x and y) and then store the result to the left-hand side. Without let keyword, if we just type in sum=$x+$y, we’ll get 2+2 as output and not 4.  

Get user input and store

The program will ask for user input (to enter some number), store the number to a variable, and then display it on the terminal screen.

#!/bin/bash
echo Enter some number:
read number
echo The number you entered is $ 

Above we have another new command called read which is responsible for getting input from the user. Without read command, the program would execute to the end without asking and waiting for your (user) input.  

Conclusion

Remember, a variable is a named storage location which contains a value. The value could be stored either temporarily or permanently in BASH. This wraps up our variable lesson in BASH scripting. And by the way, make sure to practice the example scripts so you can better grasp variable concepts before moving on to the next chapter. In our next chapter, we will learn about comments in BASH.

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