Numeric variables in Bash

Introduction to Bash Scripting

Alex Scriven

Data Scientist

Numbers in other languages

Numbers are not built in natively to the shell like most REPLs (console) such as R and Python

In Python or R you may do:

>>> 1 + 4

It will return what you want!

Numbers in the shell

Numbers are not natively supported:

(In the terminal)

1 + 4

bash: 1: command not found

Introducing expr

expr is a useful utility program (just like cat or grep)

This will now work (in the terminal):

expr 1 + 4

Nice stuff!

expr limitations

expr cannot natively handle decimal places:

(In terminal)

expr 1 + 2.5

expr: not a decimal number: '2.5'

Fear not though! (There is a solution)

Introducing bc

bc (basic calculator) is a useful command-line program.

You can enter it in the terminal and perform calculations:

Using BC program in shell

Getting numbers to bc

Using bc without opening the calculator is possible by piping:

echo "5 + 7.5" | bc

12.5

bc scale argument

bc also has a scale argument for how many decimal places.

echo "10 / 3" | bc

echo "scale=3; 10 / 3" | bc

Note the use of ; to separate 'lines' in terminal

3.333

Numbers in Bash scripts

We can assign numeric variables just like string variables:

dog_name='Roger'
dog_age=6

echo "My dog's name is $dog_name and he is $dog_age years old"

Beware that dog_age="6" will work, but makes it a string!

My dog's name is Roger and he is 6 years old

Double bracket notation

A variant on single bracket variable notation for numeric variables:

expr 5 + 7
echo $((5 + 7))

12
12

Beware this method uses expr, not bc (no decimals!)

Shell within a shell revisited

Remember how we called out to the shell in the previous lesson?

Very useful for numeric variables:

model1=87.65
model2=89.20

echo "The total score is $(echo "$model1 + $model2" | bc)"

echo "The average score is $(echo "($model1 + $model2) / 2" | bc)"

The total score is 176.85
The average score is 88

Let's practice!

Introduction to Bash Scripting