Working with numbers in Bash

We distinguish:

  • arithmetic expansion $(( ))
    • returns the result of literal numbers that can then be stored in a variable
  • artihmetic evaluation (( ))
    • perform calculations on existing variables

An example of expansion:

a=3
((a += 3))
echo $a
# 6
((a++))
# 7

Note: we do not use a dollar-sign when referring to variables within arithmetic evaluation, there is no need. If we do, we get an error. This is because we are using an expansion, therefore the variables are already being interpreted as variables.

Declaring variables as integers

It is good practice to safeguard against Bash treating numbers as strings to declare them as integers in addition to using arithmetic evaluation, e.g:

declare -i b=3

Whilst this isn’t a strict type, it means we can do this:

b=$b+4
echo b
# 7

Without getting 3+4 in return

No decimals in bash

Bash does not support decimal calculations natively. This is what you’d get for example:

echo $(( 1/3 ))
# 0 (not 0.33)

So work with decimals you should use awk or bc (“basic calculations”).

Example of using bc:

declare -i c=1
declare -i d=3
e=$(echo "scale=3; $c/$d" | bc)
# 0.333

Random numbers

Generate a pseudo-random number between 1 and 20:

echo $(( 1 + RANDOM % 10))
# 18
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