## ⚔️ - Previous: [[Variables]] - Next: [[Base Values]] **Numbers, numbers, numbers.**  # Bash Numbers In this section, you will learn how to work with numbers in Bash. - [[Numbers#Declaring Integer Variables|Declaring Integer Values]] - [[Base Values|Using Different Bases]] - [[Arithmetic]] - [[Decimal Point Values]] ## Supported Numbers - Supported: Decimal Integers (_no decimal point values_) - e.g. `1`, `2`, `3`, `4`, `5`, `100`, `1000`, etc - _All the way up to _`9,223,372,036,854,775,808` - _Note: Bash does not support using the `,` in numbers_ - Supported: Octal numbers - Supported: Hexadecimal numbers - Supported: Any other base between 2 and 64 ## Unsupported Bash does **not** natively support numbers with **decimal points**: - Unsupported: `3.14`, `1.1`, `2.1`, `4.20`, etc ℹ️ There are ways to work with [[Decimal Point Values]] using Bash. This will be covered later in the lesson. # Declaring Integer Variables The *simplest* way to declare variables with numeric values in Bash is by using the `[variable name]=[variable value]` syntax, as shown in [[Variables|the previous section]]: ```shell dragon_strength=10 goblin_strength=5 ``` This is **not** how we will be declaring integers, however. **Here is why:** ```shell dragon_strength=10 echo $dragon_strength dragon_strength="Changed it to text" echo $dragon_strength ``` Try the above in the terminal and you will get this output: ``` 10 Changed it to text ``` **Why is this a problem?** Your program may perform arithmetic, e.g. it may multiply `10 * dragon_strength`. What will happen if the program tries to multiply `10 * "Changed it to text"`? Kaboom! Instead, to **ensure** your variables are integer values, we will use `declare`. # declare -i varname The correct way to work with integers in Bash is to use `declare -i`. ( _The `-i` stands for `integer`_ ) I have updated the previous example to demonstrate how it works: ```shell declare -i dragon_strength=10 echo $dragon_strength dragon_strength="Changed it to text" echo $dragon_strength ``` Try the above in the terminal and you will get this output: ``` 10 bash: Changed it to text: syntax error in expression (error token is "it to text" 10 ``` It breaks! _How wonderful!_ That's great, because it's not a number! Using `declare -i [variable name]` declares a variable **_as an integer_** and it will always be an integer, regardless of what value it's set to. Once a variable has been declared via `declare -i`, it can somewhat safely[^1] be set in other ways, e.g. using the `[variable name]=[variable value]` syntax. > ℹ️ Setting a `declare -i` variable to a non-number will **not** always result in an error. When an integer is set to `A - Z` text _without spaces_, it will _**set the value to 0**_ instead of raising an explicit error: > ```shell > declare -i strength=10 > echo $strength > strength=SomeTextHere > echo $strength > ``` > > The above example will output `10` and then `0`. > > You can use [[Arithmetic#Let|let]] or [[Arithmetic#Double Parentheses|(( ... ))]] to force an error! ## Declaring Without a Value You can also `declare -i dragon_strength` _without_ providing a value. This declares that the `dragon_strength` variable is, _and will always be_, an integer. Then, later code can modify the value using simple `[variable name]=[variable value]` syntax: ```shell declare -i x y # <--- declares 2 integer variables, x and y echo "x is $x and y is $y" x=10 # <--- set the values of the declared integer variables y=20 echo "x is $x and y is $y" ``` Running the above code in a terminal will output: ``` x is and y is x is 10 and y is 20 ``` This is often useful when you _don't know the value_ of a variable (_the program will set the value later_), but you want to be **_certain_** that the variable is treated as an integer. > ℹ️ `declare -i` can declare multiple integer variables at once: > - `declare -i foo` > - `declare -i foo=20 bar` > - `declare -i foo=20 bar=40 baz=60` ## Declaring With an Expression When declaring a variable with `declare -i`, an expression may be provided to set the initial value of the variable: ```shell declare -i a="1 + 4" b="a + 1" c="b * 2" echo $a echo $b echo $c ``` If you run the example above in the terminal, you will get the output: ``` 5 6 12 ``` This will be covered more in the upcoming [[Arithmetic]] section. > ⚠️ Warning: `declare -i x="Hello"` does **not fail**. > > It is best practice to use `(( ... ))` or `let` instead: > ```shell > declare -i x > > (( x = Hello )) # <-- this fails! ✓ > let x=Hello # <-- this fails! ✓ > let "x = Hello" # <-- this fails! ✓ > ``` ## Modifying Integer Variables Once a variable has been **declared as an integer** using `declare -i`, there are 3 common ways to set or modify the variable's value: 1. Simple: `[variable name]=[variable value]` (_Not recommended_) 3. Double Parentheses: `(( [variable name] = arithmetic expression ))` 2. Let: `let [variable name]="[arithmetic expression]"` These will each be reviewed in the upcoming [[Arithmetic]] section. **But let's take a quick sneak-peek at what they look like!** ### Simple x=[value] This is the same method seen above, it is the simplest method: ```shell declare -i x=5 x=10 x=20 ``` *This method only supports "static values", e.g. `10` or `20`, and does not perform arithmetic expressions.* > **This is not recommended.** > **Please use `(( ... ))` or `let`.** > ℹ️ Note: if you try to set `x="Hello"`, this will **not** cause an error. > Both `(( x = Hello ))` and `let "x = Hello"` **do** fail (_which is good_) ### Double Parentheses (( expression )) Double parentheses `(( ... ))` perform [[Arithmetic]] in Bash. ```shell declare -i x=10 echo $x (( x = x + 5 )) echo $x ``` If you run the example above in the terminal, you will get the output: ```shell 10 15 ``` ### Let x="expression" `let` allows you to easily set the value of a variable to the result of an [[Arithmetic]] expression. ```shell declare -i x=10 echo $x let x="x + 5" echo $x ``` If you run the example above in the terminal, you will get the output: ```shell 10 15 ``` # Bash Numbers In this section, you learned how to **declare** integer variables in Bash. Next up, you will learn more about using numbers in Bash! - [[Base Values|Using Different Bases]] _(Optional)_ - [[Arithmetic]] - [[Decimal Point Values]] After numbers, you'll move on to: - [[Text]] variables - [[Lists]] variables [^1]: Even if a variable is declared with `declare -i`, trying `varname=Hello` will set the variable to `0` _but_ it will not _fail_ (_it returns a 0 exit code_). If you want to ensure that your script fails when an invalid value is given to an integer value, use `let` or `(( ... ))` to assign variable values. This is the safest and recommended way.