# Variables

> Source: https://learn-bash.net/variables/
> Part of Learn Bash, free to read.

Shell variables are created once they are assigned a value. A variable can contain a number, a character or a string of characters. Variable name is case sensitive and can consist of a combination of letters and the underscore “_”.
Value assignment is done using the “=” sign. Note that no space permitted on either side of = sign when initializing variables.

```bash
PRICE_PER_APPLE=5
MyFirstLetters=ABC
greeting='Hello        world!'
```

Referencing the variables

A backslash “\” is used to escape special character meaning

```bash
PRICE_PER_APPLE=5
echo "The price of an Apple today is: \$HK $PRICE_PER_APPLE"
```

Encapsulating the variable name with ${} is used to avoid ambiguity

```bash
MyFirstLetters=ABC
echo "The first 10 letters in the alphabet are: ${MyFirstLetters}DEFGHIJ"
```

Encapsulating the variable name with “” will preserve any white space values

```bash
greeting='Hello        world!'
echo $greeting" now with spaces: $greeting"
```

Variables can be assigned with the value of a command output. This is referred to as substitution. Substitution can be done by encapsulating the command with `` (known as back-ticks) or with $()

```bash
FILELIST=`ls`
FileWithTimeStamp=/tmp/my-dir/file_$(/bin/date +%Y-%m-%d).txt
```

Note that when the script runs, it will run the command inside the $() parenthesis and capture its output.