In this chapter, we'll learn how to work with conditionals and then loops in Bash using the if, for, and while shell keywords. These allow us to execute commands conditionally and/or repeatedly, giving us control flow for our scripts.

We will cover:

• How to use the if keyword
• The use of and the difference between the [ builtin and the [[ keyword
• When to use Bash's (( arithmetic compound command
• How to match values with the case keyword
• How shell script for loops work
• Using Bash's alternative C-style for loops
• Using while loops
• The while read -r idiom for reading data line by line
• Choosing between for and while

Shell script's syntax for these structures is somewhat unique, largely for historical reasons and to fit into its command-centered structure, and only partly resembles comparable features in other programming languages.