if statements in Bash are often used in four important ways:
1. if...then...fi statements 2. if...then...fi...else statements 3. if..elif..else..fi 4. if..then..else..if..then..fi..fi.. (Nested Conditionals)
The Recommended Resources section may give you a clearer idea of conditionals in Bash.
A quick but useful tutorial for Bash newcomers is here.
Handling input is documented and explained quite well on this page.
Different ways in which 'if' statements may be used in Bash are demonstrated here.