OSBash.004: Conditional Tests — if, then, else, fi, and [[ ]]

2026-10-18

OSBash.004: Conditional Tests — if, then, else, fi, and [[ ]]

Anime illustration of a Linux learner studying Bash conditional expressions at a desk.

This lesson introduces conditional tests in Bash: how a shell script chooses what to do when a condition is true or false. It follows OSBash.003 on variables and expansion.

Learning Objectives

Recognize if, then, else and fi; test whether a directory exists; compare strings safely; understand the meaning of an exit status.

The Basic if Statement

A Bash if statement runs a command and checks its exit status. Status zero means success; a nonzero status means failure. The shell executes the then branch only when the tested command succeeds.

if true; then echo "Condition passed"; else echo "Condition failed"; fi

Check Whether a Directory Exists

The [[ ... ]] test expression is built into Bash. The -d operator checks whether a path exists and is a directory.

if [[ -d /var/log ]]; then echo "Log directory found"; else echo "Log directory missing"; fi

Compare Two Strings

Use == inside [[ ... ]] for string equality. Quote variables to make intent clear, especially when teaching beginners.

role="admin"; if [[ "$role" == "admin" ]]; then echo "Administrator"; else echo "Standard user"; fi

Common Mistakes

Every if must end with fi. Use spaces around operators inside [[ ... ]]. Remember that [[ ... ]] is Bash syntax and is not guaranteed to work in every POSIX sh shell. Never assume a successful test grants authorization; it only checks the condition you wrote.

Knowledge Check

1. What exit status means success? 2. What does -d test? 3. Which keyword ends a Bash if statement? 4. What is the purpose of else?

Answers

1. Zero. 2. Whether the path is a directory. 3. fi. 4. It supplies commands to run when the tested condition fails.