What is the syntax for if/then/else statements in Bash?

if CONDITION; then ... else ... fi — note the semicolon (or newline) before then, and that the block closes with fi ("if" reversed).

The thing that surprises people is what the "condition" actually is: bash doesn't test true/false values, it tests a command's exit code. if runs the command and treats exit code 0 as true, anything else as false. That's why if [ -f file ] works — [ is itself a command that exits 0 when the test passes.

if CONDITION; then
    STATEMENTS
fi

if CONDITION; then
    STATEMENTS
else
    STATEMENTS
fi

Example - check if service is running:

systemctl is-active sshd > /dev/null 2>&1
if [ $? -ne 0 ]; then
    sudo systemctl start sshd
else
    echo "sshd already running"
fi

Important syntax rules:

• Space after [ and before ]
• Semicolon before then (or then on new line)
• End with fi (if backwards)

One-liner:

if [ -f /etc/passwd ]; then echo "exists"; fi