Conditionals: if/elif/else and [[ ]]

The shape

if [[ condition ]]; then
  # ...
elif [[ other ]]; then
  # ...
else
  # ...
fi

if takes a command. It runs the command and branches on exit code (0 = then, non-zero = else). [[ ... ]] is the bash/zsh test command — preferred over [ ... ] (POSIX) because it's safer with empty variables and supports more operators.

String tests

• [[ -z "$x" ]] — string is empty.
• [[ -n "$x" ]] — string is non-empty.
• [[ "$a" == "$b" ]] — equal.
• [[ "$a" != "$b" ]] — not equal.
• [[ "$x" == prefix* ]] — glob match (only inside [[ ]]).
• [[ "$x" =~ ^[0-9]+$ ]] — regex match (bash/zsh).

Numeric tests

• [[ $a -eq $b ]] — equal (eq, ne, lt, le, gt, ge).
• (( a == b )) — arithmetic context (cleaner for math).

File tests

• [[ -f path ]] — regular file exists.
• [[ -d path ]] — directory.
• [[ -e path ]] — exists (any type).
• [[ -r path ]], -w, -x — readable / writable / executable.
• [[ -s path ]] — exists and non-empty.

And / or

if [[ -f config.yaml && -r config.yaml ]]; then ...
if [[ -z "$NAME" || -z "$EMAIL" ]]; then ...

Inside [[ ]], use && and ||. Outside it, those operators chain whole commands.