If Else

Bash IF Statements

• Many beginners have an incorrect intuition about if statements brought about by seeing the very common pattern of an if keyword followed immediately by a [ or [[. This convinces people that the [ is somehow part of the if statement's syntax, just like parentheses used in C's if statement.
• This is not the case! if takes a command. [ is a command, not a syntax marker for the if statement. It's equivalent to the test command, except that the final argument must be a ]. For example:

# POSIX
if [ false ]; then echo "HELP"; fi
if test false; then echo "HELP"; fi

• Are equivalent -- both checking that the argument "false" is non-empty. In both cases HELP will always be printed, to the surprise of programmers from other languages guessing about shell syntax.
• The syntax of an if statement is:

if COMMANDS
then <COMMANDS>
elif <COMMANDS> # optional
then <COMMANDS>
else <COMMANDS> # optional
fi # required