if CONDITION; COMMANDS_TRUE...; [else; COMMANDS_FALSE...;] end
if will execute the command CONDITION. If the condition's exit status is 0, the
commands COMMANDS_TRUE will execute. If it is not 0 and else is given,
COMMANDS_FALSE will be executed.
The following code tests whether a file foo.txt exists as a regular file.
if test -f foo.txt
echo foo.txt exists
echo foo.txt does not exist