if list; then list; [ elif list; then list; ] ... [ else list; ] fi
              The if list is executed.  If its exit status is zero, the then list is executed.  Otherwise, each elif list  is  executed
              in  turn,  and if its exit status is zero, the corresponding then list is executed and the command completes.  Otherwise,
              the else list is executed, if present.  The exit status is the exit status of the last command executed, or  zero  if  no
              condition tested true.

       [[ expression ]]
              Return a status of 0 or 1 depending on the evaluation of the conditional expression expression.  Expressions are composed
              of the primaries described below under CONDITIONAL EXPRESSIONS.  Word splitting and pathname expansion are not  performed
              on the words between the [[ and ]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command sub‐
              stitution, process substitution, and quote removal are performed.  Conditional operators such as -f must be  unquoted  to
              be recognized as primaries.

       test expr
       [ expr ]
              Return a status of 0 (true) or 1 (false) depending on the evaluation of the conditional expression expr.   Each  operator
              and  operand  must  be  a separate argument.  Expressions are composed of the primaries described above under CONDITIONAL
              EXPRESSIONS.  test does not accept any options, nor does it accept and ignore an argument of -- as signifying the end  of
              options.