Several ways of shell logic and judgment

Test can be understood to be the same as [], [[]] in logic and logic or at times, it can be split into multiple []

[Cmd-a cmd]

#!/bin/bash
a=23;
If [$a-gt 18-a $a-lt]
then
        echo ' Young && handsome '
else
        echo ' Just so ... "
fi

Test cmd-a cmd

#!/bin/bash
a=23;
if test $a-gt  -a  $a-lt
then echo ' Young
        && handsome '
else
        echo ' just so. .."
Fi

[[cmd && cmd]]

#!/bin/bash
a=23;
if [[$a-gt && $a-lt]]
then
        echo ' Young && handsome '
else
        echo ' just so. .."
Fi

[cmd] && [cmd]

#!/bin/bash
a=23;
If [$a-gt] && [$a-lt]
then
        echo ' Young && handsome '
else
        echo ' just so ... "
fi