Comparisons in a shell script may either be accomplished using regular operators (such as <, > or =) or using the equivalent (-lt, -gt, -eq) for POSIX shells.
if statements take the form
if [[ condition ]] then do this elif [[ condition ]] then do this instead else do this by default fi
Note the spacing on the conditionals. There must be a space between the brackets and their contents, e.g.
if [[ $a < $b ]].