-eq
          is equal to

          if [ "$a" -eq "$b" ]

   -ne
          is not equal to

          if [ "$a" -ne "$b" ]

   -gt
          is greater than

          if [ "$a" -gt "$b" ]

   -ge
          is greater than or equal to

          if [ "$a" -ge "$b" ]

   -lt
          is less than

          if [ "$a" -lt "$b" ]

   -le
          is less than or equal to

          if [ "$a" -le "$b" ]

   <
          is less than (within double parentheses)

          (("$a" < "$b"))

   <=
          is less than or equal to (within double parentheses)

          (("$a" <= "$b"))

   >
          is greater than (within double parentheses)

          (("$a" > "$b"))

   >=
          is greater than or equal to (within double parentheses)

          (("$a" >= "$b"))

   string comparison

   =
          is equal to

          if [ "$a" = "$b" ]

          Caution

   Note the whitespace framing the =.
   if [ "$a"="$b" ] is not equivalent to the above.

   ==
          is equal to

          if [ "$a" == "$b" ]

          This is a synonym for =.