=
          is equal to

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

   ==
          is equal to

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

  !=
          is not equal to

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

   <
          is less than, in ASCII alphabetical order

          if [[ "$a" < "$b" ]]; if [ "$a" \< "$b" ]

   >
          is greater than, in ASCII alphabetical order

          if [[ "$a" > "$b" ]]; if [ "$a" \> "$b" ]

   -z
          string is null, that is, has zero length

          if [ -z "$a" ]