string
string is not null
-n string
Length of string is greater than 0
-z string
Length of string is 0
string1 = string2
string1 == string2
string1 and string2 are equal. == is used for POSIX compliance
string1 != string2
string1 and string2 are not equal
string1 > string2
string1 sorts after string2
string1 < string2
string1 sorts before string2