-a file
True if file exists.
-b file
True if file exists and is a block special file.
-d file
True if file exists and is a directory.
-e file
True if file exists.
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than zero.
string1 == string2
True if the strings are equal.
string1 != string2
True if the strings are not equal.
string1 < string2
True if string1 sorts before string2 lexicographically.