String expression

Question 1

[[ -n STRING ]]

Answer 1

Not empty string

Question 2

[[ -z STRING ]]

Answer 2

Empty string

Question 3

[[ STRING == STRING ]]

string1 == string2

Answer 3

STRINGs are equal.

== is used for POSIX compliance

Question 4

[[ STRING != STRING ]]

Answer 4

STRINGs are not equal

Question 5

[[ STRING =~ RegExp ]]

Answer 5

Regular Expression checking against the STRING