Difference between parentheses and brackets in Bash conditionals

Question

While learning a bit about bash, I come to see four types of ways of working with if statements:

• Single Parenthesis - ( ... )
• Double Parenthesis - (( ... ))
• Single Square Bracket - [ ... ]
• Double Square Brackets - [[ ... ]]

What is the difference between Parenthesis and Square Brackets in bash.

Answer 1

The tests you had listed :

• Single Parenthesis - ( ... ) is creating a subshell
• Double Parenthesis - (( ... )) is for arithmetic operation
• Single Square Bracket - [ ... ] is the syntax for the POSIX test
• Double Square Brackets - [[ ... ]] is the syntax for bash conditional expressions (similar to test but more powerful)

are not exhaustive, you can use boolean logic

if command; then ... 

too, because the commands have exit status. In bash, 0 is true and > 0 is false.

You can see the exit status like this :

command echo $? 

See :

http://wiki.bash-hackers.org/syntax/basicgrammar
http://wiki.bash-hackers.org/syntax/arith_expr
http://mywiki.wooledge.org/BashGuide/TestsAndConditionals