-- ECHO command
var = 'Hello World!'
echo 'var =' $ var
-- LOGICAL
---- && command :
---- ---- command1 && command2
---- ---- ---- if command1 is true, then execute command2; if command1 is false, then not execute command2.
---- || command :
---- ---- command1 || command2
---- ---- ---- if command1 is true, then not execute command2; if command1 is false, then execute command2.
---- && condition:
---- ---- [[ bool-expression1 && bool-expression2 ]]
---- || condition:
---- ---- [[ bool-expression1 || bool-expression2 ]]
---- -a condition:
---- ---- [ bool-expression1 -a bool-expression2 ]
---- -a condition:
---- ---- [ bool-expression1 -o bool-expression2 ]