Double pipe Double logical and Control operators

 

( expression )

Returns the value of expression. This may be used to override the normal precedence of operators.

! expression

True if expression is false.

expression1 && expression2

True if both expression1 and expression2 are true.

expression1 || expression2

True if either expression1 or expression2 is true.

The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression.

For example:

Code:
$ ls this_file_does_not_exist.txt || echo KO
ls: cannot access this_file_does_not_exist.txt: No such file or directory
KO

$ ls this_file_exist.txt && echo OK
this_file_exist.txt
OK

Ref:

https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Command-Grouping

https://pongsona.wordpress.com/2011/12/03/what-does-double-pipe-mean/