The &&, ||, and ! operators allow you to run a lot of useful commands on Linux, but you first need to get used to syntax that is a little trickier than the if-then-else command you might normally use.
To get started, I should explain that one thing the command examples in this post have in common is the use of something that I might call a shorthand “if” test. Here’s an example:
$ [ -f donuts ]
$ echo %?
The -f donuts command asks if there is a file (-f) named “donuts”. Unless we display the return code afterwards with “echo $?”, the result of the test is not displayed. In this case, it’s false (i.e., not zero), so we know the file doesn’t exist. No donuts for us!
Source:: Network World – Linux