The most secure passwords used, are those that are randomly generated. From the command line, you can randomize potential passwords in multiple ways.
First method uses openssl, in the terminal type:
openssl rand -base64 6
The output of this command will be random, and as an example look like: cG/ah3+9
Adjustments to the length of the password can be done by changing the number on the end of the string. If abnormal characters are not wanted like ^ and *, generation can be done via hex too:
openssl rand -hex 4
For more random options, pipe the randomized output of openssl through md5 and trim the md5 hash of the randomized output down to a set number of characters:
openssl rand -base64 8 |md5 |head -c8;echo
Other options are to take random input from other commands, such as date, and trim 8 characters from the current dates md5 hash:
date |md5 | head -c8; echo
Or even ping:
ping -c 1 google.com |md5 | head -c8; echo
Using the md5 method, the output of any command, or file, can be used to create a secure password.