Add shell script style

[qinkunbao]

We should have a style guide for bash scripts.

As discussed in #609, environment variables and internal shell variables are capitalized. All other variable names should be lower case. Some shell scripts in the project don't follow this convection.

Here is an example:
https://google.github.io/styleguide/shellguide.html

In addition, we can add a shell scripts format checker to the build pipeline.

[mssun]

The shellcheck will be helpful for the issue: https://github.com/koalaman/shellcheck

[mssun]

Another useful tool shfmt: https://github.com/mvdan/sh