clean-up: move all shell script code to a function and use a "main" #740

This a very generic, clean-up task. Its main purpose is to document this good practice: almost all the code in a shell script should be in a function. In other words, the smallest script should look like this:

main()
{
  do stuff
}

# The braces and bottom position let you safely edit the script while it's running
# https://hachyderm.io/@simontatham/114511220670677410
# (XKCD 303)
{
 main "$@"; exit "$?"
}

Rationales:

• More variables can be local. This reduces the risk of accidentally overwriting some user environment variables (side effects)
• Functions can be defined in any order, they don't need to be defined before being used
• It provides more line numbers in the FUNCNAME stack on failure
• moving code around functions does not change the indentation level which helps with git diff, rebase, cherry-pick, merge...
• it's very easy to comment out the last, main "$@" line and source the entire file without running it to test individual functions interactively (almost like the usual Python's __main__ trick)
• Similarly, it's very easy to copy/paste functions into an interactive shell and test them.
• It's easy to temporarily turn off an entire function call for local testing purposes by commenting out a single line.
• For the same reason, better testability, example: https://opensource.com/article/19/2/testing-bash-bats if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then run_main; fi
• https://en.wikipedia.org/wiki/Margin_(typography)#The_Digital_Page "Although margin-less web pages do still exist, today it is generally understood that having wide enough margins to provide adequate white space around text is important to the usability and readability of digital text" (unverified)

And last but not least, stating the obvious:

• A sequence of calls to well named functions is much more readable than a giant and anonymous concatenation of all these functions.

cc:

• https://hachyderm.io/@simontatham/114511220670677410
• https://google.github.io/styleguide/shellguide.html#main
• https://google.github.io/styleguide/shellguide.html#function-location
• https://steve-yegge.blogspot.com/2006/03/execution-in-kingdom-of-nouns.html
• [sof-test] add set -e to all test cases (Unless You Love Debugging) #312