Learning Shell scripting by reading the manual is impressive! I would not
recommend that approach. One can start with
https://flossmanuals.net/command-line/ (but skip the "STANDARD FILES" section
that should definitely be toward the end of "ADVANCED-ISH" or maybe even in
"ADVANCED": certainly a "bug" in the manual). Then, to learn (almost)
everything about Bash through examples: http://tldp.org/LDP/abs/html/
Manuals enter the same category as software. They are functional: works are
(indirectly) achieved through them.
https://www.gnu.org/philosophy/free-doc.html
In my opinion, GNU manuals are free as in freedom. They are under the GNU
Free Documentation License... that allows invariant sections, which have
non-functional content such as political statements (e.g. the "GNU
Manifesto": the original reason for the invariant section clause). Those
should not be modifiable. Debian disagrees:
https://www.debian.org/vote/2006/vote_001.en.html#amendmenttexta
