Shell – Difference between ls -l and ll

For one, --help is not a command, it is an argument that is often given to a command to get help using it. Meanwhile, man is a command, short for "manual". Manual pages are installed by many programs, and are a common way to find help about commands, as well as system calls, (e.g. fork()).

If a program installs a manual page, it can always be accessed via the man command, whereas --help is just a common convention, but need not be enforced—it could be just (and only) -h.

man also typically uses a pager, such as less, automatically, which can make viewing and searching through the information much easier.

Finally, you mention Bash programming in your question—none of this is unique to Bash. Bash doesn't care about the commands themselves or their arguments for the most part.

Substitution is almost synonymous with expansion in this context because their meanings overlap. Neither is quite a complete sub category of the other, although in the GNU Manual section you reference there are substitutions that are considered as part of an overall expansion.

An expansion is extracting the value of an identifier. E.g., if this=that, when we expand this we get that. An expansion that doesn't involve substitution is pre-determined in that the value used already exists and must simply be retrieved, although this includes combining retrieved/explicit values (such as with "arithmetic expansion").

A substitution creates a value as the result of an explicit input/output operation. E.g., if this=$(foo bar), this is the result of executing foo bar and capturing its output.¹ Although the value resulting from a substitution maybe completely predictable, it is different from the one retrieved in a normal expansion because it does not actually exist until the substitution takes place -- it is produced.

Substitutions come in two flavours, command and process, which are sort of symmetrical:

# Command substitution
foo=$(ls)
# Process substitution
wc <(ls)

The "command" in first one is ls, as is the "process" in the second one. We might say what is being substituted is really the end of a pipe. Process substitution overlaps with redirection. However, this is probably a bit too under-the-hood restrictive technically, which brings us to the footnote...

1. foo bar in this case could be an internal shell function, in which case it there's no interprocess IO. The existence of shell built-ins less obviously obscures this difference. In terms of content the input and output will be the same.