for filename in *.txt ; do ...

has no issue with spaces in filenames. If *.txt matches "foo bar.txt", then that's what the filename variable is set to. In the body of the loop you have to make sure you have "$filename".

You don't need play games with IFS to correctly process filesystem entry names expanded from a pattern.

Wildcards are not variables. Wildcards don't get expanded in quotes. Variables get expanded in double quotes but not single quotes. $@ obeys all the same expansion rules as all other variables. Command substitution with both $() and `` follow the same rules as variables.

No, $@ is special where $* is regular. Consider the following script foo.sh and call it like so:

    ./foo.sh one "a b" two

The $* part will print one line, the $@ part will print three lines.

    #!/bin/bash
    for x in "$*"; do
        echo $x
    done
    for x in "$@"; do
        echo $x
    done

I use "$@" often, but to this day I don't fully understand how $@ works...

Unquoted $@ works exactly like unquoted $* .

Quoted "$* " separates the parameters using the first character stored in IFS, or with nothing if IFS is unset/null. Usually, the first character of IFS is space, giving the impression that "$* " means "separate with spaces"; i.e. that it's just an ordinary quote job around $* (i.e. that it is "regular", in your words).

Quoted "$@" does ... what you clearly understand.

> $@ obeys all the same rules as all other variables.

That's hardy the case. Most other variables do not represent the positional parameters, and don't have the logic of "$@" which effectively produces "$1" "$2" "$3" ...

I was refering to the expansion behavior, which was the point in contention. I've clarified the original comment.