Re: printf '%s\n' "$@" versus <<< redirection

[Greg Wooledge]

On Sun, Feb 19, 2023 at 10:00:17AM +0000, goncholden wrote:
> theone()
> (
>     for arg in "$@"
>     do
>         while IFS='\n' read -r vl
>         do
>             printf '## vl: %s ##\n' "$vl"
>         done <<< "$arg"
>     done
> )

The IFS value you're using there contains a backslash and a lowercase n,
not a newline character.  Since you're passing a single variable name
to read, and since your IFS doesn't contain any whitespace characters,
that IFS does exactly nothing at all.

unicorn:~$ theone "no bananas"
## vl: no bananas ##

It doesn't even trim leading lowercase n's, because they're not
whitespace.

On the other hand, it also doesn't trim leading newlines (even though
those *are* whitespace), because you haven't included newline in your
IFS, but also because your read command stops reading at each newline,
and therefore newlines will never appear in the lines you're reading.

> var1="
> First Argument
> Another Line"
> 
> var2="
> Second Argument
> Another Line"
> 
> theone "$var1" "$var2"

If your intent was to strip the leading newlines from each of those
variables, you can see that it doesn't work:

unicorn:~$ theone $'\nFirst Argument\nAnother Line'
## vl:  ##
## vl: First Argument ##
## vl: Another Line ##

Even if you fixed your IFS (by writing IFS=$'\n' instead), it still
wouldn't work.  The first read command in the loop reads an empty
string, and applying IFS word splitting to an empty string will never
do anything.