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

[Kerin Millar]

On Sun, 19 Feb 2023 16:25:13 +0000
goncholden <goncholden@protonmail.com> wrote:

> 
> ------- Original Message -------
> On Sunday, February 19th, 2023 at 3:17 AM, Greg Wooledge <greg@wooledge.org> 
> wrote:
> 
> 
> > On Sat, Feb 18, 2023 at 10:46:08AM +0000, goncholden wrote:
> > 
> > > My intention is to use prinf line by line on arguments containing 
> > > newlines. With a newline also introduced between arguments $1 $2 $3 etc.
> > 
> > 
> > This is quite unique. I don't believe I've ever seen someone try to
> > write a command where each argument is a group of lines, and all of
> > the groups of lines are supposed to be concatenated together to form
> > one bigger group of lines.
> > 
> > For this goal, printf '%s\n' "$@" seems to be the correct choice.
> > 
> > The <<< "$@" construct is nonsensical. Whatever it does (which is
> > pretty hard to predict, since it doesn't have a real definition), it
> > will not serve your goal.
> > 
> > If you want to avoid a pipeline which would cause your processing loop
> > to run in a subshell, then your syntax of choice would be:
> > 
> > while read ...
> > do
> > ...
> > done < <(printf '%s\n' "$@")
> 
> I have found that nested loops are most clear Consequently, I have either 
> this one
> 
> Code:
> 
> # Loop over arguments
> for arg in "$@"; do
>   # Loop over lines
>   printf '%s\n' "$arg" |
>     while IFS= read -r vl ; do
>       ...
>     done
> done
> 
> or this
> 
> Code:
> 
> # Loop over arguments
> for arg in "$@"; do
>   # Loop over lines
>   while IFS= read -r vl ; do
>     ...
>   done < <(printf '%s\n' "$arg")
> done

The use of a process substitution - as in your second example - is typical in 
bash because, by having the while command be run in the initial shell, it's 
immune to the issue of 'disappearing' variables. See 
https://mywiki.wooledge.org/BashFAQ/024 for further details. Whether that truly 
matters in your case depends on what "..." is exactly.

-- 
Kerin Millar