Re: Not so useless use of cat

[Dennis Williamson]

On Sep 12, 2014 7:12 PM, "Bob Proulx" <bob@proulx.com> wrote:
>
> Ralf Goertz <me@myprovider.invalid> wrote:
>
> Since you have used an invalid address I assume you are reading the
> mailing list via a web archive or other means and did not CC you.
>
> > Why do I need cat (the second on) here?
>
> You don't.
>
> > $ echo first >file1
> > $ echo second >file2
> > $ (for i in file[12] ; do cat "$i" > /dev/stdout ; done) | cat > both
>
> Why do you want >/dev/stdout there?
>
> > $ cat both
> > first
> > second
>
> Okay.
>
> > If I omit the "| cat" after the loop I get
>
>   (for i in file[12] ; do cat "$i" > /dev/stdout ; done) > both
>
> > $ cat both
> > second
>
> Because the >/dev/stdout truncates the output.  It writes the first.
> Then the second one truncates the file and then writes the second.
> Remove that from the line and it will print both lines.
>
>   (for i in file[12] ; do cat "$i" ; done) > both
>
> > Even when using ">> both" instead of "> both" only "second" makes it
> > into both. Why?
>
> All that the >, >>, 2>&1, types of redirections do is to attach the
> associated files with different files.  Once associated the further
> actions such as truncation happen to those files.  Therefore the
> truncation associated with >/dev/null truncates the file that on the
> far right was >both.
>
> Try this:
>
>   (for i in file[12] ; do cat "$i" >> /dev/stdout ; done) > both
>
> But really I think you are better off forgetting about >/dev/stdout.
> That is almost like the useless use of cat.
>
> And the subshell isn't needed either.  Use a list.
>
>   { for i in file[12] ; do cat "$i" ; done ;} > both
>
> > I would have thought that using a subshell should be
> > enough to protect the both from being overwritten.
>
> The subshell isn't really part of the issue.  The issue is that the
> file descriptor is attached and truncated and that is orthogonal to
> process boundaries.
>
> Hope that helps!
>
> In the future please consider asking questions such as these on
> help-bash@gnu.org for general discussion of bash shell scripting.  If
> you expose a bug then report the bug to the bug reporting address.
> The help-bash list is for help with bash scripting and this would have
> been perfect there.
>
> Bob
>

There's no need for the curly braces and the last semicolon.

Note that the loop in this case can be replaced by

cat file[12] > both

I failed to spot that in my earlier reply.