Re: is there a way to save stdout and stderr to two bash variables?

[Peng Yu]

On Tue, May 12, 2020 at 5:19 AM Andreas Kusalananda Kähäri <
address@hidden> wrote:

> On Tue, May 12, 2020 at 04:44:09AM -0500, Peng Yu wrote:
> > On Mon, May 11, 2020 at 7:45 PM Koichi Murase <address@hidden>
> > wrote:
> >
> > > 2020-05-12 6:13 Peng Yu <address@hidden>:
> > > >
> > > > But this solution still involves using external files if I read the
> > > > code correctly? Thanks.
> > >
> > > Yes, as already explained in the original post of `ble/util/assign'.
> > > If you don't want to use external files, you can instead use nested
> > > command substitutions and variable exports by `declare -p':
> > >
> > >   function cmd {
> > >     echo this is stderr >&2
> > >     echo this is stdout
> > >   }
> > >
> > >   eval -- "$(
> > >     { stderr=$(
> > >         { stdout=$(cmd); } 2>&1
> > >         declare -p stdout >&3); } 3>&1
> > >     declare -p stderr )"
> > >
> > >   echo "($stdout)($stderr)"
> > >
> > > If you feel it is cumbersome to write it every time, again you can
> > > wrap it in a function:
> > >
> > >   function upvars {
> > >     while (($#)); do
> > >       unset "$1"
> > >       printf -v "$1" %s "$2"
> > >       shift 2
> > >     done
> > >   }
> > >   function save-stdout-stdin {
> > >     eval -- "$(
> > >       { printf -v "$2" %s "$(
> > >         { printf -v "$1" %s "$(eval -- "$3")"; } 2>&1
> > >         declare -p "$1" >&3)"; } 3>&1
> > >       declare -p "$2" )"
> > >     upvars "$1" "${!1}" "$2" "${!2}"
> > >   }
> > >
> > >   save-stdout-stdin a b cmd
> > >   echo "($a)($b)"
> > >
> > > However, these solutions require at least three forks which are slower
> > > than the external file accesses.  There is no solution with neither
> > > forks nor external file accesses.  In my opinion, there is no reason
> > > to refrain from external files as far as the files are created in
> > > memory (tmpfs such as /dev/shm or /tmp) and the permissions are
> > > properly mainined.
> > >
> > > Also, if you do not allow even internal usages of external files at
> > > all, you cannot use here documents and here strings as they also use
> > > temporary files internally.  For example, you can confirm this by the
> > > following command.
> > >
> > > $ ls -la /dev/fd/0 <<< Here
> > > lr-x------. 1 murase murase 64 2020-05-12 09:42:22 /dev/fd/0 ->
> > > /tmp/sh-thd.ZLmXgN (deleted)
> >
> >
> > I didn’t know that this was the case. I always thought heredoc was in
> > memory.
> >
> > Chet, There was not an in memory implementation of here doc possible in
> > bash? How much performance difference it could be comparing in memory and
> > temp file (suppose temp file is not in a RAM disk)?
>
> Theoretically, I don't think you can assume that a here-document can be
> stored in RAM as its size is only limited by the storage medium that
> hold the script that it is part of.


I am not sure this statement is relevant to most real usage. I mostly use
heredoc for short strings, it would be costly to use external files instead
of in memory strings.

>
>
> --
> Andreas (Kusalananda) Kähäri
> SciLifeLab, NBIS, ICM
> Uppsala University, Sweden
>
> .
>
-- 
Regards,
Peng