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

[Andreas Kusalananda Kähäri]

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.

-- 
Andreas (Kusalananda) Kähäri
SciLifeLab, NBIS, ICM
Uppsala University, Sweden

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