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

[Peng Yu]

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)?

>
>
> --
> Koichi
>
-- 
Regards,
Peng