Re: Ordering bug when handling redirection with file descriptors

[Chet Ramey]

On 1/31/21 3:40 PM, Érico Nogueira wrote:
> Hi! I have the below test case which works with dash, zsh, and has been 
> reported to work at least up to bash 4.4.12. After 4.4.23 it definitely no 
> longer works (I have also tested with 5.1.4).
>
> #!/bin/sh
>
> fn() {
>      echo a >&3
> }
>
> b=`fn 3>&1 1>&4 4>&-` 4>&1

I can't find a bash version where this has ever `worked'.  The `1>&4' means
to dup fd 4 to fd 1, and at the time this is called, fd 4 is invalid. Bash
always does word expansions and variable assignments before redirections
when executing simple commands, so when the command substitution is called,
the `outer' redirection has not been performed.

> While exploring similar issues, I found that
>
> fn() {
>      echo a >&3
> }
>
> echo `fn 3>&1 1>&4 4>&-` 4>&1

Bash is clearly correct here; the command substitution is performed before
the `outer' redirection, and fd 4 is invalid when the shell attempts
`1>&4'.


--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/