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Re: Ordering bug when handling redirection with file descriptors

From: Chet Ramey
Subject: Re: Ordering bug when handling redirection with file descriptors
Date: Mon, 1 Feb 2021 09:49:05 -0500
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.6.1

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).


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

``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/

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