RE: set -a leads to truncated output from ps

[Alain BROSSARD]

Thanks Chet,

    Your explanation is crystal clear and shows how bash follows its 
documentation and vice versa.
However, how many users of bash in the world could have this understanding of 
how set -a, checkwinsize and COLUMNS interact with each causing "PS" to change 
its behavior in mid-flight (so to speak).
    In my case, this behavior breaks at least half-dozen scripts which are 
provided by a third-party just because I called grep in the site configuration 
script. I do retract the fact that this is a bug, but this is still not really 
an acceptable behavior from my point of view.
    COLUMN is clearly meant to be an internal variable of bash given the 
description of how it is used; checkwinsize is automatically set and probably 
unknown to 99% of bash script writers as is the fact that an external command, 
grep in my case, will impact the behavior of ps if called within -a.  Add to 
this that it is extremely difficult to check whether -a is set or not without 
using an external command.

    Honestly, I don't know where to go with this issue. Bash's behavior should 
be consistent and easily understood in order to have reliable scripts. Clearly 
this case doesn't respect this.
    I would suggest to put LINES and COLUMN out of scope of the behavior of 
"set -a", or at the very least make all these interactions explicit within the 
man page.  If it had been documented, I would have saved myself many hours of 
work as I did read the documentation once I had pinned down that "set -a" is 
the command which caused those script to fail. Though the real culprit, in the 
end, might be checkwinsize default behavior if one wanted to blame a single 
member of this trio.

Regards,
Alain Brossard





Alain BROSSARD 
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-----Original Message-----
From: Chet Ramey <chet.ramey@case.edu> 
Sent: Thursday, June 13, 2024 8:44 PM
To: Alain BROSSARD <abrossard@reyl.com>; Andreas Schwab 
<schwab@linux-m68k.org>; Alain BROSSARD via Bug reports for the GNU Bourne 
Again SHell <bug-bash@gnu.org>
Cc: chet.ramey@case.edu
Subject: Re: set -a leads to truncated output from ps

On 6/13/24 1:45 PM, Alain BROSSARD via Bug reports for the GNU Bourne Again 
SHell wrote:
> Hi Andreas,
> 
>       Sure, the variables explicitly assigned to get exported, that is the 
> expected behavior.
> But there is nothing which explains why the variable COLUMNS becomes an 
> environment variable.

OK. The description of COLUMNS says:

COLUMNS
       Used by the select compound command to  determine  the  terminal
       width  when  printing selection lists.  Automatically set if the
       checkwinsize option is enabled or in an interactive  shell  upon
       receipt of a SIGWINCH.


> 
>      Run “ps ax” twice and the results of the second execution is different 
> from the first if and only if “set -a” was called fiirst. This is NOT an 
> expected behavior.

You have checkwinsize set. The description of checkwinsize reads:

checkwinsize
        If  set, bash checks the window size after each external
        (non-builtin) command and,  if  necessary,  updates  the
        values  of LINES and COLUMNS.  This option is enabled by
        default.

So the first invocation of ps, since it's a non-builtin command, makes bash 
query the terminal and adjust LINES and COLUMNS.

> Do the following and you won’t get a problem:
> 
> set -a
> var1=xx
> var2=xx
> set +a

All builtin commands.

>      All is fine, but do the following and COLUMNS gets exported:
> 
> set -a
> echo $PATH | grep xxx/scripts || PATH=${PATH}:/newdir
>     ps ax   is now truncated, definitely NOT expected

The value of COLUMNS gets modified, and auto-exported since you have `set -a' 
enabled.

--
``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/