Re: Odd Behavior: $SHLVL decreases in some situations with a pipe

[Chet Ramey]

On 6/27/22 8:02 AM, H. Thiele wrote:

> Bash Version: 5.1
> Patch Level: 16
> Release Status: release
>
> Description:
> In some situations (see Repeat-By) the value of the $SHLVL (Shell Level) 
> variable may
> unexpectedly decrease by 1, e.g. when the stdout output of "printenv" is 
> redirected to
> via a pipe.

As you surmise, the behavior is related to bash optimizing away forks and
using an implicit `exec' in the pipeline. I changed this particular case
back last September as the result of

https://lists.gnu.org/archive/html/help-bash/2020-10/msg00012.html

which contains a pretty good discussion of the issue.


> Reddit user Electronic_Youth points out some other, perhaps related issue: if 
> one sets
> SHLVL to a value higher than 999 a warning is generated when a new shell is 
> started, but
> more interesting a warning is generated two times when printenv is used 
> (perhaps suggesting
> some code runs twice?) I reproduced that double warning like this:
>
>     $ SHLVL=1001
>     $ printenv SHLVL | cat
>     bash: warning: shell level (1000) too high, resetting to 1
>     bash: warning: shell level (1000) too high, resetting to 1
>     1

It's slightly related. You get two messages because bash-5.1 tries to
optimize away the exec in both subshells, and notices that SHLVL is too
high when trying to adjust it.

--
``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/