Re: "builtin jobs" does not output to stdout.

[Koichi Murase]

2023年2月11日(土) 23:57 岡部将幸 <okabe_m@hmi.aitech.ac.jp>:
> Description:
> builtin command "jobs" behaves differntly depending on
> the presence or absence of "builtin".
> "jobs" outputs to stdout, but "builtin jobs" does not
> output to stdout.

This seems to be reproduced in all the Bash versions from 1.14.7 to
devel. The problem is related to the condition of whether subshell
builtins inherit the job information. In particular, this is related
to the following test in the source code [1]:

[1] https://git.savannah.gnu.org/cgit/bash.git/tree/execute_cmd.c?h=devel#n5236

$ sleep 1000 &
$ : | jobs
[1]+  Running                 sleep 1000 &
$ : | command jobs
[1]+  Running                 sleep 1000 &
$ echo "$(jobs)"
[1]+  Running                 sleep 1000 &

In these cases, the `jobs' builtin inherits the job information of the
main shell.

$ : | { jobs; }
$ (jobs)

In these cases, the `jobs' builtin shouldn't print the job information
of the main shell. So, the `jobs' builtin in subshells actually
inherits the job information only in a limited case, which is
implemented in [1]. Now we call the `jobs' builtin through the
`builtin' builtin in the reported code, so it bypasses the condition
tested in [1]. We can test many other combinations of such indirect
builtin calls, where it doesn't inherit the job information if at
least one `builtin' is contained in the chain:

$ : | command jobs
[1]+  Running                 sleep 1000 &
$ : | command command jobs
[1]+  Running                 sleep 1000 &
$ : | command command command jobs
[1]+  Running                 sleep 1000 &
$ : | builtin jobs
$ : | command builtin jobs
$ : | builtin builtin jobs
$ : | builtin command jobs
$ : | builtin builtin builtin jobs

--
Koichi