From: Harald van Dijk <firstname.lastname@example.org> To: Jilles Tjoelker <email@example.com> Cc: Steffen Nurpmeso <firstname.lastname@example.org>, DASH shell mailing list <email@example.com>, Denys Vlasenko <firstname.lastname@example.org> Subject: Re: dash 0.5.11.2, busybox sh 1.32.0, FreeBSD 12.2 sh: spring TTOU but should not i think Date: Wed, 23 Dec 2020 20:18:24 +0000 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <20201221162442.GA26001@stack.nl> On 21/12/2020 16:24, Jilles Tjoelker wrote: > Also, simply entering the command > trap "echo TTOU" TTOU > in an interactive shell makes it behave strangely. Created jobs > immediately stop, "fg" works once but after that the shell tends to get > stuck quickly. Good catch, there is some work to be done there too. > This seems a good approach, but certain writes to the terminal may need > the same treatment, for example the error message when fork fails for > the second and further elements of a pipeline. This also makes me wonder > why SIGTTOU is ignored at all by default. True. This is hard to create a reliable test case for, but there is only limited code that can get executed while a job-control-enabled shell may not be in the foreground process group. If fork fails halfway through a pipeline, it may also be a problem that some of the commands in the pipeline may still run. > In mksh, the issue is resolved slightly differently: setting a trap on > TTOU pretends to work but the signal disposition remains set to ignored. zsh also rejects traps on TTOU, but does so explicitly: zsh: can't trap SIGTTOU in interactive shells Amusingly, it prints this in any shell where job control is enabled, regardless of whether the shell is interactive. > Tradition is for job control shells to be a process group leader, but I > don't really know why. Changing this will not fix the issue entirely > anyway since the shell must perform tcsetpgrp() from the background when > a foreground job has terminated. I've been thinking more about this, and I suspect it's a another conflation between interactive mode and job control. In an interactive shell that's not executing any external program, you want any Ctrl-C to only send SIGINT to that shell, not to any other process. In order for that to work, it needs to be its own process group. This should, in my opinion, *only* happen for interactive shells, not for job-control-enabled non-interactive shells. Consider sh -c 'sh -mc "while :; do :; done"; echo bye' The behaviour I would want is that Ctrl-C kills the parent shell, so that this does not print "bye". Different shells behave differently. > What is definitely required, though, is that the shell not read input or > alter terminal settings if it is started in the background (possibly > unless the script explicitly ignored SIGTTOU). This is what the loop > with tcgetpgrp() does. Definitely. Cheers, Harald van Dijk
next prev parent reply other threads:[~2020-12-23 20:19 UTC|newest] Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-12-19 17:28 Steffen Nurpmeso 2020-12-19 22:21 ` Steffen Nurpmeso 2020-12-19 23:52 ` Harald van Dijk 2020-12-21 16:24 ` Jilles Tjoelker 2020-12-21 19:43 ` Steffen Nurpmeso 2020-12-23 20:18 ` Harald van Dijk [this message] 2020-12-24 15:29 ` Jilles Tjoelker 2021-01-10 23:56 ` Harald van Dijk 2021-01-06 4:46 ` Herbert Xu 2021-01-06 4:45 ` [PATCH] jobs: Block signals during tcsetpgrp Herbert Xu 2021-01-06 21:16 ` Harald van Dijk 2021-01-06 22:41 ` Jilles Tjoelker 2021-01-07 7:36 ` Denys Vlasenko
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