Re: [Help-bash] Command Substition (using tail) in exec line stopped working

[Erik Ackermann]

Firstly, I should have titled "Process substitution" not command
substitution.

So I used strace to attach to the substituted process, and it just seems
like both `head` and `tail` are killed before they write out, but cat is
much faster.

Here is a trace of "head -n1" which is killed faster than it can write, if
I add a sleep or block to script.sh it completes fine:

sudo strace -r -p 60210
Process 60210 attached
     0.000000 read(0, "SUCCESS\n", 8192) = 8
     1.641432 lseek(0, 0, SEEK_CUR)     = -1 ESPIPE (Illegal seek)
     0.000126 fstat(0, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
     0.000136 --- SIGHUP {si_signo=SIGHUP, si_code=SI_KERNEL} ---
     0.000170 +++ killed by SIGHUP +++

Why is the substituted process killed with SIGHUP.. why don't we just close
the FD and let the process continue to run? Was this behavior changed?  I'm
pretty sure this used to be the behavior at some point, will try to dig up
an old bash.

On Fri, Oct 14, 2016 at 11:35 AM, Erik Ackermann <
address@hidden> wrote:

> So I just posted a more detailed, formatted version on stackexchange, but
> wanted to reach out here as well, particularly since this might be a
> regression, or at least a new behavior I was not aware of.
>
> http://unix.stackexchange.com/questions/316479/bash-command-
> substitution-in-exec-line-some-commands-stopped-working
>
> Basically though, this command works (script.sh writes to /tmp/log):
>
>   exec ./script.sh >(cat > log)
>
> While this one does not work (file created but no contents):
>
>   exec ./script.sh >(tail > log)
>
> Where script.sh contains:
>
>   #!/bin/bash
>   echo "fd: $1" >/tmp/out
>   ls -l "$1" >>/tmp/out
>   echo "SUCCESS" > "$1"
>
> In both cases /tmp/out shows a writable fd.
>
> It's a boiled down example of something that's been in my .bashrc for a
> long time, but just noticed it stopped working on my machine recently. Is
> there something special about the tail/head commands that makes this not
> work..?
>
> Bash version: GNU bash, version 4.3.11(1)-release (x86_64-pc-linux-gnu)
>