Re: subprocess

[Pier Paolo Grassi <pierpaolog@xxxxxxxxx>]

thanks for the debugging technique and the explanation, very interesting.
What would be the best way to learn more about these mechanisms without
digging in the source code of the shell or the kernel? I am not very
comfortable around c
regards

Il giorno sab 6 giu 2020 alle ore 20:19 Bart Schaefer <
schaefer@xxxxxxxxxxxxxxxx> ha scritto:

> On Fri, Jun 5, 2020 at 6:01 PM Daniel Shahaf <d.s@xxxxxxxxxxxxxxxxxx>
> wrote:
> > > On 6/5/20, Pier Paolo Grassi <pierpaolog@xxxxxxxxx> wrote:
> > > > efe() cat $1
> > > > efe <(seq 1 10) | wc -l
> > > > cat: /proc/self/fd/13: No such file or directory
> > > > 0
> > > >
> > > > gives error, instead these all works fine:
> > > > efe =(seq 1 10)| wc -l
> > > > cat <(seq 1 10)| wc -l
> >
> > And this,
> >
> > % () { cat $1 } <(seq 1 10) | wc -l
>
> You can see what's happening if you do it this way:
>
> % efe() ls -l ${${1:+$1:h}:-/proc/self/fd}
> % efe <(seq 1 10) | cat
> total 0
> lrwx------ 1 schaefer schaefer 64 Jun  6 10:59 0 -> /dev/pts/0
> l-wx------ 1 schaefer schaefer 64 Jun  6 10:59 1 -> pipe:[1945962]
> lrwx------ 1 schaefer schaefer 64 Jun  6 10:59 2 -> /dev/pts/0
> lr-x------ 1 schaefer schaefer 64 Jun  6 10:59 3 -> /proc/5533/fd
> % () { ls -l $1:h } <(seq 1 10) | cat
> total 0
> lrwx------ 1 schaefer schaefer 64 Jun  6 11:00 0 -> /dev/pts/0
> l-wx------ 1 schaefer schaefer 64 Jun  6 11:00 1 -> pipe:[1945988]
> lr-x------ 1 schaefer schaefer 64 Jun  6 11:00 11 -> pipe:[1944505]
> lrwx------ 1 schaefer schaefer 64 Jun  6 11:00 2 -> /dev/pts/0
> lr-x------ 1 schaefer schaefer 64 Jun  6 11:00 3 -> /proc/5537/fd
>
> A named function on the left side of a pipeline becomes its own "job",
> where all descriptors above 10 are closed.  An anonymous function
> stays in the parent shell (which is still a forked subshell, because
> of the pipeline) so those descriptors are not closed.
>
> % efe =(seq 1 10) | cat
> total 4
> -rw------- 1 schaefer schaefer 21 Jun  6 11:09 zshkDpnRj
>
> In that case a real file has been created and is referenced by name,
> so the descriptors closing does not matter.
>
> % efe <<(seq 1 10) | cat
> total 0
> lr-x------ 1 schaefer schaefer 64 Jun  6 11:13 0 -> pipe:[1946804]
> l-wx------ 1 schaefer schaefer 64 Jun  6 11:13 1 -> pipe:[1946579]
> lrwx------ 1 schaefer schaefer 64 Jun  6 11:13 2 -> /dev/pts/0
> lr-x------ 1 schaefer schaefer 64 Jun  6 11:13 3 -> /proc/5599/fd
>
> For <<(...) no file is created at all, instead the standard input of
> the function is changed.
>
> > Mikael Magnusson wrote on Fri, 05 Jun 2020 12:25 +0200:
> > >
> > > This also works,
> > > % efe <(seq 1 10) > >(wc -l)
> > > as does this,
> > > % wc -l < <(efe <(seq 1 10))
>
> In both of these cases, there is no pipeline, so efe stays in the
> parent shell and all the descriptors remain open.
>