Re: [Help-bash] When pipes fail (and when not)

[Paul Wagner]

Dear Bob,

a really really huge Thank You for the effort you put into this 
explanation!

> Therefore I conclude that your Android environment is mistakenly
> installing a SIG_IGN handler for SIGPIPE in the environment.

With that as a working hypothesis, could I install my own SIGPIPE 
handler then and kill the subshell tee runs in?  Something like

for i in {0..9}; do echo $i; sleep 1; done | { trap 'kill $$' pipe; tee 
foo; } | dd bs=1 count=10

To experiment with that, I tried (under Cygwin, where the SIGPIPE is 
working fine)

trap 'echo foo >&2' pipe
for i in {0..9}; do echo $i; sleep 1; done | dd bs=1 count=10

which terminates after '4' but never writes 'foo', making me wonder if 
the trap handler ever got called.  When I try

trap '' pipe
for i in {0..9}; do echo $i; sleep 1; done | dd bs=1 count=10

as you did in your expose, I get the expected behaviour of

0
1
2
3
4
10+0 records in
10+0 records out
10 bytes copied, 4.10439 s, 0.0 kB/s
-bash: echo: write error: Broken pipe
-bash: echo: write error: Broken pipe
-bash: echo: write error: Broken pipe
-bash: echo: write error: Broken pipe
-bash: echo: write error: Broken pipe

so setting the trap handler actually works in this case, but why not in 
the former one?  Or did I miss something else?

> For what it is worth I tried this on Android using Termux and
> received the same result.  I feel confident that the parent
> environment ignoring SIGPIPE is the problem.

Interesting is that inside Termux the behaviour is the same.  I am not 
familiar with Termux, but GNUroot uses a proot-environment, and from 
what I read in the FAQ Termux might operate the same way.  I will 
probably have a look at the proot sources, might find a clue there.

> > P.S.: I noticed that with GNURoot on android, process substitution 
> > does not
> > work, probably because /dev can not be written?  Is that related to 
> > the
> > problem above?
>
> No.  I don't think so.  That would be a different problem.  And
> strictly speaking that isn't a portable operation anyway.

Regarding process substitution not working, in the meantime I found out 
that /dev is just lacking the /dev/fd-directory, but /proc has it, so on 
a rooted Android device a simple 'ln -s /proc/self/fd /dev/fd' before 
starting the proot-environment does the trick.

Again, a big Thank You!

Paul

P.S.
> P.S. Using {0..9} as in "for i in {0..9}" is really not a good way to
> do this.  It expands to be the full set.
>
>   for i in 0 1 2 3 4 5 6 7 8 9
>
> For 10 items this is okay.  But people get used to doing that and then
> do it for 10 zillion items.  That eats up a *LOT* of memory.

I get your point.  I just wanted to make the example *really* short, and 
I never intended more than 10 iterations.

> And next I am of the opinion that counting numbers should be whole
> counting numbers starting at 1 not 0 and proceeding on from there.

Totally agree.  I'm just *very* lazy, and typing 0..9 saved me one 
keystroke compared to 1..10 ;-)