Controlling the terminal while in the middle of a pipe

Question

Say I have 3 commands linked through pipes:

A | B | C

I would like B to output error messages to stderr and some progress to the terminal. If stderr is the same as the terminal, then that is easy: Simply send the progress information to stderr. But what if stderr is redirected?

A | B 2>/tmp/file | C

Then I still want the progress information to go to the terminal, while error messages go to the file.

Can I now somehow identify the terminal that stderr would have ended up on had it not been redirected?

My initial idea would be to find the tty using tty and then opening that and writing to it. However, if I call tty from inside B tty says: not a tty, which I believe is because of the pipe between A and B.

I would like to do this from Perl.

You can use a `tee` programm: **tee** - *read from standard input and write to standard output and files* — Costas, Dec 13 '14 at 11:35
You cannot use tee. tee makes a copy of the stream. That is not the wanted behaviour. — Ole Tange, Dec 14 '14 at 06:49

Ole Tange · Answer 1 · 2014-12-14T06:48:36.950

1

Linux specific:

echo |perl -e '$p=getppid; `echo foo > /proc/$p/fd/2`'

If you here redirect stderr 'foo' is still printed on the terminal:

echo |perl -e '$p=getppid; `echo foo > /proc/$p/fd/2`' 2>/dev/null

edited Dec 14 '14 at 06:48

answered Dec 13 '14 at 12:22

Ole Tange

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Controlling the terminal while in the middle of a pipe

1 Answers1