pty.spawn directs stderr to stdout

**Donn Cave** · Apr 11 '08, 08:05 PM

Re: pty.spawn directs stderr to stdout

In article <mailman.262.12 07895473.17997. python-list@python.org >,
Wilbert Berendsen <wbsoft@xs4all. nlwrote:

Hi,
>
using pty.spawn() it seems that stderr output of the spawned process is
directed to stdout. Is there a way to keep stderr separate and only direct
stdin and stdout to the pty?

There is, of course.

First, you have to decide where you want unit 2 ("stderr") to go, and
then get the spawned process to redirect it there. If a disk file
will do, then your question is just "how do I redirect error output
to a disk file, in ___" (fill in the blank with language used to
implement the spawned process - UNIX shell? Python? C?)

More likely, you want the spawned process' error output to go wherever
the parent's error output was going. This is a little trickier.

Ideally, your spawned shell script can conveniently take a new
parameter that identifies the new file descriptor unit number for
error output. In this case, use fd2 = os.dup(2) to get a new
duplicate, add a parameter like -e str(fd2), and in the spawned
process, redirect from that unit - in UNIX shell, exec 2>&$fd2

Or you could use an environment variable to identify the backup
error unit, if the command line parameter option isn't available
for some reason.

Donn Cave, donn@u.washingt on.edu

**Donn Cave** · Jun 27 '08, 04:17 PM

Re: pty.spawn directs stderr to stdout

In article <mailman.262.12 07895473.17997. python-list@python.org >,
Wilbert Berendsen <wbsoft@xs4all. nlwrote:

Hi,
>
using pty.spawn() it seems that stderr output of the spawned process is
directed to stdout. Is there a way to keep stderr separate and only direct
stdin and stdout to the pty?

There is, of course.

First, you have to decide where you want unit 2 ("stderr") to go, and
then get the spawned process to redirect it there. If a disk file
will do, then your question is just "how do I redirect error output
to a disk file, in ___" (fill in the blank with language used to
implement the spawned process - UNIX shell? Python? C?)

More likely, you want the spawned process' error output to go wherever
the parent's error output was going. This is a little trickier.

Ideally, your spawned shell script can conveniently take a new
parameter that identifies the new file descriptor unit number for
error output. In this case, use fd2 = os.dup(2) to get a new
duplicate, add a parameter like -e str(fd2), and in the spawned
process, redirect from that unit - in UNIX shell, exec 2>&$fd2

Or you could use an environment variable to identify the backup
error unit, if the command line parameter option isn't available
for some reason.

Donn Cave, donn@u.washingt on.edu

**Wilbert Berendsen** · Jun 27 '08, 04:17 PM

Re: pty.spawn directs stderr to stdout

Op vrijdag 11 april 2008, schreef Donn Cave:

More likely, you want the spawned process' error output to go wherever
the parent's error output was going. This is a little trickier.

I ended up writing a small script that basically reimplements fork() from the
pty module, where then STDERR is not dup'ed to the pty.

I'm currently trying to combine subprocess.Pope n with pty.openpty ...

tx,
Wilbert Berendsen

--

Wilbert Berendsen – “Plays Pretty Just For You”

http://www.wilbertberendsen.nl/

"You must be the change you wish to see in the world."
-- Mahatma Gandhi