how to get messages originally output to stderr after it has been

Hi All,

Have a look at this script:
STDERR=/tmp/.stderr
exec 3<>$STDERR 2>&3;
date >&2
ls abc.k | gawk '{print 5}'
gcc
ld
read -u3 aa bb cc
echo $aa-$bb-$cc
exec 3<&-
echo -----------------------
echo
echo -----------------------
cat $STDERR

All error messages are redirected to the file /tmp/.stderr. But why
`read' get nothing from file descriptor 3?
How to modify this script to have `read' get something from file
descriptor 3?
BTW, must file descriptor 3 be associated with a real file in the
system? If the line
exec 3<>$STDERR 2>&3;
is modified to
exec 2>&3;
bash will report a `Bad file descriptor' error.

Best Regard
PRC
Apr 8, 2008

Re: how to get messages originally output to stderr after it has been redirected?

PRC wrote:

> Hi All,
>
> Have a look at this script:
> STDERR=/tmp/.stderr
> exec 3<>$STDERR 2>&3;
> date >&2
> ls abc.k | gawk '{print 5}'
> gcc
> ld
> read -u3 aa bb cc
> echo $aa-$bb-$cc
> exec 3<&-
> echo -----------------------
> echo
> echo -----------------------
> cat $STDERR

I assume you're using bash...

> All error messages are redirected to the file /tmp/.stderr. But why
> `read' get nothing from file descriptor 3?
> How to modify this script to have `read' get something from file
> descriptor 3?

Not exactly sure why (hopefully someone more fd-savvy will explain that),
but it seems that using two different descriptors for input and output
works, ie

$ cat script.sh
STDERR=/tmp/.stderr
exec 3>"$STDERR" 4<"$STDERR" 2>&3
date >&2
ls abc.k | gawk '{print 5}'
gcc
ld
read aa bb cc <&4
echo $aa-$bb-$cc
exec 3<&- 4<&-
echo -----------------------
echo
echo -----------------------
cat "$STDERR"

$ ./script.sh
Tue-Apr-8 14:02:16 CEST 2008
-----------------------

-----------------------
Tue Apr 8 14:02:16 CEST 2008
ls: cannot access abc.k: No such file or directory
gcc: no input files
ld: no input files

> BTW, must file descriptor 3 be associated with a real file in the
> system? If the line
> exec 3<>$STDERR 2>&3;
> is modified to
> exec 2>&3;
> bash will report a `Bad file descriptor' error.

That is correct, since there is no descriptor 3 open.

--
All the commands are tested with bash and GNU tools, so they may use
nonstandard features. I try to mention when something is nonstandard (if
I'm aware of that), but I may miss something. Corrections are welcome.

Re: how to get messages originally output to stderr after it has been

In article <9887299b-2fe7-4706-8764-90bd1ea97a21@p39g2000prm.googlegroups.com>,
PRC wrote:
>STDERR=/tmp/.stderr
>exec 3<>$STDERR 2>&3;
>date >&2
>ls abc.k | gawk '{print 5}'
>gcc
>ld
>read -u3 aa bb cc
>echo $aa-$bb-$cc
>exec 3<&-
>echo -----------------------
>echo
>echo -----------------------
>cat $STDERR
>
>All error messages are redirected to the file /tmp/.stderr. But why
>`read' get nothing from file descriptor 3?

Because the file pointer is positioned after the last data written - the end of
the file. If you use ksh93, you can see this by adding just above the read
line:

echo $(3<#)

which will show the current offset.

>How to modify this script to have `read' get something from file
>descriptor 3?

If you're using ksh93, add just before the read::

3<#((0))

This will reposition the file pointer to offset 0, the start of the file.

In other shells - close and reopen fd 3.

>BTW, must file descriptor 3 be associated with a real file in the
>system? If the line
>exec 3<>$STDERR 2>&3;
>is modified to
>exec 2>&3;
>bash will report a `Bad file descriptor' error.

You need to dup a file descriptor from an existing file descriptor.
And yes, a file descriptor can only be created for something that supports
file operations (open, etc.).

John
--
John DuBois spcecdt@armory.com KC6QKZ/AE http://www.armory.com/~spcecdt/

Re: how to get messages originally output to stderr after it has been redirected?

PRC writes:

> ls abc.k | gawk '{print 5}'

Odd bit of scripting.....