Pipe Find command "stderr" to a command - unix

Question

Hi I have a peculiar problem and I'm trying hard to find (pun intended) a solution for it.
$> find ./subdirectory -type f 2>>error.log
I get an error, something like, "find: ./subdirectory/noidea: Permission denied" from this command and this will be redirected to error.log.
Is there any way I can pipe the stderr to another command before the redirection to error.log?
I want to be able to do something like
$> find ./subdirectory -type f 2 | sed "s#\(.*\)#${PWD}\1#" >> error.log
where I want to pipe only the stderr to the sed command and get the whole path of the find command error.
I know piping doesn't work here and is probably not the right way to go about.
My problem is I need both the stdout and stderr and the both have to be processed through different things simultaneously.
EDIT:
Ok. A slight modification to my problem.
Now, I have a shell script, solve_problem.sh
In this shell script, I have the following code
ErrorFile="error.log"
for directories in `find ./subdirectory -type f 2>> $ErrorFile`
do
field1=`echo $directories | cut -d / -f2`
field2=`echo $directories | cut -d / -f3`
done
Same problem but inside a shell script. The "find: ./subdirectory/noidea: Permission denied" error should go into $ErrorFile and stdout should get assigned to the variable $directories.

Answer 1

Pipe stderr and stdout simultaneously - idea taken from this post:
(find /boot | sed s'/^/STDOUT:/' ) 3>&1 1>&2 2>&3 | sed 's/^/STDERR:/'
Sample output:
STDOUT:/boot/grub/usb_keyboard.mod
STDERR:find: `/boot/lost+found': Brak dostępu
Bash redirections like 3>&1 1>&2 2>&3 swaps stderr and stdout.
I would modify your sample script to look like this:
#!/bin/bash
ErrorFile="error.log"
(find ./subdirectory -type f 3>&1 1>&2 2>&3 | sed "s#^#${PWD}: #" >> $ErrorFile) 3>&1 1>&2 2>&3 | while read line; do
field1=$(echo "$line" | cut -d / -f2)
...
done
Notice that I swapped stdout & stderr twice.
Small additional comment - look at -printf option in find manual page. It might be useful to you.

Answer 2

If you need to redirect stderr to stdout so that the following command in the pipe gets it as its input, then you can use 2>&1.
For more information, please have a look at the all about redirection how-to.
Edit: Even if you need to edit pass stdout further in the pipe, you can use sed to filter error messages and write them to a file:
$ find . -type f 2>&1 | sed '/^find:/{
> w error.log
> d
> }'
In this example:
in find command stderr is redirected to stdout
in sed command errors from find that match a regular expression are written to a file (w error.log) and removed from output (d).
any command in the pipeline following the pipeline will received the output from find.
Note: This will work as lon as all the error messages from find start with find:. Otherwise, the regular expression in sed should be modified to properly match all cases.

Answer 3

You can try this (on bash), which appears to work:
find ./subdirectory -type f 2> >(sed "s#\(.*\)#${PWD}\1#" >> error.log)
This does the following:
2> redirects stderr to
>(...) a process substitution (running sed, which appends to error.log)