Actually the way you presented your problem you were writing into a a file - "error.log". You just happened to redirect stderr to point to that file.

You might picture UNIX processes (running commands) like garden hoses - you put something in on top (this is stdin) then inside something is done with it and finally something pours out at bottom (stdout). You can point both - stdin and stdout - to some file or device. In case of stdin from this device or file is being read, in case of stdout output gets written there. You can even connect stdout of one process to stdin of another process - this is called "pipeline" and looks like this:

Code:

command1 | command2

So far, so basic. Now, the garden hoses of UNIX processes are in fact Y-shaped: the normal flow is like explained above, but for diagnostic messages there is a small outlet in the middle (to stay in the hose picture) where errors, diagnostic messages, etc. are shown. This is good, because if the diagnostic output is separated from the normal output it is possible to process it via different processes.

So, this is what "stderr" is normally used for, but in fact it is just a so-called "I/O descriptor" - a possible outlet where a process can output something and in this respect not different from "stdin" or "stdout"*). You can use stderr in the same way you use stdout and if you write:

Code:

echo "hello world" >&2

you will see "hello world" appear on stderr. If this is inside a script and you redirect the stderr output of it to some file:

Code:

./script 2>/some/logfile

you will find "hello world" in this file.

Per default "stdout" is I/O-descriptor 1 and "stderr" is I/O-descriptor 2 (this is why it is "1>" and "2>" in redirections) but you can use other I/O-descriptors too - you just have to open and close them, just like i did for I/O-descriptor 3 in my sample above.

But the question "include variable in stderr" makes no sense: you can output everything to stderr the same way you output something to stdout or anywhere else - it is just a different part of the y-shaped hose you use.

I hope this helps.

bakunin
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*) in fact there are differences: i.e. stdout is buffered, stderr is not, but this is not relevant here