How to Redirect a Unix program to output to Java not a log

Question

Hi all!
Okay, here is what I am trying and I cannot seem to get a handle on where I should start.
I have a Unix program that basically outputs to a log file when a error occurs. (std_err_file: /Errorfile)
What I am trying to do is have this entry redirected to a java program that will take the log stream and then use it in some fashion. Perhaps (std_err_file: >java readlog Errorfile)? Not sure if this will work, and I need to get a better understanding of what type of stream to use to read this data.
Can anyone give me a pointer in the "write" direction??
Thanks in advance...

Answer 1

Just read it in like you would keyboard input.
Programs have three IO streams permanently attached
Standard In (stdin) is an Input Stream.
Standard Out (stdout) is an Output Stream.
Standard Error (stderr) is a Second Output Stream.
Think of it this way:
Each STDxxx is like a hose. When you invoke a program, by default the
hoses are attached as follows:
stdin is attached to the keyboard for input.
stdout is attached to the console window for output.
stderr is attached to the console window for output.
In java, the streams System.in, System.out, System.err are the other ends of these hoses.
Now, in Unix (and similarly in dos or NTshell) you can connect these hoses in different ways using a "pipe" (hence the hose metaphor)
'Program1 | Program2' will take the stdout stream from Program1, and disconnect it from the console window. It will also disconnect the stdin stream from the keyboard, and then, it will slip in a piece of pipe between the two hoses, thus connecting them.
So, program2 can read the output on its System.in stream in exactly the same way as it would if it was connected to the keyboard. In fact, Program2 has absolutely no knowledge of what is at the other end of that hose, just that there is data coming down it.
So, the leaves us with the question "What is stderr for?" It's for error messages. It is often convenient to use a pipe or a file redirect (similar to a pipe) to send data to a file, but errors to the screen.
A programmer can facilite this by sending all true output to System.out, and all error messages to System.err
Example: (in class Bob)
System.out.println("Hello Bob");
System.err.println("Warning: Bob is hacking again.");
java Bob | someProgram
// pipe the stdout of Bob to someProgram
// leave stderr attached to the keyboard.
Will send the message Hello Bob to someProgram, but the Error message will appear on the console for the system administrator to read.
Do a google on "Pipes and redirects" for more info.