UNIX for Dummies Questions & Answers

Hi. I have a situation where I need to constantly read a command's output in order to loop through it to determine something that is happening over our system. The command td <filename> interactively prints 'filename' as text is written to it and this is redirected to standard output. I have been trying to loop through this output interactively within a shell to catch any new messages to the files but without success. I have tried with:

Code:

#! /bin/ksh
while read line
do
        echo $line
done < td filename

exit 0

but it complains with

Code:

./test.sh[5]: syntax error at line 5 : `filename' unexpected

what I want to, is feed while with the output of the command interactively, which I believe will in turn run for ever (which I expect and want) thus achieving what I want. But as you can see, it doesn't work like this. I have also tried with for with not success.

If anyone could come with something similar to what I want, please advice. It will be very appreciated.

Thanks!

give the output of td filename to while

as

`td filename`

matrixmadhan, thanks for your prompt reply. I have done what you suggested, but apparently the shell hangs there and does nothing (no output or anything). I even tried redirecting echo's to a file and the file doesn't even gets created, which I believe that it hangs before it goes into the loop.

then your process is not hanging, blocking on a read.

try executing
td filename
alone in the shell, then you would be clear with the input required and the output you target.

Hi matrixmadhan. Thanks again. I dont understand at all your statemnet about 'blocking on a read', could you please ellaborate more on that ?

Here is an example of what I am trying to do.
I executed on a terminal

Code:

$ td test

See how it waits to show you there are no more 'trace to debug' on the file test. I then send a message from another terminal, that writes to this file.

Code:

$ echo "hello" > test
$ echo "world" > test

Then, in my first terminal I see

Code:

$ td test
hello
world

td just seem to do something like "I am watching this file for you, write to it, and I will show you what did you write". I am trying to do the same thing to catch any new message written to some file.

I just see that td is just a wrapped tail -f under the covers. So, this actually make things easier, since I just want to, as new lines are added to this output, verify them to apply my criteria based on what I found on these. Still, if I do `tail -f file` while just seem to hang there.

td filename | while read line ; do

has a fair shot at doing what you want. But it depends on how td is written. It may be line buffered only when writting to a tty device. When writing to a pipe, it may be fully buffered. Use "man setvbuf" if you don't understand buffering.