Breaking out of "tail -f" that's being read by a "while read" loop in HP-UX

Question

Hi All,

I'm trying to write a (sh -bourne shell) script that processes lines as they are written to a file. I'm attempting to do this by feeding the output of tail -f into a while read loop. This tactic seems to be proper based on my research in Google as well as this question dealing with a similar issue, but using bash.

From what I've read, it seems that I should be able to break out of the loop when the file being followed ceases to exist. It doesn't. In fact, it seems the only way I can break out of this is to kill the process in another session. tail does seem to be working fine otherwise as testing with this:

touch file
tail -f file | while read line
do
  echo $line
done

Data I append to file in another session appears just file from the loop processing written above.

This is on HP-UX version B.11.23.

Thanks for any help/insight you can provide!

Answer 1

I don't know about HP-UX tail but GNU tail has the --follow=name option which will follow the file by name (by re-opening the file every few seconds instead of reading from the same file descriptor which will not detect if the file is unlinked) and will exit when the filename used to open the file is unlinked:

tail --follow=name test.txt

Answer 2

If you want to break out, when your file does not exist any more, just do it:

 test -f file || break

Placing this in your loop, should break out.

The remaining problem is, how to break the read line, as this is blocking.

This could you do by applying a timeout, like read -t 5 line. Then every 5 second the read returns, and in case the file does not longer exist, the loop will break. Attention: Create your loop that it can handle the case, that the read times out, but the file is still present.

EDIT: Seems that with timeout read returns false, so you could combine the test with the timeout, the result would be:

  tail -f test.file | while read -t 3 line || test -f test.file; do 
          some stuff with $line
  done

Answer 3

I've never been happy with this answer but I have not found an alternative either:

kill $(ps -o pid,cmd --no-headers --ppid $$ | grep tail | awk '{print $1}')

Get all processes that are children of the current process, look for the tail, print out the first column (tail's pid), and kill it. Sin-freaking-ugly indeed, such is life.