Breaking "tail -f" that is read by "while read" loop in HP-UX
I am trying to write a (sh -bourne shell) script that processes lines as they are written to a file. I am trying to do this by feeding the output tail -f
into a loop while read
. This tactic seems to be correct, based on my research on Google and also this question , which addresses a similar problem, but using bash.
From what I've read, it seems like I have to break out of the loop when the executable ceases to exist. This is not true. Actually, it seems the only way I can get out of this is by killing the process in another session. tail
seems to work fine, otherwise testing with this:
touch file tail -f file | while read line do echo $ line done
The data I add to file
in another session is displayed as soon as the file from the above processing loop.
This is HP-UX B.11.23 version.
Thanks for any help / understanding you can provide!
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If you want to break out when your file no longer exists, just do it:
test -f file || break
Placing this in your loop should break out.
The rest of the problem is how to split the read line as it blocks.
This can be done using a timeout like read -t 5 line. Then every 5 seconds, the read returns, and in case the file no longer exists, the loop is interrupted. Note: Create your loop so it can handle the case that the read time is not working but the file is still present.
EDIT: It seems that with a timeout, read returns false, so you can combine the test with a timeout, the result is:
tail -f test.file | while read -t 3 line || test -f test.file; do
some stuff with $line
done
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I don't know about HP-UX tail
, but GNU tail
has a parameter --follow=name
that will follow the file by name (by reopening the file every few seconds instead of reading from the same file a descriptor that won't detect if the file is detached) and exits when the file name used to open the file will be disabled:
tail --follow=name test.txt
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If you are not using GNU tail , it will not be able to complete its action by following the file. The -f option is really only for interactive monitoring - indeed, I have a book that says "-f" is unlikely to be used in shell scripts. "
But to solve the problem, I'm not entirely sure if this is not a very difficult way to do it, but I figured that you can send tail to a FIFO, then have a function or script that would check the file for existence and kill off the tail if it was detached ...
#! / bin / sh
sentinel ()
{
while true
do
if [! -e $ 1]
then
kill $ 2
rm / tmp / $ 1
break
fi
done
}
touch $ 1
mkfifo / tmp / $ 1
tail -f $ 1> / tmp / $ 1 &
sentinel $ 1 $! &
cat / tmp / $ 1 | while read line
do
echo $ line
done
There has been some naive testing and it looks like everything is fine and don't leave garbage around you.
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I've never been happy with this answer, but I haven't found an alternative:
kill $(ps -o pid,cmd --no-headers --ppid $$ | grep tail | awk '{print $1}')
Get all processes that are children of the current process, find the tail, print the first column (tail pid) and kill it. Sin-damn-ugly indeed, that's life.
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The following approach uses the command tail -f file
echos its process id plus a custom string prefix (here tailpid:
) in a loop while
where a custom string prefix string starts another (given one) while
that checks every 5 seconds if more exists file
. If not, it tail -f file
will be killed and the subshell containing the background loop while
will exit.
# cf. "The Heirloom Bourne Shell",
# http://heirloom.sourceforge.net/sh.html,
# http://sourceforge.net/projects/heirloom/files/heirloom-sh/ and
# http://freecode.com/projects/bournesh
/usr/local/bin/bournesh -c '
touch file
(tail -f file & echo "tailpid: ${!}" ) | while IFS="" read -r line
do
case "$line" in
tailpid:*) while sleep 5; do
#echo hello;
if [ ! -f file ]; then
IFS=" "; set -- ${line}
kill -HUP "$2"
exit
fi
done &
continue ;;
esac
echo "$line"
done
echo exiting ...
'
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