In this code statement (tail -f -n +1 s_GenerateXMLDataFile.log& echo $! > $$.tailpid) | awk ', could you please clarify if my understanding is wrong
1) you enclosed the statement in parantheses because you don't want to print the standard output to the teriminal instead you are piping the output to the awk script?
2) I don't understand what would be the output of echo $! as it is printing nothing when i execute at shell prompt? and why did you use the single ampersand symbol & between tail and echo commands as we would generally use double ampersand symbols && to combine to commands.
In this code statement trap 'kill $(cat $$.tailpid);rm -f $$.tailpid' EXIT
3) what is the difference between EXIT in capitals & exit in smaller case? i see both working
4) why you didn't put exit statement after commands before EXIT signal?
Hello my requirement has changed as UI developer is having difficulties to capture the output of your shell script, now i have to get each value which is returned from this awk script into a shell variable and need to update that value into the table, with latest count as it gets populated in the log file.
For example if this is going to be output from the awk/shell script.
starts tail asynchronously (AKA in the background). This tail will copy the current contents of the file to standard output and then will copy newly added data to standard output shortly after it is written to the file. This tail will run forever unless it is killed by trying to write to a pipe that has been closed (which won't happen even though awk terminates and closes the read end of the pipe when it sees the last line of the file because no new data will be added to the file after awk terminates) or it is manually terminated by a signal.
Code:
echo $! > $$.tailpid
write the process ID of the last job started in the background (i.e. tail) into a file whose name is the process ID of the shell running this command followed by the string .tailpid.
When awk completes, it will exit, but tail will continue running. The:
says that just before the script exits, it should execute the command:
Code:
kill $(cat $$.tailpid);rm -f $$.tailpid
which will get the process ID of the tail command out of the file created by the echo $?, send that process a SIGTERM signal and remove the file created by the echo.
------------------------------------------------------------------------
If we assume that you named the earlier script
Code:
getcounts
and removed the
Code:
echo 'log file complete'
from it, and then create another script (e.g., readcounts):
Code:
#!/bin/bash
while read count
do echo 'Do whatever'
echo 'you want'
echo "with $count."
done
Then you can use:
Code:
./getcounts | ./readcounts
This User Gave Thanks to Don Cragun For This Post:
Doing ./getcounts | ./readcounts is causing the second script (readcounts) waiting untill the getcounts script finishes reading the whole log file, but i wanted to capture the output of the awk script or (getcounts) script real time output into a variable or something to insert into the table. Please help.
Testing the scripts I gave you on Mac OS X with a program that writes one line into s_GenerateXMLDataFile.log every two seconds and the
Code:
getcounts | readcounts
pipeline was printing three lines very quickly after tail forwarded a line to awk that contained the data needed to print another count.
Of course it is possible that tail -f or awk on your system buffers output if the output isn't directed to a terminal although it would be VERY strange if tail did that (buffering contradicts the intent of if). You haven't told us what system you're using.
If getcounts is printing counts in real time, we know that tail -f isn't buffering. So, if awk really is buffering, you could change:
please let me do some more testing and i will get back to you.
Btw i am using the Red Hat Enterprise Linux Server release 5.9 (Tikanga)
Thank you.
---------- Post updated 06-11-14 at 09:06 AM ---------- Previous update was 06-10-14 at 09:41 AM ----------
May i please know why do you need to explicitly close the shell script, wouldn't it exit by itself after execution when you pipe the output of the awk script.
---------- Post updated 06-11-14 at 09:06 AM ---------- Previous update was 06-10-14 at 09:41 AM ----------
May i please know why do you need to explicitly close the shell script, wouldn't it exit by itself after execution when you pipe the output of the awk script.
close("./readcounts")
Thank you.
You tell us that awk is buffering output sent through a pipe. By closing the pipe to readcounts after writing each count, the buffering problem is eliminated. Unfortunately, it also means that readcounts has to be invoked for each count (instead of once for all counts).
This User Gave Thanks to Don Cragun For This Post:
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TERMINAL TYPE: prod
SYSTEM: nb11cu51
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06-06-2014
