Removing lines from a file being used by another process using SHELL script
Hi All,
Need a small help in writing a shell script which can delete a few lines from a file which is currently being used by another process.
File gets appended using tee -a command due to which its size is getting increased.
Contents like :
Code:
[INFO] 25/09/2012 05:18 Run ID:56579677-1
My requirement is to remove lines which are more than 1 month old...
for this i will cut month from the line i.e. 09 and backtrack to fetch the previous value i.e. 8
and remove all lines from top till the row fetched...
Query is can i remove the lines in the same file ? (what if another process comes to write in that file using tee -a)..
Thanks,
Nik
Last edited by Scrutinizer; 09-25-2012 at 04:39 AM..
Reason: code tags
I would guess that it depends on how you remove the top lines. This is at the bash prompt...
Code:
$ echo x > file1
$ ( ( echo a ; sleep 30 ; echo b ) | tee -a file1 ) &
[1] 7000
$ a
$ sed -i 1d file1
sed: cannot rename ./sedCDnEEq: Permission denied
$ cat file1
x
a
$ awk '{a[NR]=$0}END{close(FILENAME);for(i=2;i<=NR;i++)print a[i]>FILENAME}' file1
$ cat file1
a
$ b
[1]+ Done ( ( echo a; sleep 30; echo b ) | tee -a file1 )
$ cat file1
a
b
$
In the example above, I created a file then submitted a background process to append to the file. Using sed -i to remove the first line didn't work, but awk worked okay.
To have a real feel of what's happening, reset the original file content and use some sleeps, as in:
Code:
flock file -c "sed '1,/\/08\/2012/ d' <file >tmpfile; cat tmpfile >file; sleep 10" & \
sleep 3; echo "sed has worked; cat has worked, here is the file:"; cat file; \
echo; echo "waiting the last 7 seconds for tee..."; \
flock file -c "echo yadayadayada |tee -a file >/dev/null"; \
echo "tee has been able to write just now. Here is the file:"; cat file
To sum it up:
a) to delete your old lines, use flock file -c "sed ... <file >tmpfile; cat tmpfile >file";
b) prepend flock file -c "... to your tee -a file command accessing the file.
--
Bye
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