I wish to delete lines from a log file. Should take a date string as a variable. Find the date in the file, get the line number for the string. Delete lines from 1 to that (line number - 1), all from within a shell script. Is that possible ?.

Thnks,

umal,

Yes you can. Use sed, cat and grep then you shou,d be able to do that or sed and awk.

line=`grep -n "${datevariable}" log_filename|cut -d":" -f1`
sed "1,${line} d" log_filename

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I tried this out , sed "1,${line} d" log_filename

This leaves the log_filename as it is. I would have to
sed "1,${line} d" log_filename > log_filename.new
mv log_filename log_filename.old
mv log_filename.new log_filename

the log file is constantly being written to, so how can I guarantee there wont be any loss of data...

I wish to delete lines from the log as with "vi". Besides wont inodes change ?

What does ruby mean ?

i dont have idea bt ruby
but as far as maintaining the same file inode, try this

perl -pie "s/.....
the idea here is to use perl with flags -pie, doubt if you have perl installed.
I have posted this solution of perl -pie on numerous occassions where a temp file creation is to be omitted.
you can find it in some searches.

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War doesnt determine who is right, it determines who is left