Searching file contents in last n lines of shell script

Question

I use the grep file content command and am doing something.
However, the file size grows continuously every second. (will be more than 500 MB)
Due to a performance problem, I want to grep the contents of the file in the last N lines, and not the contents of the entire file.

if grep -q "SOMETHING" "/home/andy/log/1.log"; then
    ps -ef | grep "127.0.0.1:50000" | awk '{print $2}' | xargs kill; cat /dev/null > /home/andy/log/1.log
fi

How can I change the contents of a script to a grep file in the last N lines?
Thank!

Answer 1

you can use tail -n to get the last n lines of the file.

So, if you want to look only at the last 100 lines, you can make your script work like this:

if tail -n 100 "/home/andy/log/1.log" | grep -q "SOMETHING"; then
...

Answer 2

tail -c ()

# prev_size is known
curr_size=$(stat -c %s file.log)
if tail -c $((curr_size - prev_size)) | grep -q pattern; then ...
    # ...
fi
prev_size=$curr_size
# loop

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