Retrieve last 100 lines logs

You can use tail command as follows:

tail -100 <log file>   > newLogfile

Now last 100 lines will be present in newLogfile

EDIT:

More recent versions of tail as mentioned by twalberg use command:

tail -n 100 <log file>   > newLogfile

"tail" is command to display the last part of a file, using proper available switches helps us to get more specific output. the most used switch for me is -n and -f

SYNOPSIS

tail [-F | -f | -r] [-q] [-b number | -c number | -n number] [file ...]

Here

-n number : The location is number lines.

-f : The -f option causes tail to not stop when end of file is reached, but rather to wait for additional data to be appended to the input. The -f option is ignored if the standard input is a pipe, but not if it is a FIFO.

Retrieve last 100 lines logs

To get last static 100 lines       tail -n 100 <file path>To get real time last 100 lines     tail -f -n 100 <file path>

Look, the sed script that prints the 100 last lines you can find in the documentation for sed (https://www.gnu.org/software/sed/manual/sed.html#tail):

$ cat sed.cmd1! {; H; g; }1,100 !s/[^\n]*\n//$p$ sed -nf sed.cmd logfilename

For me it is way more difficult than your script so

tail -n 100 logfilename

is much much simpler. And it is quite efficient, it will not read all file if it is not necessary. See my answer with strace report for tail ./huge-file: https://unix.stackexchange.com/questions/102905/does-tail-read-the-whole-file/102910#102910