Bash edit file and keep last 500 lines

Question

I am looking to create a cron job that opens a directory loops through all the logs i have created and deletes all lines but keep the last 500 for example.

I was thinking of something along the lines of

tail -n 500 filename > filename

Would this work?

I also not sure how to loop through a directory in bash

Thanks in advance.

Answer 1

In bash you loop over files in a directory, e.g. like this:

cd target/directory

for filename in *log; do
    echo "Cutting file $filename"
    tail -n 500 $filename > $filename.cut
    mv $filename.cut $filename
done

Answer 2

DIR=/path/to/my/dir # log directory
TMP=/tmp/tmp.log # temporary file
for f in `find ${DIR} -type f -depth 1 -name \*.log` ; do
  tail -n 500 $f > /tmp/tmp.log
  mv /tmp/tmp.log $f
done

Answer 3

Think about using logrotate.
It will not do want you want (delete all lines but the last 500), but it can take care of too big logfiles (normally by comressing the old ones and deleting them at some point). Should be widely available.