Truncate a file with sed

Log files grow over time but you don't want to fill your disk. Often a log file is just for simple debug and does not need to be rotated or kept in /var/log/, sometimes a simple debug log just goes well in /dev/shm/. In this case it is important to keep it short and trim off the top of the file from time to time.

I don't know why but this problem seems very difficult for a lot of people and they end up writing long complex multi line blocks of code to truncate a file and just keep the end of it. It is super simple with sed.

# only keep the last 100 lines of the log file
sed -i /dev/shm/my_debug.log -e :a -e '$q;N;100,$D;ba'