On a linux server, I want to reduce the size of a log file which is several GB big. Cutting off the top half, or maybe the first million lines would work.
This comes up in interviews quite often...
Are you looking to truncate the file without disrupting the processes? Is any of the information in the log file valuable? If so, I usually "zero" the file with a simple bash string.
: > /var/log/badlogfile
This comes up in situations where you may have an application that can't be restarted in a controlled manner. Let's say it's a financial trading application and the program can't be halted or restarted during the trading day. However, the log files are growing at some obscene rate due to an application bug. Truncating the log files using the method above or below can keep the system running.
If it's actively being written to you don't really have much you can do by way of truncate. Your only options are to blank the file (you could copy it elsewhere first.)
echo "" >/var/log/fileYouWantToEmpty
That way the file ends up empty but is still the same file/inode so it won't disrupt the program that is logging.