How do you keep only the last n lines of a log file?

It is possible like this, but as others have said, the safest option is the generation of a new file and then a move of that file to overwrite the original.

The below method loads the lines into BASH, so depending on the number of lines from tail, that's going to affect the memory usage of the local shell to store the content of the log lines.

The below also removes empty lines should they exist at the end of the log file (due to the behaviour of BASH evaluating "$(tail -1000 test.log)") so does not give a truly 100% accurate truncation in all scenarios, but depending on your situation, may be sufficient.

$ wc -l myscript.log
475494 myscript.log

$ echo "$(tail -1000 myscript.log)" > myscript.log

$ wc -l myscript.log
1000 myscript.log

The utility sponge is designed just for this case. If you have it installed, then your two lines can be written:

tail -n 1000 myscript.log | sponge myscript.log

Normally, reading from a file at the same time that you are writing to it is unreliable. sponge solves this by not writing to myscript.log until after tail has finished reading it and terminated the pipe.

Install

To install sponge on a Debian-like system:

apt-get install moreutils

To install sponge on a RHEL/CentOS system, add the EPEL repo and then do:

yum install moreutils

Documentation

From man sponge:

sponge reads standard input and writes it out to the specified file. Unlike a shell redirect, sponge soaks up all its input before writing the output file. This allows constructing pipelines that read from and write to the same file.

definitely "tail + mv" is much better! But for gnu sed we can try

sed -i -e :a -e '$q;N;101,$D;ba' log