Question

I delete all file in /var/log, however, when I restart mysql service, it failed to start now.
help!

Rather than invoking init scripts through /etc/init.d, use the service(8)
utility, e.g. service mysql restart

Since the script you are attempting to invoke has been converted to an
Upstart job, you may also use the stop(8) and then start(8) utilities,
e.g. stop mysql ; start mysql. The restart(8) utility is also available.
start: Job failed to start

Answer 1

Try creating Directory Manually & Assign the Permission to Mysql.

Ex. mkdir /var/log/mysql 
Chown -R mysql:mysql /var/log/mysql

& Restart the mysql Service again it might be slove your problem.