Controlled with signals:
master process:
TERM, INT fast shutdown
QUIT graceful shutdown
HUP changing configuration, keeping up with a changed time zone (only for FreeBSD and Linux), starting new worker processes with a new configuration, graceful shutdown of old worker processes
USR1 re-opening log files
USR2 upgrading an executable file
WINCH graceful shutdown of worker processes
worker process:
TERM, INT fast shutdown
QUIT graceful shutdown
USR1 re-opening log files
WINCH abnormal termination for debugging (requires debug_points to be enabled)
Changing Configuration (FreeBSD 4.x)
ps axw -o pid,ppid,user,%cpu,vsz,wchan,command | egrep ‘(nginx|PID)‘
Rotating Log-files
In order to rotate log files, they need to be renamed first. After that USR1 signal should be sent to the master process.
Upgrading Executable on the Fly
In order to upgrade the server executable, the new executable file should be put in place of an old file first. After that USR2 signal should be sent to the master process.
/usr/local/nginx/logs/nginx.pid.oldbin
原文：http://www.cnblogs.com/thlzhf/p/5175607.html