docker run, docker exec and logs
When you docker exec you can see you have several process
/ # ps -efPID USER TIME COMMAND 1 root 0:00 nginx: master process nginx -g daemon off; 6 nginx 0:00 nginx: worker process 7 root 0:00 /bin/sh 17 root 0:00 ps -ef/ # and in Linux, each process has its own stdin, stdout, stderr (and other file descriptors), in /proc/pid/fd
and so, with your docker exec (pid 7) you display something in
/proc/7/fd/1
If you do ls -ltr /proc/7/fd/1, it displays something like/proc/4608/fd/1 -> /dev/pts/2 which means output is being sent to terminal
while your nginx process (pid 1) displays his output in
/proc/1/fd/1
If you do ls -ltr /proc/1/fd/1, it displays something like /proc/1/fd/1 -> pipe:[184442508] which means output is being sent to docker logging driver