If I do :
docker run --name nginx -d nginx:alpine /bin/sh -c 'echo "Hello stdout" > /dev/stdout'
I can see "Hello stdout" when I do :
docker logs nginx
But when the container is running (docker run --name nginx -d nginx:alpine) and I do :
docker exec nginx /bin/sh -c 'echo "Hello stdout" > /dev/stdout'
or when I attach the container with :
docker exec -it nginx /bin/sh
and then :
echo "Hello stdout" > /dev/stdout
I can't see anything in docker logs. And since my Nginx access logs are redirected to /dev/stdout, I can't see them as well.
What is happening here with this stdout ?
When you docker exec you can see you have several process
/ # ps -ef
PID 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