The ENTRYPOINT and CMD Instructions

The entry point instruction sets an executable to be run 0:00 each time a container starts. 0:04 If you're always going to want the container to run the same executable, 0:06 whether that Python, MongoDB, Redis or whatever, you should set it here. 0:10 Just like the run instruction, both shell and exec forms, 0:15 that is the form with the square brackets, are supported. 0:19 But the exec form is preferred, 0:23 because the command can be shutdown more cleanly when the container stops. 0:24 Unlike run, you can have only one entrypoint instruction per Dockerfile. 0:29 If there are additional arguments that you always want passed to the command, 0:34 you can add them after the executable. 0:38 Here's the Dockerfile for the Apache image, again. 0:41 It gives us the entrypoint instruction to run the Apache server. 0:44 By default, Apache runs in the background, but we want to run it in the foreground so 0:48 we can simple press Ctrl+C to stop it. 0:52 To do that, we need to pass -D and 0:55 FOREGROUND arguments to the apache2ctl executable. 0:58 So this entrypoint runs the Apache2ctl executable, and 1:02 ensures that -D and FOREGROUND are passed to it as arguments. 1:06 The CMD or command instruction is a list 1:11 of arguments that get passed to the entrypoint executable. 1:14 Or, if no entrypoint has been set, 1:19 the first argument is treated as the executable to be run. 1:21 As with entrypoint, you can only have one command instruction per Dockerfile. 1:25 So this command instruction combined with the entrypoint instruction, 1:30 causes a command of python3 app.py to be rung when the container starts. 1:34 The user can override the value of the command instruction by passing additional 1:41 arguments when they call docker run. 1:45 If we call docker run with the same command line we've been using to run our 1:48 Python web app all along, We'll see the usual message in our web browser. 1:52 But if we stop the container, And 2:02 then call docker run again with an additional alternate.py argument. 2:05 Our default command value of app.py will be overridden, and 2:12 the container will end up running a command of python3 alternate.py. 2:16 We've set up alternate.py to contain a web app that's just like app.py but 2:21 prints a different message. 2:27 And if we visit local host port 8080 in our browser, 2:29 we'll see it's the alternate app that's running. 2:32 Like run an entrypoint, the command instruction also has a shell form. 2:36 Whatever command you type in shell form will be executed in the /bin/sh shell. 2:41 Here's a minimal Dockerfile that just uses the shell to 2:48 echo a message to the terminal. 2:51 If we build it with docker build -t echo, 2:54 In the current directory, and run it with docker run echo, 3:00 The container will simply print that message and exit. 3:07