WEBVTT 00:01.260 --> 00:03.420 Hello and welcome to this lecture. 00:03.420 --> 00:05.410 My name is Mumshad Mannambath. 00:05.730 --> 00:11.340 In this section we will talk about Command and Arguments in a Pod Definition file. 00:11.370 --> 00:16.590 This is not listed as a required topic in the certification curriculum but I think it's important to 00:16.590 --> 00:20.720 explain as it is a topic that is usually overlooked. 00:20.730 --> 00:25.330 Let us first refresh our memory on commands in containers and docker. 00:25.380 --> 00:29.230 We will then translate this into pods in the next lecture. 00:29.400 --> 00:35.010 In this lecture we will look at commands arguments and entry points in Docker. 00:35.010 --> 00:37.280 Let's start with a simple scenario. 00:37.320 --> 00:41.220 Say you were to run a docker container from an Ubuntu image. 00:41.220 --> 00:47.670 When you run the “docker run ubuntu” command, it runs an instance of Ubuntu image and exits immediately. 00:48.300 --> 00:52.830 If you were to list the running containers you wouldn't see the container running. 00:53.010 --> 00:58.620 If you list all containers including those that are stopped you will see that the new container you 00:58.620 --> 01:01.390 ran is in an exited state. 01:01.470 --> 01:05.470 Now why is that unlike virtual machines. 01:05.470 --> 01:09.130 Containers are not meant to host an operating system. 01:09.130 --> 01:16.270 Containers are meant to run a specific task or process such as to host an instance of a web server or 01:16.270 --> 01:23.730 application server or a database or simply to carry out some kind of computation or analysis once the 01:23.730 --> 01:25.040 task is complete. 01:25.110 --> 01:32.730 The container exits a container only lives as long as the process inside it is alive. 01:32.730 --> 01:38.520 If the web service inside the container is stopped or crashes the container exits. 01:38.520 --> 01:42.600 So who defines what process is run within the container. 01:42.600 --> 01:48.660 If you look at the docker file for popular Docker images like NGINX you will see an instruction 01:48.660 --> 01:55.170 called CMD which stands for command that defines the program that will be run within the container when 01:55.170 --> 01:57.980 it starts. For the NGINX image 01:57.990 --> 02:01.680 it is the nginx command, for the mysql image 02:01.740 --> 02:04.120 it is the mysqld command. 02:04.170 --> 02:10.830 What we tried to do earlier was to run a container with a plain Ubuntu Operating System 02:10.830 --> 02:18.940 Let us look at the docker file for this image you will see that it uses bash as the default command. 02:18.940 --> 02:24.210 Now bash is not really a process like a web server or database server. 02:24.210 --> 02:31.650 It is a shell that listens for inputs from a terminal if it cannot find a terminal it exits. 02:31.650 --> 02:38.250 When we ran the Ubuntu container earlier Docker created a container from the Ubuntu image and launched 02:38.280 --> 02:45.480 the bash program by default Docker does not attach a terminal to a container when it is run. 02:45.720 --> 02:53.340 And so the bash program does not find the terminal and so it exits since the process that was started 02:53.340 --> 02:58.920 when the container was created finished the container exits as well. 02:58.920 --> 03:03.680 So how do you specify a different command to start the container. 03:03.690 --> 03:10.980 One option is to append a command to the docker run command and that way it overrides the default command 03:10.980 --> 03:13.260 specified within the image. 03:13.260 --> 03:20.530 In this case I run the docker run ubuntu command with the “sleep 5” command as the added option. 03:20.700 --> 03:27.960 This way when the container starts it runs the sleep program, waits for 5 seconds and then exits. 03:27.960 --> 03:30.840 But how do you make that change permanent. 03:30.870 --> 03:35.260 Say you want the image to always run the sleep command when it starts. 03:35.460 --> 03:42.180 You would then create your own image from the base Ubuntu image and specify a new command. 03:42.240 --> 03:49.140 There are different ways of specifying the command either the command simply as is in a shell form or 03:49.170 --> 03:52.080 Or in a JSON array format like this. 03:52.080 --> 03:59.990 But remember, when you specify in a JSON array format, the first element in the array should be the executable. 04:00.000 --> 04:08.100 In this case the sleep program do not specify the command and parameters together like this the command 04:08.190 --> 04:12.580 and its parameters should be separate elements in the list. 04:12.720 --> 04:20.240 So I now build by new image using the docker build command, and name it as ubuntu-sleeper. 04:20.250 --> 04:26.270 I could now simply run the docker Ubuntu sleeper command and get the same results. 04:26.430 --> 04:30.780 It always sleeps for five seconds and exits. 04:30.780 --> 04:35.420 But what if I wish to change the number of seconds it sleeps currently. 04:35.640 --> 04:39.830 It is hard coded to five seconds as we learned before. 04:39.840 --> 04:45.160 One option is to run the docker run command with the new command appended to it. 04:45.240 --> 04:52.320 In this case sleep 10 and so the command that will be run at startup will be sleep 10 but it doesn't 04:52.320 --> 04:53.650 look very good. 04:53.670 --> 04:58.870 The name of the image Ubuntu sleeper in itself implies that the container will sleep. 04:58.980 --> 05:02.410 So we shouldn't have to specify the sleep command again. 05:02.430 --> 05:10.640 Instead we would like it to be something like this Docker run Ubuntu sleeper 10 we only want to pass 05:10.640 --> 05:16.460 in the number of seconds the containers should sleep and sleep command should be invoked automatically 05:17.180 --> 05:21.420 and that is where the entry point instruction comes into play. 05:21.530 --> 05:27.620 The entry point instruction is like the command instruction as in you can specify the program that will 05:27.620 --> 05:32.960 be run when the container starts and whatever you specify on the command line. 05:32.960 --> 05:39.860 In this case 10 will get appended to the entry point so the command that will be run when the container 05:39.860 --> 05:42.760 starts is sleep 10. 05:42.800 --> 05:45.250 So that's the difference between the two. 05:45.470 --> 05:52.460 In case of the CMD instruction the command line parameters passed will get replaced entirely, whereas 05:52.490 --> 05:57.770 in case of entry point the command line parameters will get appended. 05:57.770 --> 06:04.220 Now, in the second case what if I run the ubuntu-sleeper without appending the number of 06:04.220 --> 06:13.680 seconds then the command at startup will be just sleep and you get the error that the operant is missing. 06:13.760 --> 06:17.300 So how do you configure a default value for the command. 06:17.300 --> 06:24.260 If one was not specified in the command line that's where you would use both entry point as well as 06:24.260 --> 06:26.180 the command instruction. 06:26.180 --> 06:32.810 In this case the command instruction will be appended to the entry point instruction so at startup the 06:32.810 --> 06:34.760 command would be sleep 5. 06:34.910 --> 06:41.330 If you didn't specify any parameters in the command line if you did then that will override the command 06:41.330 --> 06:42.530 instruction. 06:42.530 --> 06:48.740 And remember for this to happen you should always specify the entry point and command instructions in 06:48.740 --> 06:52.030 a JSON format. Finally, 06:52.110 --> 07:00.120 What if you really really want to modify the entry point during runtime say from sleep to an imaginary 07:00.120 --> 07:02.380 sleep 2.0 command. 07:02.460 --> 07:09.600 Well in that case you can override it by using the entry point option in the docker run command. 07:09.600 --> 07:15.770 The final command at startup would then be sleep 2.0 10. 07:15.820 --> 07:19.930 Well that's it for this lecture and I will see you in the next.