1 00:00:01,260 --> 00:00:03,420 Hello and welcome to this lecture. 2 00:00:03,420 --> 00:00:05,410 My name is Mumshad Mannambath. 3 00:00:05,730 --> 00:00:11,340 In this section we will talk about Command and Arguments in a Pod Definition file. 4 00:00:11,370 --> 00:00:16,590 This is not listed as a required topic in the certification curriculum but I think it's important to 5 00:00:16,590 --> 00:00:20,720 explain as it is a topic that is usually overlooked. 6 00:00:20,730 --> 00:00:25,330 Let us first refresh our memory on commands in containers and docker. 7 00:00:25,380 --> 00:00:29,230 We will then translate this into pods in the next lecture. 8 00:00:29,400 --> 00:00:35,010 In this lecture we will look at commands arguments and entry points in Docker. 9 00:00:35,010 --> 00:00:37,280 Let's start with a simple scenario. 10 00:00:37,320 --> 00:00:41,220 Say you were to run a docker container from an Ubuntu image. 11 00:00:41,220 --> 00:00:47,670 When you run the “docker run ubuntu” command, it runs an instance of Ubuntu image and exits immediately. 12 00:00:48,300 --> 00:00:52,830 If you were to list the running containers you wouldn't see the container running. 13 00:00:53,010 --> 00:00:58,620 If you list all containers including those that are stopped you will see that the new container you 14 00:00:58,620 --> 00:01:01,390 ran is in an exited state. 15 00:01:01,470 --> 00:01:05,470 Now why is that unlike virtual machines. 16 00:01:05,470 --> 00:01:09,130 Containers are not meant to host an operating system. 17 00:01:09,130 --> 00:01:16,270 Containers are meant to run a specific task or process such as to host an instance of a web server or 18 00:01:16,270 --> 00:01:23,730 application server or a database or simply to carry out some kind of computation or analysis once the 19 00:01:23,730 --> 00:01:25,040 task is complete. 20 00:01:25,110 --> 00:01:32,730 The container exits a container only lives as long as the process inside it is alive. 21 00:01:32,730 --> 00:01:38,520 If the web service inside the container is stopped or crashes the container exits. 22 00:01:38,520 --> 00:01:42,600 So who defines what process is run within the container. 23 00:01:42,600 --> 00:01:48,660 If you look at the docker file for popular Docker images like NGINX you will see an instruction 24 00:01:48,660 --> 00:01:55,170 called CMD which stands for command that defines the program that will be run within the container when 25 00:01:55,170 --> 00:01:57,980 it starts. For the NGINX image 26 00:01:57,990 --> 00:02:01,680 it is the nginx command, for the mysql image 27 00:02:01,740 --> 00:02:04,120 it is the mysqld command. 28 00:02:04,170 --> 00:02:10,830 What we tried to do earlier was to run a container with a plain Ubuntu Operating System 29 00:02:10,830 --> 00:02:18,940 Let us look at the docker file for this image you will see that it uses bash as the default command. 30 00:02:18,940 --> 00:02:24,210 Now bash is not really a process like a web server or database server. 31 00:02:24,210 --> 00:02:31,650 It is a shell that listens for inputs from a terminal if it cannot find a terminal it exits. 32 00:02:31,650 --> 00:02:38,250 When we ran the Ubuntu container earlier Docker created a container from the Ubuntu image and launched 33 00:02:38,280 --> 00:02:45,480 the bash program by default Docker does not attach a terminal to a container when it is run. 34 00:02:45,720 --> 00:02:53,340 And so the bash program does not find the terminal and so it exits since the process that was started 35 00:02:53,340 --> 00:02:58,920 when the container was created finished the container exits as well. 36 00:02:58,920 --> 00:03:03,680 So how do you specify a different command to start the container. 37 00:03:03,690 --> 00:03:10,980 One option is to append a command to the docker run command and that way it overrides the default command 38 00:03:10,980 --> 00:03:13,260 specified within the image. 39 00:03:13,260 --> 00:03:20,530 In this case I run the docker run ubuntu command with the “sleep 5” command as the added option. 40 00:03:20,700 --> 00:03:27,960 This way when the container starts it runs the sleep program, waits for 5 seconds and then exits. 41 00:03:27,960 --> 00:03:30,840 But how do you make that change permanent. 42 00:03:30,870 --> 00:03:35,260 Say you want the image to always run the sleep command when it starts. 43 00:03:35,460 --> 00:03:42,180 You would then create your own image from the base Ubuntu image and specify a new command. 44 00:03:42,240 --> 00:03:49,140 There are different ways of specifying the command either the command simply as is in a shell form or 45 00:03:49,170 --> 00:03:52,080 Or in a JSON array format like this. 46 00:03:52,080 --> 00:03:59,990 But remember, when you specify in a JSON array format, the first element in the array should be the executable. 47 00:04:00,000 --> 00:04:08,100 In this case the sleep program do not specify the command and parameters together like this the command 48 00:04:08,190 --> 00:04:12,580 and its parameters should be separate elements in the list. 49 00:04:12,720 --> 00:04:20,240 So I now build by new image using the docker build command, and name it as ubuntu-sleeper. 50 00:04:20,250 --> 00:04:26,270 I could now simply run the docker Ubuntu sleeper command and get the same results. 51 00:04:26,430 --> 00:04:30,780 It always sleeps for five seconds and exits. 52 00:04:30,780 --> 00:04:35,420 But what if I wish to change the number of seconds it sleeps currently. 53 00:04:35,640 --> 00:04:39,830 It is hard coded to five seconds as we learned before. 54 00:04:39,840 --> 00:04:45,160 One option is to run the docker run command with the new command appended to it. 55 00:04:45,240 --> 00:04:52,320 In this case sleep 10 and so the command that will be run at startup will be sleep 10 but it doesn't 56 00:04:52,320 --> 00:04:53,650 look very good. 57 00:04:53,670 --> 00:04:58,870 The name of the image Ubuntu sleeper in itself implies that the container will sleep. 58 00:04:58,980 --> 00:05:02,410 So we shouldn't have to specify the sleep command again. 59 00:05:02,430 --> 00:05:10,640 Instead we would like it to be something like this Docker run Ubuntu sleeper 10 we only want to pass 60 00:05:10,640 --> 00:05:16,460 in the number of seconds the containers should sleep and sleep command should be invoked automatically 61 00:05:17,180 --> 00:05:21,420 and that is where the entry point instruction comes into play. 62 00:05:21,530 --> 00:05:27,620 The entry point instruction is like the command instruction as in you can specify the program that will 63 00:05:27,620 --> 00:05:32,960 be run when the container starts and whatever you specify on the command line. 64 00:05:32,960 --> 00:05:39,860 In this case 10 will get appended to the entry point so the command that will be run when the container 65 00:05:39,860 --> 00:05:42,760 starts is sleep 10. 66 00:05:42,800 --> 00:05:45,250 So that's the difference between the two. 67 00:05:45,470 --> 00:05:52,460 In case of the CMD instruction the command line parameters passed will get replaced entirely, whereas 68 00:05:52,490 --> 00:05:57,770 in case of entry point the command line parameters will get appended. 69 00:05:57,770 --> 00:06:04,220 Now, in the second case what if I run the ubuntu-sleeper without appending the number of 70 00:06:04,220 --> 00:06:13,680 seconds then the command at startup will be just sleep and you get the error that the operant is missing. 71 00:06:13,760 --> 00:06:17,300 So how do you configure a default value for the command. 72 00:06:17,300 --> 00:06:24,260 If one was not specified in the command line that's where you would use both entry point as well as 73 00:06:24,260 --> 00:06:26,180 the command instruction. 74 00:06:26,180 --> 00:06:32,810 In this case the command instruction will be appended to the entry point instruction so at startup the 75 00:06:32,810 --> 00:06:34,760 command would be sleep 5. 76 00:06:34,910 --> 00:06:41,330 If you didn't specify any parameters in the command line if you did then that will override the command 77 00:06:41,330 --> 00:06:42,530 instruction. 78 00:06:42,530 --> 00:06:48,740 And remember for this to happen you should always specify the entry point and command instructions in 79 00:06:48,740 --> 00:06:52,030 a JSON format. Finally, 80 00:06:52,110 --> 00:07:00,120 What if you really really want to modify the entry point during runtime say from sleep to an imaginary 81 00:07:00,120 --> 00:07:02,380 sleep 2.0 command. 82 00:07:02,460 --> 00:07:09,600 Well in that case you can override it by using the entry point option in the docker run command. 83 00:07:09,600 --> 00:07:15,770 The final command at startup would then be sleep 2.0 10. 84 00:07:15,820 --> 00:07:19,930 Well that's it for this lecture and I will see you in the next.