1 00:00:00,360 --> 00:00:01,410 In this lesson, 2 00:00:01,410 --> 00:00:03,469 we're going to cover the concept of services 3 00:00:03,469 --> 00:00:05,520 in the Linux operating system. 4 00:00:05,520 --> 00:00:07,920 Now, services are programs or processes 5 00:00:07,920 --> 00:00:10,170 that run in your servers at all times, 6 00:00:10,170 --> 00:00:12,150 usually, from the time you boot up the server 7 00:00:12,150 --> 00:00:13,740 until you shut it down. 8 00:00:13,740 --> 00:00:16,560 These services are used to provide persistent support 9 00:00:16,560 --> 00:00:17,880 for requests and monitoring 10 00:00:17,880 --> 00:00:21,030 from other processes or from external clients. 11 00:00:21,030 --> 00:00:23,760 Some examples of well-known services are Apache 12 00:00:23,760 --> 00:00:24,960 for running a web server 13 00:00:24,960 --> 00:00:27,540 and Postfix for running an email server. 14 00:00:27,540 --> 00:00:29,730 Just like any other components of Linux though, 15 00:00:29,730 --> 00:00:31,770 services can be managed as well. 16 00:00:31,770 --> 00:00:33,360 If you're a system administrator, 17 00:00:33,360 --> 00:00:35,790 you're going to be dealing with services regularly. 18 00:00:35,790 --> 00:00:37,710 If you're a normal desktop user though, 19 00:00:37,710 --> 00:00:39,360 you may come across the need to restart 20 00:00:39,360 --> 00:00:43,380 or perform operations to run services as things go wrong. 21 00:00:43,380 --> 00:00:46,200 In this lesson, you're going to learn how to start, stop, 22 00:00:46,200 --> 00:00:49,500 and restart services in any Linux distribution. 23 00:00:49,500 --> 00:00:52,590 Now, most services are implemented in the form of daemons, 24 00:00:52,590 --> 00:00:55,350 which are running programs that operate in the background 25 00:00:55,350 --> 00:00:57,270 without the need for human intervention, 26 00:00:57,270 --> 00:00:59,430 which means that daemons are going to lie dormant 27 00:00:59,430 --> 00:01:00,810 until an event triggers them 28 00:01:00,810 --> 00:01:03,450 to take some kind of function or activity. 29 00:01:03,450 --> 00:01:05,850 Now, the init system is the first process 30 00:01:05,850 --> 00:01:07,680 that starts outside of the kernel, 31 00:01:07,680 --> 00:01:09,810 and it's generally going to be the backend service 32 00:01:09,810 --> 00:01:11,940 that controls when and how other services 33 00:01:11,940 --> 00:01:13,470 are going to be started. 34 00:01:13,470 --> 00:01:16,290 The init daemon refers to a configuration file 35 00:01:16,290 --> 00:01:19,530 and initiates the processes that are listed in that file. 36 00:01:19,530 --> 00:01:20,880 This prepares your system 37 00:01:20,880 --> 00:01:23,100 to run the required software for you. 38 00:01:23,100 --> 00:01:25,230 The init daemon is going to run continuously 39 00:01:25,230 --> 00:01:27,150 until the system is powered off. 40 00:01:27,150 --> 00:01:30,090 And the programs on your system are not able to run 41 00:01:30,090 --> 00:01:31,413 without the init daemon. 42 00:01:32,280 --> 00:01:34,650 In Linux, you can initialize the system 43 00:01:34,650 --> 00:01:37,530 through SysVinit and systemd. 44 00:01:37,530 --> 00:01:38,760 The method that's going to be active 45 00:01:38,760 --> 00:01:40,740 on your Linux system is going to affect 46 00:01:40,740 --> 00:01:43,650 how you manage services on that system. 47 00:01:43,650 --> 00:01:46,980 For example, if you use the systemd software suite, 48 00:01:46,980 --> 00:01:48,690 it's going to provide for an init method 49 00:01:48,690 --> 00:01:50,340 for initializing the system. 50 00:01:50,340 --> 00:01:53,100 It's also going to provide for tools for managing services 51 00:01:53,100 --> 00:01:56,310 on that system that derive from the init daemon. 52 00:01:56,310 --> 00:01:58,860 The systemd suite was designed as a replacement 53 00:01:58,860 --> 00:02:01,380 for other methods like SysVinit 54 00:02:01,380 --> 00:02:03,900 and is now the dominant init method that's used 55 00:02:03,900 --> 00:02:05,970 in modern Linux distributions. 56 00:02:05,970 --> 00:02:08,729 The systemd suite is going to offer several improvements 57 00:02:08,729 --> 00:02:10,590 over older methods as well. 58 00:02:10,590 --> 00:02:13,110 For example, it supports parallelization, 59 00:02:13,110 --> 00:02:16,380 like starting programs at the same time for quicker boot up 60 00:02:16,380 --> 00:02:18,360 and reducing shell overhead. 61 00:02:18,360 --> 00:02:22,200 In systemd, control groups are also used to track processes 62 00:02:22,200 --> 00:02:24,990 instead of process IDs, or PIDs, 63 00:02:24,990 --> 00:02:26,610 which provides a better isolation 64 00:02:26,610 --> 00:02:29,580 and categorization for all of your processes. 65 00:02:29,580 --> 00:02:32,010 Now, systemd is going to use unit files 66 00:02:32,010 --> 00:02:34,110 to determine how it's going to handle units, 67 00:02:34,110 --> 00:02:37,710 which are system resources that systemd can actually manage. 68 00:02:37,710 --> 00:02:40,260 Resources can include network services, 69 00:02:40,260 --> 00:02:42,990 block storage devices, peripheral devices, 70 00:02:42,990 --> 00:02:45,210 file systems, and many others. 71 00:02:45,210 --> 00:02:46,530 Daemons are going to access 72 00:02:46,530 --> 00:02:48,990 and manage all of these different resources for you, 73 00:02:48,990 --> 00:02:50,850 and unit files can also be used 74 00:02:50,850 --> 00:02:53,730 to set system environment variables and parameters 75 00:02:53,730 --> 00:02:55,500 as well as which values are going to be passed 76 00:02:55,500 --> 00:02:58,980 from a parent process to a child process that it creates. 77 00:02:58,980 --> 00:03:01,380 By adding directives for an environment variable, 78 00:03:01,380 --> 00:03:03,420 you're going to be able to make it easier for a service 79 00:03:03,420 --> 00:03:06,930 or other unit to work with custom values as well. 80 00:03:06,930 --> 00:03:09,210 In systemd, targets are a method 81 00:03:09,210 --> 00:03:12,600 of grouping unit configuration files together, typically, 82 00:03:12,600 --> 00:03:15,690 in order to represent specific modes of operations. 83 00:03:15,690 --> 00:03:18,210 Each dot target file is going to be used 84 00:03:18,210 --> 00:03:20,070 to determine one of several ways 85 00:03:20,070 --> 00:03:21,660 in which the system can operate, 86 00:03:21,660 --> 00:03:24,060 such as running with just a command line environment, 87 00:03:24,060 --> 00:03:26,220 running with a graphical desktop environment, 88 00:03:26,220 --> 00:03:29,250 initiating a system shutdown, and many others. 89 00:03:29,250 --> 00:03:32,130 Therefore, you can activate a target in order to boot 90 00:03:32,130 --> 00:03:35,040 into a desired environment if you want to. 91 00:03:35,040 --> 00:03:36,810 Now, another command we need to talk about 92 00:03:36,810 --> 00:03:39,810 is systemctl, or system control. 93 00:03:39,810 --> 00:03:41,910 The system control command enables you 94 00:03:41,910 --> 00:03:44,370 to control the systemd init daemon, 95 00:03:44,370 --> 00:03:46,380 and you can view running services, 96 00:03:46,380 --> 00:03:48,930 enable and disable services to run at boot 97 00:03:48,930 --> 00:03:50,580 or in the current session, 98 00:03:50,580 --> 00:03:53,280 and you can determine the status of these services 99 00:03:53,280 --> 00:03:55,590 as well as managing the system target. 100 00:03:55,590 --> 00:03:57,930 You can do all of this through various subcommands 101 00:03:57,930 --> 00:04:00,930 that are issued along with the systemctl command, 102 00:04:00,930 --> 00:04:03,750 including status service, which is going to be used 103 00:04:03,750 --> 00:04:06,270 to retrieve the current status of a given service, 104 00:04:06,270 --> 00:04:09,090 enable service, which is used to enable a service 105 00:04:09,090 --> 00:04:12,120 to be started on boot, disable service, 106 00:04:12,120 --> 00:04:14,100 which is going to be used to disable a service 107 00:04:14,100 --> 00:04:17,850 so it no longer is started up on boot, start service, 108 00:04:17,850 --> 00:04:20,310 which is used to activate a service immediately, 109 00:04:20,310 --> 00:04:22,200 stop service, which is used 110 00:04:22,200 --> 00:04:25,770 to deactivate a service immediately, restart service, 111 00:04:25,770 --> 00:04:28,170 which is used to restart a service immediately, 112 00:04:28,170 --> 00:04:31,560 set-default target, which is used to set the default target 113 00:04:31,560 --> 00:04:33,780 for the system to be used on boot, 114 00:04:33,780 --> 00:04:36,720 isolate target, which is used to force the system 115 00:04:36,720 --> 00:04:40,680 to immediately change the provided target, mask unit file, 116 00:04:40,680 --> 00:04:42,870 which is used to prevent the provided unit file 117 00:04:42,870 --> 00:04:44,940 from being enabled or activated 118 00:04:44,940 --> 00:04:48,240 even when it's done manually, and daemon-reload, 119 00:04:48,240 --> 00:04:51,030 which is used to reload the systemd init daemon, 120 00:04:51,030 --> 00:04:53,220 including all of the unit files. 121 00:04:53,220 --> 00:04:55,140 To run the system control command, 122 00:04:55,140 --> 00:04:59,700 type in systemctl, space, the options, space, 123 00:04:59,700 --> 00:05:02,850 the subcommand, space, and the arguments. 124 00:05:02,850 --> 00:05:04,140 The system control command 125 00:05:04,140 --> 00:05:06,240 has many different options you can use, 126 00:05:06,240 --> 00:05:10,110 including -t unit file type, which is an option that's used 127 00:05:10,110 --> 00:05:13,800 to specify the unit file type to perform the operation on. 128 00:05:13,800 --> 00:05:16,560 The -a option is used to list all the unit files 129 00:05:16,560 --> 00:05:18,930 or properties regardless of their state. 130 00:05:18,930 --> 00:05:21,360 The dash no-reload option is used 131 00:05:21,360 --> 00:05:23,670 to prevent the reloading of configuration changes 132 00:05:23,670 --> 00:05:25,980 when enabling or disabling a service. 133 00:05:25,980 --> 00:05:29,280 The --no-ask-password option is used 134 00:05:29,280 --> 00:05:31,620 to prevent users from being asked to authenticate 135 00:05:31,620 --> 00:05:34,110 when they're performing privileged operations. 136 00:05:34,110 --> 00:05:36,810 The --runtime option is going to be used 137 00:05:36,810 --> 00:05:38,310 to make changes temporary, 138 00:05:38,310 --> 00:05:41,790 this way they won't be present after you reboot the system. 139 00:05:41,790 --> 00:05:44,790 The -H remote host option is going to be used 140 00:05:44,790 --> 00:05:47,220 to execute the operation on a remote host 141 00:05:47,220 --> 00:05:50,220 that's specified by its host name or IP address. 142 00:05:50,220 --> 00:05:53,880 And the --no-pager option is used to prevent the output 143 00:05:53,880 --> 00:05:56,460 from being piped into a paging utility. 144 00:05:56,460 --> 00:05:58,290 There are lots of different instances 145 00:05:58,290 --> 00:06:00,810 when a system administrator needs to work in a GUI 146 00:06:00,810 --> 00:06:02,400 for a short period of time, 147 00:06:02,400 --> 00:06:05,400 and you can do this by using the systemctl command 148 00:06:05,400 --> 00:06:07,020 to change your target. 149 00:06:07,020 --> 00:06:09,210 Now, it's common to configure a Linux server 150 00:06:09,210 --> 00:06:12,000 to boot up by default to the command line environment. 151 00:06:12,000 --> 00:06:15,750 And you do this by using the multi-user dot target file. 152 00:06:15,750 --> 00:06:17,670 An administrator though might want to switch 153 00:06:17,670 --> 00:06:19,380 to the graphical user environment, 154 00:06:19,380 --> 00:06:22,200 which is known as the graphical dot target file. 155 00:06:22,200 --> 00:06:24,390 And we can do this for a particular task. 156 00:06:24,390 --> 00:06:26,340 And then we can switch the server back 157 00:06:26,340 --> 00:06:30,600 to the multi-user dot target file when the task is complete. 158 00:06:30,600 --> 00:06:32,790 The system administrator would then leave the server 159 00:06:32,790 --> 00:06:35,190 in a more efficient command line configuration 160 00:06:35,190 --> 00:06:36,720 after the task is completed, 161 00:06:36,720 --> 00:06:39,780 which lowers the resources required to run that server 162 00:06:39,780 --> 00:06:42,060 instead of leaving that graphical user interface 163 00:06:42,060 --> 00:06:43,350 up and running. 164 00:06:43,350 --> 00:06:46,110 In most cases, the systemctl command 165 00:06:46,110 --> 00:06:48,090 is going to be used to control services, 166 00:06:48,090 --> 00:06:50,730 but there are some additional systemd commands 167 00:06:50,730 --> 00:06:52,530 that you can also use too. 168 00:06:52,530 --> 00:06:55,620 For example, hostnamectl will enable you 169 00:06:55,620 --> 00:06:57,600 to view the system's network host name 170 00:06:57,600 --> 00:07:00,270 and other information about the system's hardware 171 00:07:00,270 --> 00:07:02,100 and the Linux kernel it's running. 172 00:07:02,100 --> 00:07:03,300 You can also use the command 173 00:07:03,300 --> 00:07:05,190 to change the system's host name. 174 00:07:05,190 --> 00:07:09,000 To run this command, type hostnamectl, the options, 175 00:07:09,000 --> 00:07:11,100 the subcommand, and the arguments. 176 00:07:11,100 --> 00:07:15,060 For example, if you want to set your host name to server01, 177 00:07:15,060 --> 00:07:20,060 you need to enter hostnamectl, space, set-hostname, space, 178 00:07:20,790 --> 00:07:24,870 server01 at the command line, and then hit enter. 179 00:07:24,870 --> 00:07:27,210 Now, earlier I mentioned System V init, 180 00:07:27,210 --> 00:07:28,920 which is an older init method 181 00:07:28,920 --> 00:07:31,620 that has largely been replaced by systemd. 182 00:07:31,620 --> 00:07:33,420 However, some distributions 183 00:07:33,420 --> 00:07:35,940 will still support System V init. 184 00:07:35,940 --> 00:07:37,410 Just like systemd, 185 00:07:37,410 --> 00:07:40,020 SysVinit is going to provide you with various tools 186 00:07:40,020 --> 00:07:42,720 to manage services and the state of the system. 187 00:07:42,720 --> 00:07:45,360 Now, one of the major differences between systemd 188 00:07:45,360 --> 00:07:49,590 and System V init is that System V init uses runlevels. 189 00:07:49,590 --> 00:07:51,720 They control the state of the operating system 190 00:07:51,720 --> 00:07:55,140 in much the same way that systemd targets are going to be used. 191 00:07:55,140 --> 00:07:56,160 And they're going to determine 192 00:07:56,160 --> 00:07:58,260 what type of daemons should be running 193 00:07:58,260 --> 00:08:01,560 in order to create a specific type of environment for you. 194 00:08:01,560 --> 00:08:04,350 In fact, the systemd targets were actually created 195 00:08:04,350 --> 00:08:06,480 to map the existing runlevels. 196 00:08:06,480 --> 00:08:08,820 When we use something like runlevel zero, 197 00:08:08,820 --> 00:08:10,530 this shuts down the system. 198 00:08:10,530 --> 00:08:13,800 Runlevel one starts up a single-user mode. 199 00:08:13,800 --> 00:08:16,440 Runlevel two starts up a multi-user mode 200 00:08:16,440 --> 00:08:18,030 without remote networking. 201 00:08:18,030 --> 00:08:20,490 Runlevel three starts multi-user mode 202 00:08:20,490 --> 00:08:22,020 with remote networking. 203 00:08:22,020 --> 00:08:24,418 Runlevel four is not really used. 204 00:08:24,418 --> 00:08:27,030 Runlevel five though starts multi-user mode 205 00:08:27,030 --> 00:08:30,360 with networking and a graphical user interface capability, 206 00:08:30,360 --> 00:08:33,330 and runlevel six is going to reboot your system. 207 00:08:33,330 --> 00:08:36,270 Now, a single-user mode is going to boot the operating system 208 00:08:36,270 --> 00:08:39,900 into an environment where the superuser must log in. 209 00:08:39,900 --> 00:08:42,630 This means you're logging in as the root user. 210 00:08:42,630 --> 00:08:45,600 Networking here is disabled in single-user mode, 211 00:08:45,600 --> 00:08:48,180 and most partitions are not going to be mounted. 212 00:08:48,180 --> 00:08:50,730 Typically, this is going to be used to troubleshoot issues 213 00:08:50,730 --> 00:08:52,740 with networking or issues that prevent you 214 00:08:52,740 --> 00:08:55,740 from booting into your normal multi-user environment. 215 00:08:55,740 --> 00:08:58,200 The telinit command will enable you to switch 216 00:08:58,200 --> 00:09:00,570 to the current runlevel of the system. 217 00:09:00,570 --> 00:09:02,850 Now, on the systemd environments, 218 00:09:02,850 --> 00:09:05,190 the telinit command is going to be translated 219 00:09:05,190 --> 00:09:08,520 into the appropriate target request for you automatically. 220 00:09:08,520 --> 00:09:10,800 The runlevel command will print the previous 221 00:09:10,800 --> 00:09:12,960 and current runlevels of the system, 222 00:09:12,960 --> 00:09:15,540 each one being separated by a space. 223 00:09:15,540 --> 00:09:17,820 So to use the telinit command, 224 00:09:17,820 --> 00:09:22,820 simply type in T-E-L-I-N-I-T, the options, and the runlevel. 225 00:09:23,700 --> 00:09:26,130 Now, there is a special file on your system 226 00:09:26,130 --> 00:09:28,680 if you're using a SysVinit system. 227 00:09:28,680 --> 00:09:33,680 This is known as the slash E-T-C slash I-N-I-T-T-A-B file. 228 00:09:34,290 --> 00:09:35,850 This is the inittab file, 229 00:09:35,850 --> 00:09:38,430 and it's going to store details of various processes 230 00:09:38,430 --> 00:09:42,540 related to your system initialization on a SysVinit system. 231 00:09:42,540 --> 00:09:43,980 It's also going to store details 232 00:09:43,980 --> 00:09:46,290 of the different runlevels that are in use. 233 00:09:46,290 --> 00:09:48,720 The init daemon is going to read from this file 234 00:09:48,720 --> 00:09:51,030 to determine what runlevel to boot into, 235 00:09:51,030 --> 00:09:52,380 what daemons to start, 236 00:09:52,380 --> 00:09:55,020 and what to do if the runlevel changes. 237 00:09:55,020 --> 00:10:00,020 Every entry inside the /etc/inittab file takes the format 238 00:10:00,090 --> 00:10:04,990 of id:rstate:action:process. 239 00:10:06,210 --> 00:10:09,600 The id is just a unique identifier for that entry. 240 00:10:09,600 --> 00:10:13,260 The rstate defines what runlevels they're going to apply to. 241 00:10:13,260 --> 00:10:16,080 And the action specifies one of several tasks 242 00:10:16,080 --> 00:10:19,230 that determine how SysVinit is going to handle the command 243 00:10:19,230 --> 00:10:21,660 that's defined in the process field. 244 00:10:21,660 --> 00:10:25,260 The /etc/init.d directory is going to store 245 00:10:25,260 --> 00:10:28,560 all of your initialization scripts for all of your services. 246 00:10:28,560 --> 00:10:31,010 These scripts control the initiation of services 247 00:10:31,010 --> 00:10:32,260 in a particular runlevel. 248 00:10:33,100 --> 00:10:37,110 The scripts are going to be invoked from the /etc/inittab file 249 00:10:37,110 --> 00:10:39,330 when the system initialization begins 250 00:10:39,330 --> 00:10:43,080 using different symbolic links that are found in that file. 251 00:10:43,080 --> 00:10:45,660 SysVinit scripts are highly flexible 252 00:10:45,660 --> 00:10:47,370 and can be configured according to the needs 253 00:10:47,370 --> 00:10:48,990 of any given user. 254 00:10:48,990 --> 00:10:53,580 The /etc/rc.local file is also going to be executed 255 00:10:53,580 --> 00:10:55,740 at the end of the init boot process. 256 00:10:55,740 --> 00:10:58,860 Typically, it's going to be used to start custom services. 257 00:10:58,860 --> 00:11:01,290 However, this file is rarely used 258 00:11:01,290 --> 00:11:03,960 and is not even supported in some distributions 259 00:11:03,960 --> 00:11:06,480 that still use SysVinit. 260 00:11:06,480 --> 00:11:08,130 Now, another command we need to talk about 261 00:11:08,130 --> 00:11:10,380 is the check configuration command. 262 00:11:10,380 --> 00:11:14,850 This is spelled C-H-K-C-O-N-F-I-G. 263 00:11:14,850 --> 00:11:16,890 The check configuration command can be used 264 00:11:16,890 --> 00:11:19,380 to control services in each runlevel. 265 00:11:19,380 --> 00:11:21,690 It's also used to start or stop services 266 00:11:21,690 --> 00:11:23,550 during system startup. 267 00:11:23,550 --> 00:11:25,710 Some subcommands and options that can be used 268 00:11:25,710 --> 00:11:27,360 with the check configuration command 269 00:11:27,360 --> 00:11:30,450 to control your services include service on. 270 00:11:30,450 --> 00:11:32,850 This is an option which is used to enable a service 271 00:11:32,850 --> 00:11:34,470 to be started on boot. 272 00:11:34,470 --> 00:11:36,510 Another one is service off. 273 00:11:36,510 --> 00:11:38,790 This option is used to disable a service 274 00:11:38,790 --> 00:11:40,830 so it's no longer started on boot. 275 00:11:40,830 --> 00:11:43,140 Service reset is an option that's used 276 00:11:43,140 --> 00:11:45,330 to reset the status of a service, 277 00:11:45,330 --> 00:11:48,030 and the --level runlevel option is used 278 00:11:48,030 --> 00:11:49,500 to specify the runlevel 279 00:11:49,500 --> 00:11:52,080 in which to enable or disable a service. 280 00:11:52,080 --> 00:11:57,080 To use this command, simply type in C-H-K-C-O-N-F-I-G, 281 00:11:57,240 --> 00:11:59,583 the options, the service, and the subcommand. 282 00:12:00,570 --> 00:12:03,210 Now, another command we have is the service command. 283 00:12:03,210 --> 00:12:06,180 This is another way to control the SysVinit services 284 00:12:06,180 --> 00:12:08,280 through the SysVinit scripts. 285 00:12:08,280 --> 00:12:10,650 It's going to support various common subcommands 286 00:12:10,650 --> 00:12:12,240 including service status, 287 00:12:12,240 --> 00:12:14,880 which is used to print the current state of a service, 288 00:12:14,880 --> 00:12:18,090 service start, which is used to start a service immediately, 289 00:12:18,090 --> 00:12:21,030 service stop to stop a service immediately, 290 00:12:21,030 --> 00:12:24,060 service restart to restart a service immediately, 291 00:12:24,060 --> 00:12:27,630 and service reload to reread a service's configuration files 292 00:12:27,630 --> 00:12:29,490 while the service remains running. 293 00:12:29,490 --> 00:12:32,430 To run the service command, simply type in service, 294 00:12:32,430 --> 00:12:34,743 the options, the service, and the subcommand.