1 00:00:00,570 --> 00:00:01,589 In this lesson 2 00:00:01,589 --> 00:00:03,870 we're going to discuss how you can schedule tasks 3 00:00:03,870 --> 00:00:05,670 with Cron to help you automate 4 00:00:05,670 --> 00:00:08,970 and perform tasks repetitively without much effort. 5 00:00:08,970 --> 00:00:11,010 This means you'll learn how the programs can run 6 00:00:11,010 --> 00:00:13,980 at specific times, such as every day at 2:00 AM, 7 00:00:13,980 --> 00:00:16,770 and you don't even have to stay up late to run these things. 8 00:00:16,770 --> 00:00:19,350 Now to do this, we're going to use Bash Scripting. 9 00:00:19,350 --> 00:00:20,340 And Bash Scripting 10 00:00:20,340 --> 00:00:22,630 is a powerful part of system administration 11 00:00:23,553 --> 00:00:25,860 and development that can be used all the time. 12 00:00:25,860 --> 00:00:27,900 It's going to be used by system administrators, 13 00:00:27,900 --> 00:00:29,910 network administrators, developers, 14 00:00:29,910 --> 00:00:32,700 scientists, and pretty much anyone who uses Linux 15 00:00:32,700 --> 00:00:35,160 or Unix as their operating system. 16 00:00:35,160 --> 00:00:38,340 These folks can all use Bash for system administration, 17 00:00:38,340 --> 00:00:41,160 data crunching, web application deployment, 18 00:00:41,160 --> 00:00:43,620 automated backups, creating custom scripts 19 00:00:43,620 --> 00:00:46,410 for various pages, and other things like that. 20 00:00:46,410 --> 00:00:49,590 By using Bash Scripting, we can contain a set of commands 21 00:00:49,590 --> 00:00:51,540 for an appropriate runtime environment, 22 00:00:51,540 --> 00:00:52,920 which is then going to be used to automate 23 00:00:52,920 --> 00:00:55,410 the execution of whatever tasks we want. 24 00:00:55,410 --> 00:00:57,360 However, there are plenty more methods 25 00:00:57,360 --> 00:01:00,180 for automating tasks that you can perform every day as well, 26 00:01:00,180 --> 00:01:03,090 and you don't even have to use Bash Scripting necessarily. 27 00:01:03,090 --> 00:01:04,290 Now, one of the challenges 28 00:01:04,290 --> 00:01:07,230 among the many advantages of being a system administrator 29 00:01:07,230 --> 00:01:08,730 is that you have to run tasks 30 00:01:08,730 --> 00:01:10,110 when you would rather be sleeping, 31 00:01:10,110 --> 00:01:13,110 because most of our maintenance is done overnight. 32 00:01:13,110 --> 00:01:16,560 For example, some tasks include regular recurring tasks 33 00:01:16,560 --> 00:01:18,900 that you need to run overnight, like doing a backup, 34 00:01:18,900 --> 00:01:20,880 and no one is going to be expected to be using 35 00:01:20,880 --> 00:01:24,030 the computer resources during that overnight timeframe, 36 00:01:24,030 --> 00:01:26,910 so it's a perfect time to get this maintenance done. 37 00:01:26,910 --> 00:01:28,560 Instead of you having to drive to work 38 00:01:28,560 --> 00:01:30,990 and be there every night at 3:00 AM, 39 00:01:30,990 --> 00:01:33,990 you can just write a script and have it do it for you. 40 00:01:33,990 --> 00:01:35,010 Now, personally, 41 00:01:35,010 --> 00:01:37,380 I like to use two different service utilities 42 00:01:37,380 --> 00:01:39,390 that will allow me to run different commands, 43 00:01:39,390 --> 00:01:42,420 programs, and tasks at predetermined times. 44 00:01:42,420 --> 00:01:45,240 These are the Cron and at services. 45 00:01:45,240 --> 00:01:47,280 Now Cron and at are services 46 00:01:47,280 --> 00:01:49,740 that enable system administrators to schedule tasks 47 00:01:49,740 --> 00:01:52,710 to run at a specific time in the future. 48 00:01:52,710 --> 00:01:55,230 The Cron service is used to do repetitive tasks, 49 00:01:55,230 --> 00:01:58,620 like every Monday at 8:00 PM, whereas the at service 50 00:01:58,620 --> 00:02:00,540 is going to provide a onetime task 51 00:02:00,540 --> 00:02:02,640 that runs at a specific time. 52 00:02:02,640 --> 00:02:04,080 Now the at command schedules 53 00:02:04,080 --> 00:02:06,330 a command to run at a particular time 54 00:02:06,330 --> 00:02:08,490 that you normally have permission to run it at. 55 00:02:08,490 --> 00:02:10,440 So in other words, it's not going to be designed 56 00:02:10,440 --> 00:02:13,320 for repetitive or regularly scheduled tasks. 57 00:02:13,320 --> 00:02:15,810 Instead, we're going to use the at command 58 00:02:15,810 --> 00:02:18,450 that can do something like sending a simple reminder message 59 00:02:18,450 --> 00:02:20,910 or it can run a complex script. 60 00:02:20,910 --> 00:02:23,820 Either way, you know that you're going to run it one time 61 00:02:23,820 --> 00:02:27,090 and only one time, but at some time in the future. 62 00:02:27,090 --> 00:02:29,040 You can start out by running the at command 63 00:02:29,040 --> 00:02:30,930 at the command line and pass to it 64 00:02:30,930 --> 00:02:33,270 the schedule time as your option. 65 00:02:33,270 --> 00:02:35,940 Then it'll place you at a special prompt, 66 00:02:35,940 --> 00:02:38,310 and at this prompt, you can type in the command 67 00:02:38,310 --> 00:02:40,770 or the series of commands that you want to run 68 00:02:40,770 --> 00:02:42,420 at a specific time. 69 00:02:42,420 --> 00:02:43,830 When you're all done with that, 70 00:02:43,830 --> 00:02:46,260 you'll press Control + D on a new line 71 00:02:46,260 --> 00:02:49,020 and your command will be placed into the queue to run 72 00:02:49,020 --> 00:02:50,940 at the time you had specified. 73 00:02:50,940 --> 00:02:53,820 To run the at command, simply type in at, 74 00:02:53,820 --> 00:02:56,730 the options, and the time that you want it to run at. 75 00:02:56,730 --> 00:03:00,210 Some of the at command options include things like -m. 76 00:03:00,210 --> 00:03:03,120 This is an option that's used to send mail to a user 77 00:03:03,120 --> 00:03:06,796 when the job completes, regardless of the output. 78 00:03:06,796 --> 00:03:08,820 The -M option will be used 79 00:03:08,820 --> 00:03:11,610 if you want to prevent mail from being sent to the user. 80 00:03:11,610 --> 00:03:14,220 Now, the -f and the file name option 81 00:03:14,220 --> 00:03:16,590 is going to be used to read a job from a file 82 00:03:16,590 --> 00:03:18,510 rather than from standard input. 83 00:03:18,510 --> 00:03:21,630 And the -t and the time option is going to be used 84 00:03:21,630 --> 00:03:24,120 to run the job at a specific time value, 85 00:03:24,120 --> 00:03:26,580 whereas the -v option is used to display 86 00:03:26,580 --> 00:03:28,800 the time the job will be executed. 87 00:03:28,800 --> 00:03:31,620 And essentially it just means -v for view. 88 00:03:31,620 --> 00:03:34,560 So you can see what things are in the queue to run later. 89 00:03:34,560 --> 00:03:36,180 When you're using the at command, 90 00:03:36,180 --> 00:03:39,060 there's a lot of different ways to specify the time. 91 00:03:39,060 --> 00:03:41,160 For instance, there's a couple of keywords. 92 00:03:41,160 --> 00:03:42,810 For instance, if you say noon, 93 00:03:42,810 --> 00:03:44,310 that means 12:00 PM. 94 00:03:44,310 --> 00:03:47,100 If you say tea time, that means 4:00 PM. 95 00:03:47,100 --> 00:03:50,040 If you say midnight, that means 12:00 AM. 96 00:03:50,040 --> 00:03:52,230 If I type now + 3 minutes, 97 00:03:52,230 --> 00:03:54,690 that's going to specify I want the time right now, 98 00:03:54,690 --> 00:03:57,150 plus three minutes for that action to happen. 99 00:03:57,150 --> 00:04:00,630 Or now + 1 hour, would then specify the time 100 00:04:00,630 --> 00:04:02,730 plus one hour from now. 101 00:04:02,730 --> 00:04:04,890 If you use the atq command, 102 00:04:04,890 --> 00:04:06,360 this is going to show you the queue 103 00:04:06,360 --> 00:04:09,420 for all the tasks that are scheduled by the at command. 104 00:04:09,420 --> 00:04:11,280 If you type atrm, 105 00:04:11,280 --> 00:04:13,980 this will actually be used to delete a scheduled task. 106 00:04:13,980 --> 00:04:16,410 It is basically the removal command. 107 00:04:16,410 --> 00:04:19,500 Now, system administrators and users may want to have scripts 108 00:04:19,500 --> 00:04:22,200 or commands that execute on a regular basis, 109 00:04:22,200 --> 00:04:24,180 and at is not very helpful for that, 110 00:04:24,180 --> 00:04:27,150 because at only schedules things one time. 111 00:04:27,150 --> 00:04:30,210 Instead, if we want this regular type of automation, 112 00:04:30,210 --> 00:04:32,340 like every Monday at midnight, 113 00:04:32,340 --> 00:04:34,380 we would use something like Cron. 114 00:04:34,380 --> 00:04:36,480 The Cron Daemon is going to be used to manage 115 00:04:37,420 --> 00:04:39,180 these scheduled tasks called cron jobs. 116 00:04:39,180 --> 00:04:41,940 And these cron jobs will happen every single time 117 00:04:41,940 --> 00:04:43,590 at repeating interval. 118 00:04:43,590 --> 00:04:47,100 The Cron Daemon will check its crontab configuration file 119 00:04:47,100 --> 00:04:50,070 every single minute to discover whether there's any tasks 120 00:04:50,070 --> 00:04:51,480 that need to be accomplished. 121 00:04:51,480 --> 00:04:53,640 If there are, it's going to execute those. 122 00:04:53,640 --> 00:04:55,530 If not, it goes back to sleep, 123 00:04:55,530 --> 00:04:58,050 it waits a minute, and then it checks again. 124 00:04:58,050 --> 00:05:01,680 The Cron Daemon is controlled using the crontab command. 125 00:05:01,680 --> 00:05:03,750 This command assumes the current user, 126 00:05:03,750 --> 00:05:07,650 unless you use the -u option to specify somebody else. 127 00:05:07,650 --> 00:05:09,210 You can also create, view, 128 00:05:09,210 --> 00:05:12,870 and delete crontab files using the crontab command. 129 00:05:12,870 --> 00:05:14,910 Some of the options of the crontab command 130 00:05:14,910 --> 00:05:16,830 include things like -e. 131 00:05:16,830 --> 00:05:19,710 This is an option that's used to edit the crontab file 132 00:05:19,710 --> 00:05:21,600 for the current user. 133 00:05:21,600 --> 00:05:25,770 l is used to view the crontab file for the current user. 134 00:05:25,770 --> 00:05:29,220 r is going to be used to delete the current crontab file. 135 00:05:29,220 --> 00:05:32,130 And -u will be able to create a crontab file 136 00:05:32,130 --> 00:05:34,500 on behalf of a specified user. 137 00:05:34,500 --> 00:05:37,290 To run crontab, simply write crontab, 138 00:05:37,290 --> 00:05:40,740 the options, and the file or user you want to specify. 139 00:05:40,740 --> 00:05:43,050 Now, the crontab file are going to be referenced 140 00:05:43,050 --> 00:05:44,850 by the Cron Daemon to determine 141 00:05:44,850 --> 00:05:47,370 what scheduled tasks might exist. 142 00:05:47,370 --> 00:05:49,650 The crontab file are going to be managed using 143 00:05:49,650 --> 00:05:52,620 the crontab -e command, like we just talked about. 144 00:05:52,620 --> 00:05:57,000 And this command will open up the Vim text editor by default 145 00:05:57,000 --> 00:06:00,300 because this is the default editor on most Linux systems. 146 00:06:00,300 --> 00:06:02,370 This will then allow a user to specify 147 00:06:02,370 --> 00:06:06,300 when they want to run a task and what task they want to run. 148 00:06:06,300 --> 00:06:08,370 Now, when you look at the crontab file, 149 00:06:08,370 --> 00:06:10,860 you're going to see a list of every single scheduled task 150 00:06:10,860 --> 00:06:12,540 on that Linux server. 151 00:06:12,540 --> 00:06:14,790 This is going to be in the format of ***** 152 00:06:18,396 --> 00:06:19,590 and then the command. 153 00:06:19,590 --> 00:06:21,480 For example, I might have something 154 00:06:21,480 --> 00:06:26,480 that says 4523**6/home/user/scripts/exportdump.sh. 155 00:06:33,210 --> 00:06:37,230 Now what this says is that 45 minutes after the 23rd hour 156 00:06:37,230 --> 00:06:40,800 on the sixth day, I want to run this script. 157 00:06:40,800 --> 00:06:42,270 Now, when you look at these columns, 158 00:06:42,270 --> 00:06:44,010 the first column is always the minute, 159 00:06:44,010 --> 00:06:46,950 the second is the hour, the third is the day, 160 00:06:46,950 --> 00:06:48,360 the fourth is the month, 161 00:06:48,360 --> 00:06:50,430 and the fifth is the day of the week. 162 00:06:50,430 --> 00:06:54,060 So in this case, I said at 11:45 at night 163 00:06:54,060 --> 00:06:56,490 on the sixth day of the week, which is Saturday, 164 00:06:56,490 --> 00:06:59,340 you are going to do this export dump script. 165 00:06:59,340 --> 00:07:02,010 Now in that script can be anything I want, 166 00:07:02,010 --> 00:07:03,120 I can have something in there 167 00:07:03,120 --> 00:07:04,860 that's going to dump the password credentials 168 00:07:04,860 --> 00:07:05,910 on this server. 169 00:07:05,910 --> 00:07:07,650 You can set these up however you want, 170 00:07:07,650 --> 00:07:10,200 but the idea is that using crontab gives you 171 00:07:10,200 --> 00:07:13,110 a way to schedule tasks inside of Linux. 172 00:07:13,110 --> 00:07:15,667 For example, let's say there's a line that says, 173 00:07:15,667 --> 00:07:20,667 *20**2-5/path/to/command 174 00:07:23,130 --> 00:07:25,350 inside of my crontab file. 175 00:07:25,350 --> 00:07:27,660 When it executes, it's going to do this 176 00:07:27,660 --> 00:07:29,700 based on the time in this line. 177 00:07:29,700 --> 00:07:33,900 The time here is 2000 hours, which translates to 8:00 PM, 178 00:07:33,900 --> 00:07:36,090 and it's going to happen Monday through Friday, 179 00:07:36,090 --> 00:07:38,700 which is days one through five of the week. 180 00:07:38,700 --> 00:07:43,700 Now, if I had another line that said 152**/path/to/command, 181 00:07:46,320 --> 00:07:50,760 this will execute at 2:15 AM every single day, 182 00:07:50,760 --> 00:07:53,520 and then this tells me that because I have the second hour 183 00:07:53,520 --> 00:07:56,280 and the 15 minutes in that line. 184 00:07:56,280 --> 00:07:57,840 And then I'm doing it every day, 185 00:07:57,840 --> 00:08:01,560 because that final thing that says day is actually a star. 186 00:08:01,560 --> 00:08:03,450 Now, if I had another line that said, 187 00:08:03,450 --> 00:08:08,370 3041**/path/2/command, 188 00:08:10,200 --> 00:08:12,660 what do you think this is going to execute as? 189 00:08:12,660 --> 00:08:15,330 Well, this is going to be 4:30 AM, 190 00:08:15,330 --> 00:08:18,630 and then that third position tells me the day of the month. 191 00:08:18,630 --> 00:08:21,690 And in this case, it's the first day of every month. 192 00:08:21,690 --> 00:08:23,340 And so here you could see how we specify 193 00:08:23,340 --> 00:08:25,080 these different crontab lines 194 00:08:25,080 --> 00:08:26,700 to be able to specify the exact time 195 00:08:26,700 --> 00:08:28,350 and the exact days that we want, 196 00:08:28,350 --> 00:08:31,620 either by day of the month or days of the week. 197 00:08:31,620 --> 00:08:33,390 The crontab file that contains 198 00:08:33,390 --> 00:08:35,070 the scheduled task information 199 00:08:35,070 --> 00:08:36,659 can be found in two directories. 200 00:08:36,659 --> 00:08:40,077 The first is /etc/cron.d. 201 00:08:40,077 --> 00:08:44,370 And the second is /var/spool/cron. 202 00:08:44,370 --> 00:08:46,200 Both of these are directories. 203 00:08:46,200 --> 00:08:49,590 Now your route user can also schedule systemwide tasks 204 00:08:49,590 --> 00:08:53,460 by using the /etc/cron.d directory. 205 00:08:53,460 --> 00:08:57,390 Services can also add scheduled tasks to this location too. 206 00:08:57,390 --> 00:08:59,700 The root user can add tasks to the directory 207 00:08:59,700 --> 00:09:04,260 or they can add tasks to the /etc/crontab file. 208 00:09:04,260 --> 00:09:06,960 Regular users though are not allowed to populate 209 00:09:06,960 --> 00:09:09,690 the /etc/cron directory. 210 00:09:09,690 --> 00:09:13,140 So a standard user has to schedule their own tasks 211 00:09:13,140 --> 00:09:14,520 in a personal directory. 212 00:09:14,520 --> 00:09:19,050 And this is located at /var/spool/cron. 213 00:09:19,050 --> 00:09:20,610 So you could see the difference here. 214 00:09:20,610 --> 00:09:23,010 If it's a root task or a service task, 215 00:09:23,010 --> 00:09:26,850 it's going to be in the /etc/cron.d directory. 216 00:09:26,850 --> 00:09:29,040 If it's a specific user who's scheduling it, 217 00:09:29,040 --> 00:09:33,360 it's going to be in /var/spool/cron. 218 00:09:33,360 --> 00:09:35,670 Now any test listed here are going to execute 219 00:09:35,670 --> 00:09:37,590 with the standard user credentials, 220 00:09:37,590 --> 00:09:40,800 because it's only been done by the user. 221 00:09:40,800 --> 00:09:43,590 If you're doing things in the /etc directory, 222 00:09:43,590 --> 00:09:46,320 you're also going to have other default cron directories 223 00:09:46,320 --> 00:09:48,480 that administrators can use to place scripts in 224 00:09:48,480 --> 00:09:50,940 that'll be executed on a regular basis. 225 00:09:50,940 --> 00:09:52,530 These directories include things 226 00:09:52,530 --> 00:09:56,293 like /etc/cron.hourly, 227 00:09:56,293 --> 00:09:58,863 /etc/cron.daily, 228 00:09:58,863 --> 00:10:02,250 /etc/cron.weekly, 229 00:10:02,250 --> 00:10:05,600 and /etc/cron.monthly. 230 00:10:05,600 --> 00:10:07,050 As you can probably guess, 231 00:10:07,050 --> 00:10:09,690 these are scripts are going to be executed either hourly, 232 00:10:09,690 --> 00:10:12,300 daily, weekly, or monthly. 233 00:10:12,300 --> 00:10:14,010 Now the link or copy of the script 234 00:10:14,010 --> 00:10:16,380 inside of these directories will be used 235 00:10:16,380 --> 00:10:19,290 to schedule your commands, based on those time frequencies, 236 00:10:19,290 --> 00:10:21,240 and any system administrator can do it 237 00:10:21,240 --> 00:10:23,280 as long as they have the right privileges 238 00:10:23,280 --> 00:10:25,130 and permissions to those directories.