0 1 00:00:03,150 --> 00:00:07,620 In this lesson, we're going to look at what shell scripts are, why you should learn them, and how they 1 2 00:00:07,620 --> 00:00:09,810 can be helpful to you. 2 3 00:00:09,840 --> 00:00:16,200 First off, a shell script is simply a file that contains a series of Linux commands and shell statements. 3 4 00:00:16,750 --> 00:00:22,230 The shell script itself, the file, is made up of ASCII text also called plain text. 4 5 00:00:22,230 --> 00:00:26,190 This means that you need to use a text editor to create shell scripts. 5 6 00:00:26,190 --> 00:00:32,490 Some good command line text editors include vim, emacs, and nano. If you want to create shell scripts in a 6 7 00:00:32,490 --> 00:00:35,480 desktop environment using a graphical user interface, 7 8 00:00:35,670 --> 00:00:42,050 you can use text editors like Atom, Sublime Text, Notepad++, and Text Wrangler. 8 9 00:00:42,090 --> 00:00:44,610 I'm being very careful with my words here. 9 10 00:00:44,700 --> 00:00:51,780 I'm saying text editor for a reason. Do not use a word processor such as Microsoft Word, Libreoffice 10 11 00:00:51,780 --> 00:00:54,960 Writer or Apple's Pages to create shell scripts. 11 12 00:00:54,960 --> 00:01:00,400 Many word processors add additional formatting and make changes to the text that you're typing in. 12 13 00:01:00,420 --> 00:01:07,080 One common thing word processors do is they turn normal, straight quotation marks into curly quotation 13 14 00:01:07,080 --> 00:01:07,810 marks. 14 15 00:01:08,100 --> 00:01:13,830 Those straight quotation marks in shell scripts are actually part of the syntax, so using fancy or curly 15 16 00:01:13,830 --> 00:01:20,370 quotation marks will cause your shell scripts to either report an error or produce some unexpected behaviors. 16 17 00:01:20,400 --> 00:01:26,820 Also, these plain text files do not include any other types of special formatting such as bold text, italics, 17 18 00:01:26,820 --> 00:01:27,820 and so on. 18 19 00:01:28,140 --> 00:01:33,750 When a shell script is executed, it in turn executes the commands listed in the script. 19 20 00:01:33,750 --> 00:01:39,420 It starts at the top and executes the commands on each line one line at a time until the end of the 20 21 00:01:39,420 --> 00:01:40,120 file. 21 22 00:01:40,440 --> 00:01:46,080 Executing a shell script will produce the exact same result as if you had typed in each line by hand 22 23 00:01:46,290 --> 00:01:48,090 directly into the terminal. 23 24 00:01:48,150 --> 00:01:52,550 So, shell scripts are plain text files that contain a series of commands. 24 25 00:01:52,620 --> 00:01:55,920 But why would you want to save a series of commands into a script? 25 26 00:01:56,070 --> 00:01:59,550 For starters, it can save you a lot of time in a work. 26 27 00:01:59,690 --> 00:02:04,410 Any time you find yourself entering multiple shell commands in the terminal in order to accomplish a 27 28 00:02:04,410 --> 00:02:10,090 task and you'll need to do that task again in the future, it's time to write a shell script. 28 29 00:02:10,380 --> 00:02:15,030 Once you figure out the set of commands that needs to be executed in order to get the work done, you 29 30 00:02:15,030 --> 00:02:20,520 can put those commands in a shell script and simply execute the script itself the next time you need 30 31 00:02:20,520 --> 00:02:22,020 to do that task. 31 32 00:02:22,020 --> 00:02:26,820 If it's a task that has to be done on a regular basis that's even better because you can write a shell 32 33 00:02:26,820 --> 00:02:32,220 script and then schedule that script to run at a certain time and then you'll never have to do it yourself 33 34 00:02:32,220 --> 00:02:33,130 again. 34 35 00:02:33,180 --> 00:02:39,630 So, shell scripts save you time and effort by allowing you to eliminate repetitive tasks through automation. 35 36 00:02:39,870 --> 00:02:45,450 That means instead of typing the same commands over and over again you can spend that time doing more 36 37 00:02:45,450 --> 00:02:48,150 valuable and higher level tasks. 37 38 00:02:48,150 --> 00:02:52,950 Another good time to write a shell script is for a task that you have to do more than once, but it's 38 39 00:02:52,950 --> 00:02:54,760 something that you rarely do. 39 40 00:02:54,960 --> 00:02:59,350 I'll give you example of a task I had to do at one of my previous jobs. 40 41 00:02:59,400 --> 00:03:04,200 Each time a new version of this one particular application came out I would have to put a copy of the 41 42 00:03:04,200 --> 00:03:09,960 production database into our test environment so that the new version of the application could be tested 42 43 00:03:10,110 --> 00:03:13,470 against actual production data on the production side. 43 44 00:03:13,470 --> 00:03:19,860 This involves suspending rights to the database, creating a clone of the underlying SAN disks, and resuming 44 45 00:03:19,860 --> 00:03:25,290 the database. To my initial point, because I rarely did this I would end up forgetting the command to 45 46 00:03:25,290 --> 00:03:30,540 initiate the clone and all the options that were required to go with it. Each time I did this I had to 46 47 00:03:30,540 --> 00:03:36,030 spend some time rereading the man page and more or less reteaching myself how to do this process. 47 48 00:03:36,030 --> 00:03:39,370 That alone was reason enough for me to create a shell script. 48 49 00:03:39,390 --> 00:03:44,250 If we go beyond my original point of "it's something that I rarely did," there was another reason to write 49 50 00:03:44,250 --> 00:03:45,140 a shell script. 50 51 00:03:45,360 --> 00:03:51,000 You see I wanted to make sure that the production database was paused for as little time as possible. 51 52 00:03:51,000 --> 00:03:57,180 I wrote the shell script so as soon as the clone was successfully kicked off, it would resume the database. 52 53 00:03:57,180 --> 00:04:01,980 That way I didn't have to manually rechecked the clone status and then manually type in the command 53 54 00:04:01,980 --> 00:04:03,420 to resume the database. 54 55 00:04:03,420 --> 00:04:06,850 The script did it and it did it faster than a human could do it. 55 56 00:04:06,990 --> 00:04:13,250 A third take away from this one example is that since this was the company's most valuable database 56 57 00:04:13,560 --> 00:04:17,300 I really didn't want to take any chances and make a mistake. 57 58 00:04:17,430 --> 00:04:22,290 Once I tested my script in a non production environment, I could let it do the hard work of picking out 58 59 00:04:22,290 --> 00:04:23,590 all the correct disks, 59 60 00:04:23,670 --> 00:04:26,640 typing all the right commands with the right options, and so on. 60 61 00:04:26,820 --> 00:04:30,270 Using a good script can reduce the chance of errors. 61 62 00:04:30,360 --> 00:04:34,880 Many times I would have to do this at midnight when I was tired and not as alert as usual. 62 63 00:04:35,010 --> 00:04:37,590 When you're tired, the chances of making mistakes go up. 63 64 00:04:37,590 --> 00:04:42,650 So, to have some extra help in the form of a shell script was a very good thing for me. 64 65 00:04:42,720 --> 00:04:47,880 If you want to delegate a task to less experienced or knowledgeable staff then create a shell script 65 66 00:04:47,880 --> 00:04:48,510 for it. 66 67 00:04:48,510 --> 00:04:54,270 If you have some specific knowledge of how a piece of work needs to be performed, you can put that knowledge 67 68 00:04:54,300 --> 00:04:55,410 in a shell script. 68 69 00:04:55,500 --> 00:04:58,320 The shell script then becomes a black box to others, 69 70 00:04:58,410 --> 00:05:01,340 but they can still use it to get the same result. 70 71 00:05:01,350 --> 00:05:08,020 Take, for example, an employee separation process. Let's say someone from human resources submits a help 71 72 00:05:08,020 --> 00:05:13,930 desk ticket to notify the information technology department that a given employee has left the company. 72 73 00:05:14,020 --> 00:05:19,570 The helpdesk then transfers that ticket to you so you can lock the Linux account on all the Linux systems. 73 74 00:05:19,990 --> 00:05:22,420 If you created a shell script that perform the work, 74 75 00:05:22,540 --> 00:05:25,480 you could hand this task over to the help desk. 75 76 00:05:25,510 --> 00:05:28,760 They don't have to know all the details of how the shell script actually works, 76 77 00:05:28,780 --> 00:05:31,930 they just need instructions on how and when to use it. 77 78 00:05:31,930 --> 00:05:38,440 That frees you up from that particular task. If you need information from more than one command or source, 78 79 00:05:38,440 --> 00:05:43,770 that is another great time to create a shell script. If you ever find yourself making notes, 79 80 00:05:43,780 --> 00:05:46,200 that's a good sign there's an easier way to do it. 80 81 00:05:46,210 --> 00:05:48,860 For example, let's say you manage a web server 81 82 00:05:48,880 --> 00:05:55,600 and for each IP address that generated a web server error, you wanted to know their host name as well as 82 83 00:05:55,600 --> 00:05:58,860 see all the web pages that they visited. 83 84 00:05:59,080 --> 00:06:04,010 If you're doing this by hand you would have to look through the error log and note each IP address. 84 85 00:06:04,150 --> 00:06:09,760 Perhaps you would copy and paste these IP addresses and put them in a scrap file, for example, and then 85 86 00:06:09,760 --> 00:06:13,470 what you would do is look up each IP address to determine their hostname. 86 87 00:06:13,480 --> 00:06:18,060 Finally, you would search for those IP addresses in the web server access logs. 87 88 00:06:18,070 --> 00:06:23,830 Now you could write a shell script that does this entire process for you and displays it in an easy 88 89 00:06:23,830 --> 00:06:26,520 to consume and easy to read format. 89 90 00:06:26,830 --> 00:06:30,020 OK, so I've given you a couple of examples of shell scripts. 90 91 00:06:30,070 --> 00:06:35,410 When you think about it, shell scripts are really just computer programs written in the shell language. 91 92 00:06:35,410 --> 00:06:41,170 So why would you choose to write programs in shell instead of writing programs in a compiled language 92 93 00:06:41,170 --> 00:06:43,510 such as C or C++? 93 94 00:06:43,540 --> 00:06:46,550 The main reason is speed of development. 94 95 00:06:46,570 --> 00:06:51,220 For example, you can write a shell script that will perform a simple backup by first archiving a set 95 96 00:06:51,220 --> 00:06:55,560 of files and then copying that archive over the network to another server. 96 97 00:06:55,750 --> 00:07:00,310 You can do that in just a few lines of shell code and be done fairly quickly. 97 98 00:07:00,550 --> 00:07:06,140 However accomplishing that same task in C would take a lot of time and effort. 98 99 00:07:06,340 --> 00:07:10,570 When you're writing shell scripts you don't have to worry about low level programming objects such as 99 100 00:07:10,570 --> 00:07:11,960 bytes, pointers, 100 101 00:07:12,060 --> 00:07:14,160 declaring data types, and so on. 101 102 00:07:14,320 --> 00:07:21,070 The shell is a high level language that allows you to express complex operations simply and clearly. 102 103 00:07:21,550 --> 00:07:26,430 Related to the speed of development is the ease and speed of learning how to shell script. 103 104 00:07:26,530 --> 00:07:31,430 If you use the command line already, then the leap to creating shell scripts is really a small one. 104 105 00:07:31,570 --> 00:07:37,180 You just need to learn a few shell specific operators and rules and you're ready to start. 105 106 00:07:37,180 --> 00:07:41,590 Of course, if you want to learn advanced techniques and write really complicated shell scripts, it's going 106 107 00:07:41,590 --> 00:07:47,370 to take more time and effort, but the initial learning curve is relatively low for simple scripts. 107 108 00:07:47,380 --> 00:07:51,470 In all fairness, there is a trade off for the ease and speed of development. 108 109 00:07:51,550 --> 00:07:56,920 That trade off comes at the cost of performance and efficiency. That shell script that you wrote in 30 109 110 00:07:56,920 --> 00:08:00,440 minutes will not execute as fast as the equivalent program 110 111 00:08:00,550 --> 00:08:07,050 that might take days to write in C++, but the trade off is often well worth it, especially when the shell 111 112 00:08:07,050 --> 00:08:10,290 script will run fast enough for the intended purpose. 112 113 00:08:10,330 --> 00:08:16,150 You can create shell scripts for almost anything, but the real sweet spot is typically for System Administration 113 114 00:08:16,150 --> 00:08:21,970 and reporting tests where getting the job done is more important than getting the job done a few microseconds 114 115 00:08:21,970 --> 00:08:22,930 quicker. 115 116 00:08:22,930 --> 00:08:28,300 I want you to keep in mind that any task that you can do on the command line can be automated by a shell 116 117 00:08:28,300 --> 00:08:28,990 script. 117 118 00:08:28,990 --> 00:08:29,990 I want to see that again. 118 119 00:08:30,010 --> 00:08:33,960 Any task you can do in the command line can be automated by a script. 119 120 00:08:34,090 --> 00:08:39,020 Of course, some scripts are going to be harder than others, but you can do it. In this course we'll cover 120 121 00:08:39,100 --> 00:08:45,310 the simple examples all the way to the very complex situations where many people might think it's practically 121 122 00:08:45,310 --> 00:08:47,730 impossible to automate such a task. 122 123 00:08:47,950 --> 00:08:53,920 To quickly recap, a shell script is a plain text file that contains a series of commands. When a shell 123 124 00:08:53,920 --> 00:08:57,980 script is executed, it in turn executes the commands listed in the script 124 125 00:08:58,090 --> 00:09:04,090 line by line starting at the top of the script just as if you had typed the commands in yourself. 125 126 00:09:04,390 --> 00:09:07,520 Shell scripts can save you a lot of time in work. 126 127 00:09:07,540 --> 00:09:10,790 You can fully automate tedious or repetitive tasks. 127 128 00:09:11,110 --> 00:09:15,300 You can hand off work to others by providing them a script to perform an action. 128 129 00:09:15,310 --> 00:09:21,230 Scripts can also act as a form of documentation for a process or procedure that you perform in frequently. 129 130 00:09:21,340 --> 00:09:24,620 In general, shell scripts are fairly quick and easy to write. 130 131 00:09:24,730 --> 00:09:29,860 Instead of spending your time worrying about low level programming tasks, you can focus on accomplishing 131 132 00:09:29,860 --> 00:09:30,960 the goal at hand. 132 133 00:09:31,270 --> 00:09:36,050 And finally anything that you can do on the command line can be automated with the shell script.