1 00:00:00,090 --> 00:00:01,110 In this lesson, 2 00:00:01,110 --> 00:00:02,760 we're going to discuss the concepts 3 00:00:02,760 --> 00:00:05,880 that lay the groundwork for writing your own scripts. 4 00:00:05,880 --> 00:00:07,860 And we're going to do that by looking at scripting 5 00:00:07,860 --> 00:00:09,780 and programming fundamentals. 6 00:00:09,780 --> 00:00:11,760 Now, before we start to create your own scripts 7 00:00:11,760 --> 00:00:12,750 with bash though, 8 00:00:12,750 --> 00:00:13,980 you need to start getting familiar 9 00:00:13,980 --> 00:00:15,600 with some of the basic concepts 10 00:00:15,600 --> 00:00:16,860 that are shared by many scripting 11 00:00:16,860 --> 00:00:19,020 and programming languages used today. 12 00:00:19,020 --> 00:00:20,910 Bash is not only the default shell 13 00:00:20,910 --> 00:00:22,440 of the Linux operating system, 14 00:00:22,440 --> 00:00:24,870 but it's also a powerful scripting language. 15 00:00:24,870 --> 00:00:26,700 Now bash scripting is a skill 16 00:00:26,700 --> 00:00:29,430 that any system administrator really needs to learn 17 00:00:29,430 --> 00:00:31,080 because it's something that's going to help you 18 00:00:31,080 --> 00:00:33,090 leverage your time and really automate 19 00:00:33,090 --> 00:00:35,520 a lot of the things you do on a daily basis. 20 00:00:35,520 --> 00:00:36,810 By using bash scripting, 21 00:00:36,810 --> 00:00:39,030 you can leverage Linux as existing tools 22 00:00:39,030 --> 00:00:41,550 to get things done much quicker. 23 00:00:41,550 --> 00:00:45,360 Essentially, any program tool, utility or system function 24 00:00:45,360 --> 00:00:47,040 that you can call it the command line, 25 00:00:47,040 --> 00:00:50,010 you can also invoke from within a bash script. 26 00:00:50,010 --> 00:00:51,690 Similarly, bash scripts 27 00:00:51,690 --> 00:00:53,820 also support modern programming elements 28 00:00:53,820 --> 00:00:55,680 like loops and conditional statements 29 00:00:55,680 --> 00:00:58,920 to enhance the logic of the task that are being automated. 30 00:00:58,920 --> 00:01:01,320 Just as the commands inside the command line interface 31 00:01:01,320 --> 00:01:02,760 have a certain syntax, 32 00:01:02,760 --> 00:01:05,760 your scripting languages also have specific syntaxes 33 00:01:05,760 --> 00:01:07,200 that have to be used. 34 00:01:07,200 --> 00:01:08,700 When I talk about syntax, 35 00:01:08,700 --> 00:01:10,230 I'm simply referring to the rules 36 00:01:10,230 --> 00:01:12,630 that define how you're going to write your code. 37 00:01:12,630 --> 00:01:15,750 Each scripting and programming language has its own syntax, 38 00:01:15,750 --> 00:01:18,000 but many share a few commonalities. 39 00:01:18,000 --> 00:01:20,220 Because the Bash is really the default 40 00:01:20,220 --> 00:01:22,110 inside the Linux operating system, 41 00:01:22,110 --> 00:01:24,720 you're going to see that the syntax of the bash script 42 00:01:24,720 --> 00:01:26,550 is very similar to what you learned 43 00:01:26,550 --> 00:01:28,530 when you inputted things line by line 44 00:01:28,530 --> 00:01:30,630 within the command line environment. 45 00:01:30,630 --> 00:01:34,260 Now your bash script may contain shell specific instructions 46 00:01:34,260 --> 00:01:36,810 that are not compatible with other Linux shells though. 47 00:01:36,810 --> 00:01:38,190 Remember, in Linux, 48 00:01:38,190 --> 00:01:40,860 you can choose what shell environment you want to use. 49 00:01:40,860 --> 00:01:42,600 We're using bash in this course, 50 00:01:42,600 --> 00:01:44,340 but you could really use anything you want 51 00:01:44,340 --> 00:01:47,460 such as the C shell, Z shell or others. 52 00:01:47,460 --> 00:01:48,690 To specify that your script 53 00:01:48,690 --> 00:01:50,430 is being written for the bash shell, 54 00:01:50,430 --> 00:01:55,430 you need to add the line #bang/bin/bash 55 00:01:55,620 --> 00:01:57,450 at the beginning of each script. 56 00:01:57,450 --> 00:01:59,010 Anytime you hear the word bang, 57 00:01:59,010 --> 00:02:01,230 that means the exclamation mark. 58 00:02:01,230 --> 00:02:03,450 Now this line will instruct the operating system 59 00:02:03,450 --> 00:02:05,280 to use the bash shell interpreter 60 00:02:05,280 --> 00:02:06,870 when they execute this script 61 00:02:06,870 --> 00:02:08,729 on an incompatible Linux shell, 62 00:02:08,729 --> 00:02:12,540 and that way your script can still operate on that system. 63 00:02:12,540 --> 00:02:14,550 Now, another thing we're going to do inside of our scripts 64 00:02:14,550 --> 00:02:15,810 is assign variables. 65 00:02:15,810 --> 00:02:18,570 And we assign variables to symbolically associate 66 00:02:18,570 --> 00:02:21,360 a specific piece of information with a given name, 67 00:02:21,360 --> 00:02:22,710 and then we can refer to that name 68 00:02:22,710 --> 00:02:25,140 when we want to get that piece of information. 69 00:02:25,140 --> 00:02:27,690 Lots of different programming languages like C 70 00:02:27,690 --> 00:02:29,880 are going to require you to define the type of variable 71 00:02:29,880 --> 00:02:31,920 before you assign a value to it. 72 00:02:31,920 --> 00:02:34,170 This includes things like integers, floats, 73 00:02:34,170 --> 00:02:36,300 strings and other characteristics. 74 00:02:36,300 --> 00:02:38,370 Essentially, these types define exactly 75 00:02:38,370 --> 00:02:40,784 what kind of information that variable can hold. 76 00:02:40,784 --> 00:02:44,880 However, in bash, you don't have to declare variable types. 77 00:02:44,880 --> 00:02:48,060 All bash variables are treated as strings by default, 78 00:02:48,060 --> 00:02:50,340 and therefore you can put any kind of data you want 79 00:02:50,340 --> 00:02:52,440 inside of that variable. 80 00:02:52,440 --> 00:02:54,660 Now, one unique thing about assigning values 81 00:02:54,660 --> 00:02:56,160 in bash to a variable 82 00:02:56,160 --> 00:02:58,740 is that you're going to make sure there are no spaces 83 00:02:58,740 --> 00:03:01,110 before or after the equal sign. 84 00:03:01,110 --> 00:03:03,630 For example, if I was assigning Jason Dion 85 00:03:03,630 --> 00:03:05,370 to the variable myname, 86 00:03:05,370 --> 00:03:09,570 I would type in myname equals 'Jason Dion' 87 00:03:09,570 --> 00:03:13,470 and that's it, no spaces except between Jason and Dion. 88 00:03:13,470 --> 00:03:14,880 And it's okay to have the space there 89 00:03:14,880 --> 00:03:17,130 because I have the quotes around that string. 90 00:03:17,970 --> 00:03:20,190 Now, after you assign a value to a variable, 91 00:03:20,190 --> 00:03:22,800 you can reference that variable later on in your code, 92 00:03:22,800 --> 00:03:24,480 so you don't have to hard code these values 93 00:03:24,480 --> 00:03:26,220 into your script's logic. 94 00:03:26,220 --> 00:03:29,070 The act of referencing or retrieving the valuable variable 95 00:03:29,070 --> 00:03:32,430 is known as substitution or parameter expansion. 96 00:03:32,430 --> 00:03:34,590 Now, when you're referencing variables in bash, 97 00:03:34,590 --> 00:03:36,510 you're going to do this by adding a dollar sign 98 00:03:36,510 --> 00:03:38,460 to the beginning of the variable name. 99 00:03:38,460 --> 00:03:42,030 For example, if I wanted to display myname to the screen, 100 00:03:42,030 --> 00:03:44,613 I would do that by typing echo $myname, 101 00:03:45,540 --> 00:03:47,370 and this will print out Jason Dion 102 00:03:47,370 --> 00:03:50,400 into your console or command line environment. 103 00:03:50,400 --> 00:03:53,010 So note that when you're assigning the variable, 104 00:03:53,010 --> 00:03:54,330 you don't use the dollar sign, 105 00:03:54,330 --> 00:03:56,640 but when you're pulling information from that variable, 106 00:03:56,640 --> 00:03:58,530 you use the dollar sign. 107 00:03:58,530 --> 00:04:00,180 Now, in addition to storing data 108 00:04:00,180 --> 00:04:02,010 and retrieving data from a variable, 109 00:04:02,010 --> 00:04:04,830 there's other operations you can do with these variables 110 00:04:04,830 --> 00:04:07,860 to create tasks and outcomes that you're looking for. 111 00:04:07,860 --> 00:04:09,540 For example, you might need 112 00:04:09,540 --> 00:04:12,300 to be able to evaluate the condition of a variable 113 00:04:12,300 --> 00:04:14,220 and what's stored inside of it. 114 00:04:14,220 --> 00:04:16,079 This is known as a comparison, 115 00:04:16,079 --> 00:04:17,940 and we do this using operators. 116 00:04:17,940 --> 00:04:20,670 Now operators are the object that can evaluate expressions 117 00:04:20,670 --> 00:04:22,650 in a variety of different ways. 118 00:04:22,650 --> 00:04:23,970 When we look at operands, 119 00:04:23,970 --> 00:04:26,910 these are the values that are being operated on. 120 00:04:26,910 --> 00:04:29,160 So there's lots of different kinds of operators 121 00:04:29,160 --> 00:04:31,380 that we can apply inside of bash. 122 00:04:31,380 --> 00:04:34,380 We have arithmetic operators, comparison operators, 123 00:04:34,380 --> 00:04:37,140 logical operators, and string operators. 124 00:04:37,140 --> 00:04:39,060 Let's take a quick look at each of these. 125 00:04:39,060 --> 00:04:41,250 First, we have arithmetic operators. 126 00:04:41,250 --> 00:04:44,610 These include things like adding, subtracting, multiplying, 127 00:04:44,610 --> 00:04:47,970 and dividing and other more advanced mathematical functions. 128 00:04:47,970 --> 00:04:50,610 For example, if I had variable one and variable two, 129 00:04:50,610 --> 00:04:52,320 and I wanted to add them together, 130 00:04:52,320 --> 00:04:55,110 I can do that by using a double parenthesis around them. 131 00:04:55,110 --> 00:04:57,750 And so I would say dollar sign, parenthesis, parenthesis 132 00:04:57,750 --> 00:05:00,870 var one plus var two, parenthesis parenthesis. 133 00:05:00,870 --> 00:05:02,970 And this will take the values of variable one 134 00:05:02,970 --> 00:05:05,040 and variable two and add them together 135 00:05:05,040 --> 00:05:06,300 and display that to the screen 136 00:05:06,300 --> 00:05:08,670 if it was inside of an echo command. 137 00:05:08,670 --> 00:05:10,860 Now comparison operators are going to be used 138 00:05:10,860 --> 00:05:12,420 to check if things are equal, 139 00:05:12,420 --> 00:05:14,940 if something is less than or greater than the other, 140 00:05:14,940 --> 00:05:16,560 or other things like that. 141 00:05:16,560 --> 00:05:18,960 For example, if I'm going to do a comparison 142 00:05:18,960 --> 00:05:21,270 to see if variable one is greater than 143 00:05:21,270 --> 00:05:22,950 or equal to variable two, 144 00:05:22,950 --> 00:05:25,860 I would do that by typing bracket, dollar sign, 145 00:05:25,860 --> 00:05:30,860 var one -ge dollar sign var two, end bracket. 146 00:05:31,020 --> 00:05:34,410 Now notice here I'm using brackets instead of parentheses, 147 00:05:34,410 --> 00:05:36,990 that's because I'm using a comparison operator. 148 00:05:36,990 --> 00:05:38,850 And the return value from this comparison 149 00:05:38,850 --> 00:05:42,750 is either a one if it's true or a zero, if it's false. 150 00:05:42,750 --> 00:05:45,060 The next one we have is a logical operator. 151 00:05:45,060 --> 00:05:46,500 And logical operators are used 152 00:05:46,500 --> 00:05:48,270 to connect multiple values together 153 00:05:48,270 --> 00:05:49,860 so they can be evaluated. 154 00:05:49,860 --> 00:05:53,490 Normally these are things like, and, or, and not. 155 00:05:53,490 --> 00:05:56,490 For example, if I wanted to test two different conditions 156 00:05:56,490 --> 00:05:58,350 and then see if they are both true, 157 00:05:58,350 --> 00:06:00,060 I would use an and operator, 158 00:06:00,060 --> 00:06:03,300 which is symbolized by the ampersand ampersand sign. 159 00:06:03,300 --> 00:06:07,093 So I might do something like bracket dollar sign var one 160 00:06:07,093 --> 00:06:10,410 ge dollar sign var two, bracket, 161 00:06:10,410 --> 00:06:14,940 ampersand, ampersand bracket, dollar sign var three, -le 162 00:06:14,940 --> 00:06:17,460 dollar sign var four, bracket. 163 00:06:17,460 --> 00:06:20,520 This says, tell me is variable one 164 00:06:20,520 --> 00:06:22,440 greater than or equal two variable two 165 00:06:22,440 --> 00:06:26,325 and is variable three less than or equal two variable four. 166 00:06:26,325 --> 00:06:28,290 This is how you connect things together 167 00:06:28,290 --> 00:06:30,553 by using ands, ors and nots. 168 00:06:30,553 --> 00:06:32,280 When you want to use a not, 169 00:06:32,280 --> 00:06:34,920 it's going to be a bang equal sign. 170 00:06:34,920 --> 00:06:37,980 If you want to use an or, it's going to be two pipes, 171 00:06:37,980 --> 00:06:40,470 which is the up and down straight line characters, 172 00:06:40,470 --> 00:06:42,510 right above your enter key. 173 00:06:42,510 --> 00:06:44,640 Next, we have string operators. 174 00:06:44,640 --> 00:06:47,070 Now string operators are going to be used in operations 175 00:06:47,070 --> 00:06:49,440 that manipulate strings in various ways, 176 00:06:49,440 --> 00:06:51,120 including concatenating strings, 177 00:06:51,120 --> 00:06:53,190 returning a specific character in a string, 178 00:06:53,190 --> 00:06:55,410 which is known as slicing or verifying 179 00:06:55,410 --> 00:06:58,320 if special characters exist inside of a string. 180 00:06:58,320 --> 00:07:01,380 For example, if I wanted to concatenate two variables 181 00:07:01,380 --> 00:07:04,890 and put them together, I can take dollar sign var one, 182 00:07:04,890 --> 00:07:07,770 dollar sign var two, with no spaces in between them, 183 00:07:07,770 --> 00:07:10,200 and this will put them both together as a single word 184 00:07:10,200 --> 00:07:12,090 or single string when being displayed 185 00:07:12,090 --> 00:07:13,980 to the command line environment. 186 00:07:13,980 --> 00:07:16,980 Now, the next thing we want to talk about is a string literal. 187 00:07:16,980 --> 00:07:19,230 A string literal is any fixed value 188 00:07:19,230 --> 00:07:22,200 that represents a string of text within your source code. 189 00:07:22,200 --> 00:07:24,420 String literals are enclosed either single 190 00:07:24,420 --> 00:07:25,797 or double quotation marks, 191 00:07:25,797 --> 00:07:28,050 and as long as you're using them consistently, 192 00:07:28,050 --> 00:07:30,690 you can use either single or double quotation marks. 193 00:07:30,690 --> 00:07:33,210 It really doesn't matter. It's up to you. 194 00:07:33,210 --> 00:07:35,790 However, there are some cases where double quotes 195 00:07:35,790 --> 00:07:37,350 won't preserve the literal value 196 00:07:37,350 --> 00:07:39,750 of all the characters within those quotes. 197 00:07:39,750 --> 00:07:42,000 For example, let's go back to the example 198 00:07:42,000 --> 00:07:43,710 of having myname as a variable 199 00:07:43,710 --> 00:07:46,140 that contains Jason, space, Dion. 200 00:07:46,140 --> 00:07:48,240 Now let's say I wanted to substitute this variable 201 00:07:48,240 --> 00:07:50,550 into a larger literal strength. 202 00:07:50,550 --> 00:07:51,720 I can do that by typing 203 00:07:51,720 --> 00:07:55,293 echo " my name is dollar sign, myname" 204 00:07:57,180 --> 00:07:58,200 and this is going to print 205 00:07:58,200 --> 00:08:01,320 My name is Jason Dion over to your screen. 206 00:08:01,320 --> 00:08:03,750 Now, on the other hand, if I use single quotes 207 00:08:03,750 --> 00:08:08,223 and typed echo 'My name is dollar sign, myname' 208 00:08:09,180 --> 00:08:10,650 this is going to print out to the screen, 209 00:08:10,650 --> 00:08:13,260 My name is dollar sign myname 210 00:08:13,260 --> 00:08:15,630 because it's not actually parsing that variable 211 00:08:15,630 --> 00:08:18,060 because I use single quotes instead of double quotes 212 00:08:18,060 --> 00:08:19,994 in this particular example. 213 00:08:19,994 --> 00:08:21,780 In general, it's a great idea 214 00:08:21,780 --> 00:08:24,000 to always put quotes around your strings 215 00:08:24,000 --> 00:08:25,800 when you're assigning them to a variable. 216 00:08:25,800 --> 00:08:27,150 This is especially important 217 00:08:27,150 --> 00:08:29,160 if you have spaces in the variable, 218 00:08:29,160 --> 00:08:32,220 such as Jason space Dion, that I was assigning 219 00:08:32,220 --> 00:08:34,200 into the myname variable. 220 00:08:34,200 --> 00:08:35,280 If you don't do that, 221 00:08:35,280 --> 00:08:37,320 it's only going to sign the first word 222 00:08:37,320 --> 00:08:38,850 instead of the entire phrase. 223 00:08:38,850 --> 00:08:40,860 So in that case, I'd only have Jason stored 224 00:08:40,860 --> 00:08:43,110 instead of Jason space Dion. 225 00:08:43,110 --> 00:08:45,690 Now, if it's a single word, you don't need quotes. 226 00:08:45,690 --> 00:08:48,750 But again, it's just a good practice to always use quotes 227 00:08:48,750 --> 00:08:51,450 around what you're setting your variable names to. 228 00:08:51,450 --> 00:08:53,670 Now in any language, including bash, 229 00:08:53,670 --> 00:08:56,100 there are certain characters that have special meanings. 230 00:08:56,100 --> 00:08:57,990 And these characters are basically what we call 231 00:08:57,990 --> 00:08:59,460 reserved characters. 232 00:08:59,460 --> 00:09:01,320 Let's talk about a few of them here. 233 00:09:01,320 --> 00:09:03,270 First, we have an escape character. 234 00:09:03,270 --> 00:09:04,680 Now an escape character is used 235 00:09:04,680 --> 00:09:06,330 to remove that special meaning 236 00:09:06,330 --> 00:09:08,137 so that any character can be used literally 237 00:09:08,137 --> 00:09:11,580 rather than as an interpreted thing by the system. 238 00:09:11,580 --> 00:09:14,670 For example, you saw the fact that we use hashtags 239 00:09:14,670 --> 00:09:16,500 as a way to start a comment on a line 240 00:09:16,500 --> 00:09:18,600 or to describe what this script is 241 00:09:18,600 --> 00:09:21,870 by doing the hashtag bang at the beginning of the script. 242 00:09:21,870 --> 00:09:23,760 Well, what if I wanted to tell you 243 00:09:23,760 --> 00:09:25,320 that I wanted to be number one 244 00:09:25,320 --> 00:09:27,510 and put that hashtag in front of the one 245 00:09:27,510 --> 00:09:29,160 when I display it to the screen. 246 00:09:29,160 --> 00:09:31,380 If I did that normally, it's just going to go ahead 247 00:09:31,380 --> 00:09:34,770 and process everything at the end of that line as a comment. 248 00:09:34,770 --> 00:09:37,530 So I need to first escape this special meaning 249 00:09:37,530 --> 00:09:39,510 from that hashtag character. 250 00:09:39,510 --> 00:09:42,060 To do this, I can use an escape character 251 00:09:42,060 --> 00:09:44,160 such as a single back slash. 252 00:09:44,160 --> 00:09:45,900 This single back slash will be put 253 00:09:45,900 --> 00:09:47,820 right in front of that character. 254 00:09:47,820 --> 00:09:50,580 And so if I said, I am number one, 255 00:09:50,580 --> 00:09:53,700 I'm going to write that as I am single back slash, 256 00:09:53,700 --> 00:09:57,210 pound sign, one, and that way it'll process 257 00:09:57,210 --> 00:10:01,110 as the pound sign and not as a comment character. 258 00:10:01,110 --> 00:10:02,580 This is the way you can escape something 259 00:10:02,580 --> 00:10:04,740 and take away that special meaning. 260 00:10:04,740 --> 00:10:08,010 Another way to do this is by using single quotation marks. 261 00:10:08,010 --> 00:10:12,240 So if I put single quotation marks around that string of 262 00:10:12,240 --> 00:10:14,730 I am number one, it's still going to process 263 00:10:14,730 --> 00:10:16,680 as I am number one. 264 00:10:16,680 --> 00:10:18,780 Alternatively, if you wanted to use double quotes 265 00:10:18,780 --> 00:10:19,920 or no quotes, 266 00:10:19,920 --> 00:10:22,320 then you're going to have to use that escape character 267 00:10:22,320 --> 00:10:24,660 of using that single back slash. 268 00:10:24,660 --> 00:10:26,970 The next thing we're going to talk about is an array. 269 00:10:26,970 --> 00:10:29,190 Now, an array enables you to store multiple values 270 00:10:29,190 --> 00:10:30,720 in a single variable. 271 00:10:30,720 --> 00:10:33,690 This can make your code much easier to read and maintain 272 00:10:33,690 --> 00:10:34,523 because you don't have 273 00:10:34,523 --> 00:10:35,970 to have a bunch of different variables. 274 00:10:35,970 --> 00:10:38,700 You can have one that contains everything you need. 275 00:10:38,700 --> 00:10:40,560 For example, you might want to perform 276 00:10:40,560 --> 00:10:42,120 a single mathematical operation 277 00:10:42,120 --> 00:10:44,040 on dozens of different values. 278 00:10:44,040 --> 00:10:46,350 And instead of creating a variable for each value, 279 00:10:46,350 --> 00:10:48,480 you can create an array to simplify your code 280 00:10:48,480 --> 00:10:51,720 and store all those values in different parts of the array. 281 00:10:51,720 --> 00:10:53,100 Another benefit of arrays 282 00:10:53,100 --> 00:10:54,840 is that you can easily update their values 283 00:10:54,840 --> 00:10:57,060 throughout the code because it's a single thing 284 00:10:57,060 --> 00:10:59,286 instead of having multiple different variables. 285 00:10:59,286 --> 00:11:03,060 Now, when you do compound assignments inside bash arrays, 286 00:11:03,060 --> 00:11:04,260 you're going to use parentheses 287 00:11:04,260 --> 00:11:06,805 with each value being separated by a space. 288 00:11:06,805 --> 00:11:10,710 For example, let's say I created an array called my array 289 00:11:10,710 --> 00:11:12,990 and I want to store three words inside of it. 290 00:11:12,990 --> 00:11:15,330 Jason, Dion, and training. 291 00:11:15,330 --> 00:11:19,380 To do this, I'm going to type in my array equals parenthesis, 292 00:11:19,380 --> 00:11:21,570 double quote, Jason, double quote, 293 00:11:21,570 --> 00:11:23,550 double quote, Dion, double quote, 294 00:11:23,550 --> 00:11:26,940 double quote, training, double quote, and parentheses. 295 00:11:26,940 --> 00:11:29,010 Now, in addition to assigning the array all at once 296 00:11:29,010 --> 00:11:30,150 like I did here, 297 00:11:30,150 --> 00:11:32,130 I can also individually assign things 298 00:11:32,130 --> 00:11:35,010 into the specific values based on their indexes 299 00:11:35,010 --> 00:11:36,630 and I place that in a bracket. 300 00:11:36,630 --> 00:11:39,420 So for example, if I wanted to get the word, Jason, 301 00:11:39,420 --> 00:11:43,020 I would do my array bracket zero bracket equals, 302 00:11:43,020 --> 00:11:45,030 double quote, Jason double quote, 303 00:11:45,030 --> 00:11:47,730 and that assigns the word Jason into space zero, 304 00:11:47,730 --> 00:11:50,790 which is the first space in this particular array. 305 00:11:50,790 --> 00:11:52,890 I can do the same thing with Dion and training. 306 00:11:52,890 --> 00:11:55,620 by using my array one and my array two, 307 00:11:55,620 --> 00:11:57,630 and putting those values into there. 308 00:11:57,630 --> 00:11:59,160 You can always reference an array 309 00:11:59,160 --> 00:12:00,690 by wrapping it in curly braces 310 00:12:00,690 --> 00:12:02,760 when you want to get information back out of it. 311 00:12:02,760 --> 00:12:05,070 So if I wanted to pull out the name, Jason, 312 00:12:05,070 --> 00:12:08,730 I can do echo dollar sign, curly brace, my array, 313 00:12:08,730 --> 00:12:11,100 bracket, zero bracket, curly brace, 314 00:12:11,100 --> 00:12:14,220 and this would return the value of Jason from position zero, 315 00:12:14,220 --> 00:12:17,180 which is the first position inside of that array. 316 00:12:17,180 --> 00:12:19,350 Now, another thing we need to talk about quickly 317 00:12:19,350 --> 00:12:20,460 is functions. 318 00:12:20,460 --> 00:12:22,770 Now functions are a block of code that you can reuse 319 00:12:22,770 --> 00:12:24,660 to perform a specific task. 320 00:12:24,660 --> 00:12:25,500 This is really useful 321 00:12:25,500 --> 00:12:27,120 when you're trying to write efficient code. 322 00:12:27,120 --> 00:12:28,920 So you don't have to copy and paste things over 323 00:12:28,920 --> 00:12:29,910 and over again. 324 00:12:29,910 --> 00:12:31,380 And so we'll call the function 325 00:12:31,380 --> 00:12:34,260 and somehow to write out the same thing over and over again. 326 00:12:34,260 --> 00:12:36,300 Just like a variable, you can define a function 327 00:12:36,300 --> 00:12:38,640 using a unique name or identifier, 328 00:12:38,640 --> 00:12:40,890 and then you can reference that identifier 329 00:12:40,890 --> 00:12:43,650 to be able to do all the things inside that function 330 00:12:43,650 --> 00:12:45,090 based on its code. 331 00:12:45,090 --> 00:12:48,000 So in bash, there's really two ways to write functions 332 00:12:48,000 --> 00:12:50,010 and both involve placing the desired code 333 00:12:50,010 --> 00:12:52,020 between a set of curly braces. 334 00:12:52,020 --> 00:12:54,990 Now, the first method is going to be something like this. 335 00:12:54,990 --> 00:12:58,890 Function, my function name, curly brace code, curly brace, 336 00:12:58,890 --> 00:13:01,470 and all your code goes inside those curly braces. 337 00:13:01,470 --> 00:13:02,303 Now, if you're familiar 338 00:13:02,303 --> 00:13:04,050 with using object oriented programming 339 00:13:04,050 --> 00:13:06,870 and you come from a background like C or Java, 340 00:13:06,870 --> 00:13:09,570 you might be more comfortable using the second method. 341 00:13:09,570 --> 00:13:12,210 And the second method is my function name, 342 00:13:12,210 --> 00:13:15,240 parenthesis parenthesis, and then curly bracket, 343 00:13:15,240 --> 00:13:17,790 all the code you want, end curly bracket. 344 00:13:17,790 --> 00:13:19,620 Notice both will do the same thing, 345 00:13:19,620 --> 00:13:21,330 it's just a different way of writing it, 346 00:13:21,330 --> 00:13:23,970 depending on which method you prefer. 347 00:13:23,970 --> 00:13:24,840 Now in bash, 348 00:13:24,840 --> 00:13:27,420 when you want to pass arguments into your function, 349 00:13:27,420 --> 00:13:28,770 you're going to do it the same way you would 350 00:13:28,770 --> 00:13:30,665 with a script at the command line. 351 00:13:30,665 --> 00:13:33,690 So if I want to send an argument in such as Jason 352 00:13:33,690 --> 00:13:35,430 into the my function name, 353 00:13:35,430 --> 00:13:39,210 I would simply enter my function name, space, Jason. 354 00:13:39,210 --> 00:13:41,220 And that means Jason is the first argument 355 00:13:41,220 --> 00:13:43,560 that's being passed into my function name. 356 00:13:43,560 --> 00:13:45,780 And then the code will execute upon it. 357 00:13:45,780 --> 00:13:47,010 Now in the world of programming, 358 00:13:47,010 --> 00:13:48,570 we often want to comment our code 359 00:13:48,570 --> 00:13:50,580 so other people can know what we're doing. 360 00:13:50,580 --> 00:13:52,830 And as I said before, the hashtag symbol 361 00:13:52,830 --> 00:13:55,290 is going to indicate that everything after that, 362 00:13:55,290 --> 00:13:57,720 on that particular line is part of the comment 363 00:13:57,720 --> 00:14:00,270 and will not be executed by the system. 364 00:14:00,270 --> 00:14:02,370 You should refrain from commenting on a line of code 365 00:14:02,370 --> 00:14:03,630 whose purpose is obvious, 366 00:14:03,630 --> 00:14:06,450 because too many comments can corrupt your source code. 367 00:14:06,450 --> 00:14:08,130 But in general, it is a good idea 368 00:14:08,130 --> 00:14:10,770 to at least comment what the function is that you're doing 369 00:14:10,770 --> 00:14:13,080 or what the logic of the next several lines is, 370 00:14:13,080 --> 00:14:14,340 so somebody else coming behind you 371 00:14:14,340 --> 00:14:16,350 can understand what you're doing. 372 00:14:16,350 --> 00:14:18,870 Another thing we need to talk about is metacharacters. 373 00:14:18,870 --> 00:14:20,880 Now metacharacters are special characters 374 00:14:20,880 --> 00:14:24,510 that the shell will by default interpret in a certain way. 375 00:14:24,510 --> 00:14:27,330 These characters have to be escaped or enclosed in quotes 376 00:14:27,330 --> 00:14:29,340 if you want them to be interpreted literally. 377 00:14:29,340 --> 00:14:31,470 Similar to the hashtag I just mentioned. 378 00:14:31,470 --> 00:14:33,990 Now let's talk about these different metacharacters, 379 00:14:33,990 --> 00:14:36,120 because it's important to know what characters 380 00:14:36,120 --> 00:14:37,830 are reserved in Linux. 381 00:14:37,830 --> 00:14:39,840 The first one is the greater than symbol, 382 00:14:39,840 --> 00:14:41,910 which is used in output redirection. 383 00:14:41,910 --> 00:14:44,160 We also have the greater than greater than symbol, 384 00:14:44,160 --> 00:14:46,080 which is used in output redirection too, 385 00:14:46,080 --> 00:14:47,340 but in a different manner. 386 00:14:47,340 --> 00:14:48,600 We have the less than symbol, 387 00:14:48,600 --> 00:14:50,490 which is used in input redirection. 388 00:14:50,490 --> 00:14:52,140 We have the less than less than symbol, 389 00:14:52,140 --> 00:14:54,270 which is used for input redirection as well, 390 00:14:54,270 --> 00:14:55,830 but a different context. 391 00:14:55,830 --> 00:14:57,090 We have the up and down bar, 392 00:14:57,090 --> 00:15:00,420 which is used to pipe content from one command into another. 393 00:15:00,420 --> 00:15:01,650 We also have the double quote, 394 00:15:01,650 --> 00:15:04,350 which is used for defining weak literal strings. 395 00:15:04,350 --> 00:15:05,310 We have the single quote, 396 00:15:05,310 --> 00:15:08,160 which is used for defining strong literal strings. 397 00:15:08,160 --> 00:15:09,510 We have the backwards tick mark, 398 00:15:09,510 --> 00:15:11,610 which is used for breaking out of a literal string 399 00:15:11,610 --> 00:15:13,830 to run the command that's inside of it. 400 00:15:13,830 --> 00:15:15,270 We have a single backwards slash 401 00:15:15,270 --> 00:15:17,760 which is used to escape all these other characters. 402 00:15:17,760 --> 00:15:18,990 We have the equal sign, 403 00:15:18,990 --> 00:15:20,610 which is used for variable assignment. 404 00:15:20,610 --> 00:15:21,720 We have the dollar sign, 405 00:15:21,720 --> 00:15:23,700 which is used for variable substitution 406 00:15:23,700 --> 00:15:26,190 and pulling information back out of a variable. 407 00:15:26,190 --> 00:15:29,040 We also have the hashtag, which is used for commenting. 408 00:15:29,040 --> 00:15:30,270 We have the double pipe, 409 00:15:30,270 --> 00:15:32,580 which is used for logical or operations. 410 00:15:32,580 --> 00:15:33,690 The double ampersand, 411 00:15:33,690 --> 00:15:35,940 which is used for logical and operations. 412 00:15:35,940 --> 00:15:38,490 The star, which is used for wildcard matching. 413 00:15:38,490 --> 00:15:39,360 The question mark, 414 00:15:39,360 --> 00:15:40,920 which is used for wildcard matching 415 00:15:40,920 --> 00:15:42,690 being applied on a single character 416 00:15:42,690 --> 00:15:44,520 instead of multiple characters. 417 00:15:44,520 --> 00:15:47,160 We have the brackets which are used for wildcard matching 418 00:15:47,160 --> 00:15:49,860 any applied character between those brackets. 419 00:15:49,860 --> 00:15:51,030 We have the curly braces, 420 00:15:51,030 --> 00:15:53,760 which is used in perimeter substitution and arrays. 421 00:15:53,760 --> 00:15:56,340 We have parentheses, which is used for grouping commands. 422 00:15:56,340 --> 00:15:58,320 We have a single ampersand by itself, 423 00:15:58,320 --> 00:16:01,020 which is used for running a process in the background. 424 00:16:01,020 --> 00:16:02,070 We have the semicolon, 425 00:16:02,070 --> 00:16:03,960 which is used for separating multiple commands 426 00:16:03,960 --> 00:16:05,250 on the same line. 427 00:16:05,250 --> 00:16:07,590 And we have the bang or exclamation character, 428 00:16:07,590 --> 00:16:10,140 which is used in referencing command history. 429 00:16:10,140 --> 00:16:12,180 All of these are considered reserve characters 430 00:16:12,180 --> 00:16:13,230 and metacharacters, 431 00:16:13,230 --> 00:16:14,730 and you should be aware of what they are 432 00:16:14,730 --> 00:16:16,230 and their functions. 433 00:16:16,230 --> 00:16:17,940 Now on Unix and Linux systems, 434 00:16:17,940 --> 00:16:20,520 programs can also pass the value to their parent process 435 00:16:20,520 --> 00:16:21,720 when they terminate. 436 00:16:21,720 --> 00:16:24,990 This is known as an exit code or exit status. 437 00:16:24,990 --> 00:16:27,300 In the Linux world, a status code of zero 438 00:16:27,300 --> 00:16:30,210 indicates that the process executed successfully, 439 00:16:30,210 --> 00:16:33,270 whereas an exit code of one or any number higher than that 440 00:16:33,270 --> 00:16:35,670 means that the process encounters some kind of an error 441 00:16:35,670 --> 00:16:37,140 when it executed. 442 00:16:37,140 --> 00:16:38,970 Now, when you're creating your own scripts, 443 00:16:38,970 --> 00:16:41,490 you want to make sure you take into account exit codes. 444 00:16:41,490 --> 00:16:42,780 Because when you write a script, 445 00:16:42,780 --> 00:16:44,670 it's often going to be used by other people 446 00:16:44,670 --> 00:16:45,960 in their scripts too, 447 00:16:45,960 --> 00:16:48,150 and you can call from one script to another. 448 00:16:48,150 --> 00:16:50,640 And by being able to have that exit code come back, 449 00:16:50,640 --> 00:16:52,680 it can tell them the status of your script, 450 00:16:52,680 --> 00:16:54,480 that it was successful or not. 451 00:16:54,480 --> 00:16:56,670 Again, if you use exit code one, 452 00:16:56,670 --> 00:16:58,260 this can cause the script to terminate 453 00:16:58,260 --> 00:16:59,580 with a failure status. 454 00:16:59,580 --> 00:17:02,040 Whereas zero would say everything was satisfactory, 455 00:17:02,040 --> 00:17:03,450 keep on going. 456 00:17:03,450 --> 00:17:05,700 These exit codes are going to exist within your scripts, 457 00:17:05,700 --> 00:17:07,440 even if you don't define them, 458 00:17:07,440 --> 00:17:10,500 but by defining them and using proper exit codes, 459 00:17:10,500 --> 00:17:12,240 you can have multiple different errors 460 00:17:12,240 --> 00:17:13,470 that you can test for, 461 00:17:13,470 --> 00:17:15,089 and then you can give that information back 462 00:17:15,089 --> 00:17:16,530 to the script that's calling you. 463 00:17:16,530 --> 00:17:19,619 For example, maybe zero is going to be successful. 464 00:17:19,619 --> 00:17:21,359 One is going to be a failure. 465 00:17:21,359 --> 00:17:23,819 Two is going to be the fact that nobody gave you an argument 466 00:17:23,819 --> 00:17:25,560 to process in the first place. 467 00:17:25,560 --> 00:17:28,050 Things like that can be used as a way to give information 468 00:17:28,050 --> 00:17:30,935 from one script to another or one program to another. 469 00:17:30,935 --> 00:17:33,150 Now, another thing you want to take advantage of 470 00:17:33,150 --> 00:17:34,680 is the ability to do redirection 471 00:17:34,680 --> 00:17:37,170 and piping within the command line environment. 472 00:17:37,170 --> 00:17:39,330 You can do this by redirecting exit codes, 473 00:17:39,330 --> 00:17:41,520 either to the standard out, standard error 474 00:17:41,520 --> 00:17:43,950 or standard in, depending on what you want to do. 475 00:17:43,950 --> 00:17:46,770 And you can also redirect data to and from different files 476 00:17:46,770 --> 00:17:48,153 by using your redirections. 477 00:17:49,200 --> 00:17:50,370 Another thing to be aware of 478 00:17:50,370 --> 00:17:53,310 is how the bash shell actually interprets commands. 479 00:17:53,310 --> 00:17:56,130 And the way it does this is to split things up into tokens, 480 00:17:56,130 --> 00:17:57,720 also known as words. 481 00:17:57,720 --> 00:17:59,190 Shell expansion is the process 482 00:17:59,190 --> 00:18:01,830 by which the shell identifies these special tokens 483 00:18:01,830 --> 00:18:04,260 and then substitutes in values for them. 484 00:18:04,260 --> 00:18:07,110 Variable substitution is a type of shell expansion 485 00:18:07,110 --> 00:18:08,580 by which the shell going to identify 486 00:18:08,580 --> 00:18:10,440 that dollar sign special character, 487 00:18:10,440 --> 00:18:11,610 and then expand the value 488 00:18:11,610 --> 00:18:14,040 into its actual value that's being stored. 489 00:18:14,040 --> 00:18:14,873 In other words, 490 00:18:14,873 --> 00:18:17,490 when I do something like echo dollar sign variable, 491 00:18:17,490 --> 00:18:19,890 the echo command doesn't see the variable, 492 00:18:19,890 --> 00:18:22,050 instead it sees what the variable represents 493 00:18:22,050 --> 00:18:23,490 and what was stored inside of it 494 00:18:23,490 --> 00:18:25,050 because bash is going to expand it, 495 00:18:25,050 --> 00:18:27,432 before passing it over to the echo command. 496 00:18:27,432 --> 00:18:29,910 Now bash is going to perform its expressions 497 00:18:29,910 --> 00:18:31,380 in a defined order. 498 00:18:31,380 --> 00:18:33,810 Very similar to the order of mathematical operations 499 00:18:33,810 --> 00:18:35,490 you learned back in high school. 500 00:18:35,490 --> 00:18:36,600 The order of expansion 501 00:18:36,600 --> 00:18:39,450 is always going to start out with brace expansion first, 502 00:18:39,450 --> 00:18:41,250 then tilde expansion, 503 00:18:41,250 --> 00:18:43,470 then parameter and variable expansion, 504 00:18:43,470 --> 00:18:47,340 then arithmetic expansion and then command substitution, 505 00:18:47,340 --> 00:18:50,340 and then word splitting and then file name expansion. 506 00:18:50,340 --> 00:18:52,620 The shell will always perform things in that order, 507 00:18:52,620 --> 00:18:55,830 as it performs its expansions and substitutions. 508 00:18:55,830 --> 00:18:57,870 Now, when do you want to include a command's output 509 00:18:57,870 --> 00:18:59,700 within an existing string of text, 510 00:18:59,700 --> 00:19:02,100 command substitution can actually be used. 511 00:19:02,100 --> 00:19:03,870 This is a method where shell expansion 512 00:19:03,870 --> 00:19:05,370 allows the output of a command 513 00:19:05,370 --> 00:19:07,260 to replace the command itself. 514 00:19:07,260 --> 00:19:11,190 For example, if I enter the command, echo double quote, 515 00:19:11,190 --> 00:19:13,890 the current directory is backwards tick mark, 516 00:19:13,890 --> 00:19:16,500 pwd backwards tick mark, quote, 517 00:19:16,500 --> 00:19:19,320 this is going to actually replace the pwd 518 00:19:19,320 --> 00:19:22,410 and replace it with what the present working directory is. 519 00:19:22,410 --> 00:19:24,060 And this would output something like, 520 00:19:24,060 --> 00:19:26,220 the current directory is /root, 521 00:19:26,220 --> 00:19:28,410 if that was your present working directory. 522 00:19:28,410 --> 00:19:29,580 Now another way to do this 523 00:19:29,580 --> 00:19:32,010 is to use the format of dollar sign parenthesis, 524 00:19:32,010 --> 00:19:33,750 command, parenthesis. 525 00:19:33,750 --> 00:19:36,030 This will perform command substitution as well. 526 00:19:36,030 --> 00:19:39,600 For example, I could type this out by doing echo, quote, 527 00:19:39,600 --> 00:19:41,520 the current directory is dollar sign parenthesis, 528 00:19:41,520 --> 00:19:44,490 pwd, parenthesis, quote. 529 00:19:44,490 --> 00:19:46,710 And this would then do the exact same thing. 530 00:19:46,710 --> 00:19:49,680 Now, another thing we need to talk about is globbing. 531 00:19:49,680 --> 00:19:51,630 Now globbing is a bass shell feature 532 00:19:51,630 --> 00:19:52,680 that's used for matching 533 00:19:52,680 --> 00:19:54,774 or expanding specific types of patterns. 534 00:19:54,774 --> 00:19:57,210 Globbing is mainly used to match file names 535 00:19:57,210 --> 00:20:00,090 and searching for content within a given file. 536 00:20:00,090 --> 00:20:02,130 Globbing is going to rely on wild card characters 537 00:20:02,130 --> 00:20:03,540 to create different patterns, 538 00:20:03,540 --> 00:20:05,130 and that way you can find the information 539 00:20:05,130 --> 00:20:06,300 you're looking for. 540 00:20:06,300 --> 00:20:09,420 Now, there are three most commonly used wild card characters 541 00:20:09,420 --> 00:20:11,400 when you're creating a globbing pattern. 542 00:20:11,400 --> 00:20:13,440 These are the star, the question mark 543 00:20:13,440 --> 00:20:15,090 and the square brackets. 544 00:20:15,090 --> 00:20:18,270 First, we have the star also known as an asterisk. 545 00:20:18,270 --> 00:20:20,670 This is going to be used to match any number of characters. 546 00:20:20,670 --> 00:20:24,895 For example, if I type in cp star.txt 547 00:20:24,895 --> 00:20:27,600 tilde/destination, 548 00:20:27,600 --> 00:20:31,200 this is going to copy all the files with a .txt extension 549 00:20:31,200 --> 00:20:34,170 because that wild card is appearing before the period. 550 00:20:34,170 --> 00:20:38,708 And that means anything .txt is going to match my criteria. 551 00:20:38,708 --> 00:20:41,310 Now it's important to realize that when you're using a star 552 00:20:41,310 --> 00:20:44,460 or an asterisk, it's not replacing a single character, 553 00:20:44,460 --> 00:20:46,140 it's replacing zero characters, 554 00:20:46,140 --> 00:20:48,810 one characters or infinite number of characters. 555 00:20:48,810 --> 00:20:51,540 So if I had a file called .txt 556 00:20:51,540 --> 00:20:56,190 or a file called one.txt or a file called Jason.txt, 557 00:20:56,190 --> 00:20:58,260 all three of those will match the wildcard 558 00:20:58,260 --> 00:21:02,100 of star.txt because they all end with .txt, 559 00:21:02,100 --> 00:21:04,320 even if there's nothing before the dot. 560 00:21:04,320 --> 00:21:06,270 The second one we have is a question mark. 561 00:21:06,270 --> 00:21:09,540 Now the question mark is used to match a single character. 562 00:21:09,540 --> 00:21:12,300 For example, if I was looking for any file 563 00:21:12,300 --> 00:21:14,790 that had a single letter .txt, 564 00:21:14,790 --> 00:21:17,100 and I wanted to copy those to my destination directory, 565 00:21:17,100 --> 00:21:21,270 I could do that by typing in cp ?.txt 566 00:21:21,270 --> 00:21:23,340 tilde/destination. 567 00:21:23,340 --> 00:21:28,005 And this is going to copy files like a.txt, b.txt, c.txt, 568 00:21:28,005 --> 00:21:30,480 but it won't do Jason.txt 569 00:21:30,480 --> 00:21:32,370 because that's too many characters. 570 00:21:32,370 --> 00:21:35,040 Note here that if I do have that question mark, 571 00:21:35,040 --> 00:21:37,380 it can also represent a zero character. 572 00:21:37,380 --> 00:21:42,137 So .txt would still match just like a.txt and b.txt, 573 00:21:43,020 --> 00:21:46,365 but Jason will not because that's too many characters. 574 00:21:46,365 --> 00:21:49,470 The third type we have is notice square brackets. 575 00:21:49,470 --> 00:21:51,750 And any characters you put between the square brackets 576 00:21:51,750 --> 00:21:53,550 can be used to match any of the characters 577 00:21:53,550 --> 00:21:55,781 that are listed inside of there. 578 00:21:55,781 --> 00:21:59,716 For example, let's say I did something like cp bracket, 579 00:21:59,716 --> 00:22:04,716 ABC bracket .txt tilde/destination. 580 00:22:04,980 --> 00:22:06,150 What will this do? 581 00:22:06,150 --> 00:22:08,490 Well, it's going to copy all the files 582 00:22:08,490 --> 00:22:12,720 that are named a.txt, b.txt or c.txt. 583 00:22:12,720 --> 00:22:14,790 Notice inside those square brackets, 584 00:22:14,790 --> 00:22:16,470 we're only going to pick one letter 585 00:22:16,470 --> 00:22:19,380 out of the combinations there and be able to use those. 586 00:22:19,380 --> 00:22:22,620 So if I wanted to put all the letters like A through Z, 587 00:22:22,620 --> 00:22:25,200 I could do that by doing a dash Z 588 00:22:25,200 --> 00:22:26,820 inside those square brackets. 589 00:22:26,820 --> 00:22:29,370 And now I'm going to find every single file 590 00:22:29,370 --> 00:22:32,190 that starts with one of the letters .txt, 591 00:22:32,190 --> 00:22:34,800 which would be very similar to using the question mark. 592 00:22:34,800 --> 00:22:37,950 But if I wanted to be able to have just a, b and c, 593 00:22:37,950 --> 00:22:41,100 putting those inside my brackets will give me those options. 594 00:22:41,100 --> 00:22:42,690 Now, the next thing we're going to talk about 595 00:22:42,690 --> 00:22:44,370 is a positional parameter. 596 00:22:44,370 --> 00:22:45,510 Now positional parameter 597 00:22:45,510 --> 00:22:47,430 is a variable within your shell script 598 00:22:47,430 --> 00:22:50,160 that's assigned to an argument when the script is invoked. 599 00:22:50,160 --> 00:22:53,640 For example, let's say I call the script dion.sh 600 00:22:53,640 --> 00:22:58,380 by typing dion.sh arg one, arg two, arg three. 601 00:22:58,380 --> 00:23:01,680 Now the arg one argument corresponds to the first position 602 00:23:01,680 --> 00:23:04,050 which we refer to as dollar sign one. 603 00:23:04,050 --> 00:23:07,170 Arg two corresponds to the second position, dollar sign two, 604 00:23:07,170 --> 00:23:09,540 and arg three corresponds to the third position, 605 00:23:09,540 --> 00:23:11,610 dollar sign three, and so on. 606 00:23:11,610 --> 00:23:13,950 Notice there's a space between each of these arguments 607 00:23:13,950 --> 00:23:15,180 and that space is used 608 00:23:15,180 --> 00:23:17,400 to separate those positional parameters. 609 00:23:17,400 --> 00:23:19,800 So let's go ahead and look at dion.sh 610 00:23:19,800 --> 00:23:21,330 this little script I have. 611 00:23:21,330 --> 00:23:22,557 I have four lines. 612 00:23:22,557 --> 00:23:25,530 The first line defines that this is a bash script 613 00:23:25,530 --> 00:23:28,800 by doing hashtag exclamation slash bin slash bash. 614 00:23:28,800 --> 00:23:30,690 And then the next three are all looking the same, 615 00:23:30,690 --> 00:23:33,090 but the only difference is which argument we're calling. 616 00:23:33,090 --> 00:23:35,220 Echo dollar sign one is the first argument, 617 00:23:35,220 --> 00:23:36,987 echo dollar sign two is the second argument 618 00:23:36,987 --> 00:23:39,810 and echo dollar sign three is the third argument. 619 00:23:39,810 --> 00:23:43,260 Now, if I call this script by entering dion.sh, 620 00:23:43,260 --> 00:23:46,560 space, Jason space, dion space, training, 621 00:23:46,560 --> 00:23:48,030 what am I going to get back? 622 00:23:48,030 --> 00:23:50,670 Well, I'm going to get three sentences back that say, 623 00:23:50,670 --> 00:23:52,140 Jason is the first argument, 624 00:23:52,140 --> 00:23:53,550 Dion is the second argument 625 00:23:53,550 --> 00:23:55,500 and training is the third argument. 626 00:23:55,500 --> 00:23:57,090 Now this is a really simple script, 627 00:23:57,090 --> 00:23:59,100 and all it's doing is echoing out information 628 00:23:59,100 --> 00:24:01,020 that we gave it from the command prompt. 629 00:24:01,020 --> 00:24:03,930 Now, what happens if I forget to give any arguments? 630 00:24:03,930 --> 00:24:06,293 Well, let's say I just run it as dion.sh, 631 00:24:06,293 --> 00:24:07,680 what's going to happen? 632 00:24:07,680 --> 00:24:09,690 Well, it's going to return the three sentences, 633 00:24:09,690 --> 00:24:12,930 but the arguments are being filled in with a blank space 634 00:24:12,930 --> 00:24:14,910 because we didn't give it any arguments. 635 00:24:14,910 --> 00:24:17,910 So we're going to get blank as the first argument, 636 00:24:17,910 --> 00:24:19,260 blank is the second argument 637 00:24:19,260 --> 00:24:21,963 and blank is the third argument displayed on our screen. 638 00:24:21,963 --> 00:24:24,600 Now, if you want to hard code the arguments into the script, 639 00:24:24,600 --> 00:24:27,570 you can do that by using something known as set. 640 00:24:27,570 --> 00:24:30,060 So we're going to modify our script by adding a new line 641 00:24:30,060 --> 00:24:34,350 that says set space dash dash space, CompTIA space, 642 00:24:34,350 --> 00:24:36,120 Dion space, training. 643 00:24:36,120 --> 00:24:38,850 Now because I've hardcoded those arguments into the script, 644 00:24:38,850 --> 00:24:41,340 I don't have to enter them at the command line. 645 00:24:41,340 --> 00:24:44,850 So I simply enter dion.sh and hit enter, 646 00:24:44,850 --> 00:24:46,920 and these three sentences are going to be returned 647 00:24:46,920 --> 00:24:49,290 to the screen with those arguments filled in. 648 00:24:49,290 --> 00:24:52,440 So now I get CompTIA is the first argument, 649 00:24:52,440 --> 00:24:54,150 Dion is the second argument, 650 00:24:54,150 --> 00:24:56,850 training is the third argument and so on. 651 00:24:56,850 --> 00:24:59,370 Now note because I hardcoded those into the script 652 00:24:59,370 --> 00:25:00,870 using the set command, 653 00:25:00,870 --> 00:25:03,720 I can no longer pass those arguments from the command line 654 00:25:03,720 --> 00:25:04,890 and overwrite them. 655 00:25:04,890 --> 00:25:07,200 So if I entered in the command I used before 656 00:25:07,200 --> 00:25:11,190 of dion.sh space, Jason space, Dion space, training, 657 00:25:11,190 --> 00:25:13,710 I'm still going to get the same three sentences out 658 00:25:13,710 --> 00:25:16,260 that I got before with those hard coded arguments, 659 00:25:16,260 --> 00:25:19,230 because it's going to say CompTIA is the first argument, 660 00:25:19,230 --> 00:25:21,240 not Jason is the first argument 661 00:25:21,240 --> 00:25:23,907 because CompTIA is hard coded inside of the script, 662 00:25:23,907 --> 00:25:25,410 and so we're going to ignore the arguments 663 00:25:25,410 --> 00:25:27,300 passed from the command line. 664 00:25:27,300 --> 00:25:30,000 Now, the next command we're going to talk about is exec. 665 00:25:30,000 --> 00:25:32,850 Now the exec command does not create a new process 666 00:25:32,850 --> 00:25:33,683 when you run it. 667 00:25:33,683 --> 00:25:36,240 Instead, it only replaces the bash shell 668 00:25:36,240 --> 00:25:38,400 with the command being executed. 669 00:25:38,400 --> 00:25:40,440 Now, this is useful if you want to prevent the user 670 00:25:40,440 --> 00:25:43,080 from returning to a parent process inside your script, 671 00:25:43,080 --> 00:25:44,880 when there's an error that's encountered. 672 00:25:44,880 --> 00:25:47,610 For example, you might want to terminate a privileged shell 673 00:25:47,610 --> 00:25:48,990 if a command fails. 674 00:25:48,990 --> 00:25:50,520 And if no command is supplied, 675 00:25:50,520 --> 00:25:51,870 the redirections are going to be used 676 00:25:51,870 --> 00:25:54,060 to modify the current shell environment. 677 00:25:54,060 --> 00:25:55,410 This can be very useful to allow us 678 00:25:55,410 --> 00:25:57,480 to change the file descriptors of a shell 679 00:25:57,480 --> 00:25:58,980 based on our desires. 680 00:25:58,980 --> 00:26:01,860 The process is going to continue even after the exec command, 681 00:26:01,860 --> 00:26:03,390 unlike the previous case, 682 00:26:03,390 --> 00:26:05,490 but now standard input, output and error 683 00:26:05,490 --> 00:26:08,790 are modified according to the redirections that we have. 684 00:26:08,790 --> 00:26:11,130 Another command we have to know about is source. 685 00:26:11,130 --> 00:26:13,650 The source command is used to execute another command 686 00:26:13,650 --> 00:26:15,810 within the current shell process. 687 00:26:15,810 --> 00:26:18,180 In this sense, it really performs the opposite function 688 00:26:18,180 --> 00:26:19,770 of the exec command. 689 00:26:19,770 --> 00:26:21,120 This is really useful when you'd like 690 00:26:21,120 --> 00:26:22,560 to stay within your current shell 691 00:26:22,560 --> 00:26:24,240 when you're executing a script. 692 00:26:24,240 --> 00:26:26,760 For example, let's say you wanted to change a directory 693 00:26:26,760 --> 00:26:28,110 inside of your script. 694 00:26:28,110 --> 00:26:30,420 Well, you'd probably want to source that first 695 00:26:30,420 --> 00:26:33,270 and by sourcing it, it means that after the script executes, 696 00:26:33,270 --> 00:26:35,850 your location will be whatever directory you change to 697 00:26:35,850 --> 00:26:37,110 inside of the script, 698 00:26:37,110 --> 00:26:38,640 because the script was operating 699 00:26:38,640 --> 00:26:41,250 just as if you were typing in those commands. 700 00:26:41,250 --> 00:26:42,660 Now for the most part, 701 00:26:42,660 --> 00:26:44,910 Linux file extensions are really optional. 702 00:26:44,910 --> 00:26:46,290 You don't need them on everything, 703 00:26:46,290 --> 00:26:48,600 but you'll notice I use .sh 704 00:26:48,600 --> 00:26:51,120 when I gave an extension to my bash script. 705 00:26:51,120 --> 00:26:52,200 Why is that? 706 00:26:52,200 --> 00:26:55,170 Well, it's because Linux checks the files metadata 707 00:26:55,170 --> 00:26:56,880 to determine what type of file it is. 708 00:26:56,880 --> 00:27:01,109 But those .shs and .txts, that helps us as humans 709 00:27:01,109 --> 00:27:03,450 know what type of a file we're looking at. 710 00:27:03,450 --> 00:27:05,430 So you don't really need to name your scripts 711 00:27:05,430 --> 00:27:07,860 with an extension, but by default, 712 00:27:07,860 --> 00:27:11,580 we do add .sh as an extension for shells scripts 713 00:27:11,580 --> 00:27:13,740 to ensure that everybody understands that that is a script 714 00:27:13,740 --> 00:27:15,510 and it has special meaning to us. 715 00:27:15,510 --> 00:27:17,970 It really is for the people and not for the computer, 716 00:27:17,970 --> 00:27:19,800 which is something to be aware of. 717 00:27:19,800 --> 00:27:22,230 Remember your ability to use any file, 718 00:27:22,230 --> 00:27:24,600 including a script is really only going to be constrained 719 00:27:24,600 --> 00:27:26,490 by the permissions assigned to that script 720 00:27:26,490 --> 00:27:27,870 and your user account. 721 00:27:27,870 --> 00:27:29,460 So even though you created a script, 722 00:27:29,460 --> 00:27:32,070 you're not going to automatically have permissions to run that 723 00:27:32,070 --> 00:27:34,140 unless you do a change mod command 724 00:27:34,140 --> 00:27:36,180 and you give it execute abilities. 725 00:27:36,180 --> 00:27:38,460 So make sure you set two permissions 726 00:27:38,460 --> 00:27:40,050 every time you write a script. 727 00:27:40,050 --> 00:27:42,120 First, you need to ensure that your user 728 00:27:42,120 --> 00:27:44,850 has the permissions to execute the script itself. 729 00:27:44,850 --> 00:27:47,220 And second, you want to make sure that you have right 730 00:27:47,220 --> 00:27:49,320 and execute bits set on the directory 731 00:27:49,320 --> 00:27:50,850 that contains that script. 732 00:27:50,850 --> 00:27:53,490 If that directory doesn't have the execute bits set, 733 00:27:53,490 --> 00:27:55,530 it's not going to allow you to be able to run that script, 734 00:27:55,530 --> 00:27:57,930 even if the script is set to execute. 735 00:27:57,930 --> 00:28:00,750 Remember, you can always control who can access the files, 736 00:28:00,750 --> 00:28:02,730 search their directories and run scripts 737 00:28:02,730 --> 00:28:04,890 using Linux's change mod command 738 00:28:04,890 --> 00:28:07,830 by being able to apply or remove, read, write, 739 00:28:07,830 --> 00:28:10,773 and execute bits onto those directories and files.