1 00:00:03,670 --> 00:00:09,090 In this lesson you're going to learn about some special variables created by the shell at a store the 2 00:00:09,090 --> 00:00:11,000 output of a command in a variable. 3 00:00:11,100 --> 00:00:16,080 And we're going to talk a little bit about the if statement first thing I'm going to do here is open 4 00:00:16,080 --> 00:00:18,090 up a terminal on my local machine. 5 00:00:18,270 --> 00:00:23,370 I'm going to go into our class folder of show class room here. 6 00:00:23,370 --> 00:00:27,210 I'm going to go into a local user CD local users. 7 00:00:27,210 --> 00:00:33,100 We're still working on creating local users and I'm going to start this virtual machine 8 00:00:41,080 --> 00:00:47,780 once the machine is booted you can connect to it with vagrant S-sh. 9 00:00:47,780 --> 00:00:53,940 Now let's move into our shared folder of Ford slash vagrant Tim get there with a cd command. 10 00:00:54,380 --> 00:01:02,510 Now let's start working on the script for this lesson Minick called this L user demo to dot S H. 11 00:01:02,510 --> 00:01:09,470 Remember we always start our scripts so the banks are going to start out with a sharp pain or spin or 12 00:01:09,530 --> 00:01:11,250 slash Bash. 13 00:01:11,360 --> 00:01:16,830 As you probably remember from the last lesson I like to put a header at the top of the file that just 14 00:01:16,850 --> 00:01:18,950 describes what the shell script is going to do. 15 00:01:18,950 --> 00:01:20,700 I'm going to put that in right now. 16 00:01:35,890 --> 00:01:41,350 So these are the two main goals that I have for this script which is to display the idea which stands 17 00:01:41,350 --> 00:01:48,730 for the user id or user identification number and the user name of the user that's executing this particular 18 00:01:48,730 --> 00:01:49,520 script. 19 00:01:49,630 --> 00:01:54,760 And the last thing we want to do in this script is to display if the user that is executing the script 20 00:01:55,030 --> 00:02:00,910 is the root user or if they are some other user they're either the root user executing the script or 21 00:02:00,910 --> 00:02:02,000 they're not. 22 00:02:02,200 --> 00:02:07,090 One of the things that can help you write shell scripts is to think about all the steps that it takes 23 00:02:07,090 --> 00:02:08,590 to accomplish your goal. 24 00:02:08,590 --> 00:02:10,180 We have the goal in the header. 25 00:02:10,300 --> 00:02:16,230 So now let's think about some steps that we need to take in order to accomplish our goal. 26 00:02:16,240 --> 00:02:18,310 A lot of people call this pseudo coding. 27 00:02:18,310 --> 00:02:24,190 And so what you do is you just write in plain English and your language of what you want to accomplish 28 00:02:24,220 --> 00:02:25,380 as a comment. 29 00:02:25,490 --> 00:02:31,750 But those comments in your file and then go back and put in the exact shell commands and syntax that 30 00:02:31,750 --> 00:02:33,060 you'll need to accomplish. 31 00:02:33,190 --> 00:02:38,250 Each one of those tasks so it's kind of like outlining before you write the shell script. 32 00:02:38,260 --> 00:02:44,280 So I'm going to go ahead and do that now the first thing I want to do is display the ID. 33 00:02:44,440 --> 00:02:49,260 The next thing I want to do is display a user name. 34 00:02:49,320 --> 00:02:56,790 And finally what I want to do is display if the user is the root user or not. 35 00:02:58,180 --> 00:03:03,850 So now we have our Hetter and our pseudo code now and start filling in the code. 36 00:03:03,850 --> 00:03:06,990 So the first thing we want to do is display the ID. 37 00:03:07,120 --> 00:03:12,280 And of course you know that the echo shell built in displays information to the screen and we're going 38 00:03:12,280 --> 00:03:21,100 to use something that the shell gives us for free I might show you here your ID is you ID as you know 39 00:03:21,100 --> 00:03:26,590 if you perceive something with a dollar sign and enclose it in curly braces that it's a variable. 40 00:03:26,590 --> 00:03:27,980 This variable is you. 41 00:03:28,000 --> 00:03:30,690 But we didn't assign a variable or set a variable. 42 00:03:30,790 --> 00:03:33,560 So where is that variable going to come from. 43 00:03:33,730 --> 00:03:37,440 Well remember that I said you get a lot of stuff quote stuff for free. 44 00:03:37,540 --> 00:03:42,940 When you use bash one of the things you get for free are special preset variables. 45 00:03:42,970 --> 00:03:47,170 So let's go ahead and write and quit our script. 46 00:03:47,170 --> 00:03:51,240 Remember before you execute the script for the first time you want to set the executable bit on it. 47 00:03:54,430 --> 00:03:56,990 And now let's execute our script and see what happens. 48 00:03:58,700 --> 00:04:01,630 Hey it says your ID is 1000. 49 00:04:01,830 --> 00:04:06,450 So let's look at the bash man page to see where this UAD comes from. 50 00:04:08,240 --> 00:04:13,070 By the way to navigate a man page you're going to use vi or VM keybindings. 51 00:04:13,070 --> 00:04:17,640 And so how to search for it is a forward slash and then the string you're searching for. 52 00:04:17,660 --> 00:04:19,420 We're looking for you. 53 00:04:19,450 --> 00:04:24,390 So we'll do that and hit enter to take us to the first instance of that. 54 00:04:24,450 --> 00:04:29,610 And if we want to keep going down the file it in to go to the next occurrence. 55 00:04:29,960 --> 00:04:35,630 And here you can see that the information about this says that it expands to the ID of the current user 56 00:04:35,750 --> 00:04:37,700 initialized at Shell startup. 57 00:04:37,700 --> 00:04:39,890 This variable is a read only. 58 00:04:39,890 --> 00:04:42,410 That means that a user can't change it. 59 00:04:42,410 --> 00:04:48,380 So if we were to check against this ID we can be fairly certain that it's going to be the actual source 60 00:04:48,380 --> 00:04:48,890 of truth. 61 00:04:48,890 --> 00:04:52,000 It's really going to have the you ID in it. 62 00:04:53,440 --> 00:04:55,100 Now let me back up a little bit. 63 00:04:55,100 --> 00:04:58,730 I'm going to hit shift and to back up to the previous match. 64 00:04:58,760 --> 00:05:05,300 Now there is a similar variable here called the you I.D. which expands to the effective user id of the 65 00:05:05,300 --> 00:05:08,030 current user initialized at Shell startup. 66 00:05:08,030 --> 00:05:10,960 This very well is also read only. 67 00:05:11,090 --> 00:05:15,230 So in reality we could use EU ID or ID. 68 00:05:15,410 --> 00:05:23,660 The only time that you ID would be different from EU ID is in the case of a set id script when a file 69 00:05:23,660 --> 00:05:26,210 has a special set id permissions set on it. 70 00:05:26,300 --> 00:05:32,330 The file actually gets executed as the owner of the file no matter what user is executing the file. 71 00:05:32,420 --> 00:05:39,880 So if its set you ID root and where the vagrant user and we execute the script are you ID is 1004 vagrant. 72 00:05:39,890 --> 00:05:43,070 But r e ID is 0 for root. 73 00:05:43,100 --> 00:05:49,880 However this is rarely seen modern Linux systems for security reasons do not allow scripts to be set. 74 00:05:49,880 --> 00:05:50,910 You ID. 75 00:05:51,020 --> 00:05:56,510 So this is just something to keep in mind for example if you're working on say a proprietary UNIX system 76 00:05:56,510 --> 00:05:57,660 that's somewhat old. 77 00:05:57,830 --> 00:06:03,950 But again if you're working on modern day Linux systems you can safely use you ID because e you ID and 78 00:06:03,950 --> 00:06:08,510 you ID are going to be the same but just know if you see that somewhere else that is where that comes 79 00:06:08,510 --> 00:06:09,050 from. 80 00:06:09,290 --> 00:06:17,310 OK let's back up even farther Monotype questionmark for a reverse search and type shell and hit enter 81 00:06:17,310 --> 00:06:18,210 here. 82 00:06:18,210 --> 00:06:24,420 So the ID and you ID that we were just reading about happened to be in the shell variables section of 83 00:06:24,420 --> 00:06:26,140 the bash man page. 84 00:06:26,370 --> 00:06:32,880 So not only do you get e ID and you ID you get several other variables that anytime you start Bash that 85 00:06:32,940 --> 00:06:34,760 all these are going to be set. 86 00:06:35,130 --> 00:06:40,010 So if you want take a moment here and just read down these variables and see what's available to you. 87 00:06:41,580 --> 00:06:48,390 OK I'm just going to hit Q to quit since we're done looking at the man page and I quickly want to demonstrate 88 00:06:48,390 --> 00:06:51,340 that to you ID is indeed read only. 89 00:06:51,420 --> 00:06:56,420 So if we do echo your I.D. It's 1000. 90 00:06:56,720 --> 00:07:03,100 If we do you ID equals let's say we're going to set it to 1000 of one hit enter. 91 00:07:03,170 --> 00:07:06,880 It says no you can't do that because it's a read only variable. 92 00:07:07,070 --> 00:07:10,820 So again we can trust the information that's in the ID variable. 93 00:07:10,880 --> 00:07:15,110 So if you remember from our script the next thing we want to do is actually display the user name. 94 00:07:15,110 --> 00:07:16,520 So how do we do that. 95 00:07:16,580 --> 00:07:22,440 Well one way to do that is with the ID command and then see this type dash a ID. 96 00:07:22,520 --> 00:07:26,360 So it is actually a program and it's not a shell built in. 97 00:07:26,390 --> 00:07:32,720 So since it's not a shell built in we can learn about it by using the man page man id by the way when 98 00:07:32,720 --> 00:07:34,610 you're looking at a man page. 99 00:07:34,640 --> 00:07:39,820 If you see things that are enclosed in square brackets that means they are optional. 100 00:07:39,980 --> 00:07:44,330 So here you can see ID with optional option an optional user. 101 00:07:44,450 --> 00:07:48,620 So actually you can run this command on its own and doesn't require any arguments because they're all 102 00:07:48,620 --> 00:07:49,430 optional. 103 00:07:49,670 --> 00:07:55,340 Additionally if you see these three dots or three periods also called an ellipsis I do believe that 104 00:07:55,340 --> 00:07:58,430 you can specify multiple of whatever precedes it. 105 00:07:58,430 --> 00:08:05,540 So you can have the command followed by multiple options and then one optional user. 106 00:08:05,690 --> 00:08:11,180 So the description here says print user and group information for the specified user or when user omitted 107 00:08:11,240 --> 00:08:13,540 for the current user. 108 00:08:13,570 --> 00:08:18,330 So I'm going to look at what our options are here dash and we'll print a name instead of a number. 109 00:08:18,340 --> 00:08:23,650 And so we are actually looking to print a name so that's useful that you print only the effective user 110 00:08:23,650 --> 00:08:24,460 ID. 111 00:08:24,490 --> 00:08:32,130 So those look like the two things that we need to use to print a user name would be Dasch you in the 112 00:08:32,490 --> 00:08:39,040 queue here and just test out the ID command with no options ID it enter and says Are you ID is 1000 113 00:08:39,130 --> 00:08:39,610 vagrant. 114 00:08:39,610 --> 00:08:41,030 Is our user name. 115 00:08:41,070 --> 00:08:46,110 The idea is 1000 which is a group ID and vagrant is the username of that group and the groups that were 116 00:08:46,120 --> 00:08:50,030 member of is also called vagrant with a group id of 1000. 117 00:08:50,380 --> 00:08:56,520 So now let's specify ideaspace dash you and that prints out the ID of 1000. 118 00:08:56,770 --> 00:09:02,770 Now if we wanted to add the dash in option we could do this ID dash you dash and and it displays our 119 00:09:02,820 --> 00:09:04,240 user name. 120 00:09:04,340 --> 00:09:08,140 Also want to point out that it doesn't matter about the order of these options. 121 00:09:08,260 --> 00:09:11,850 So I can do id dash and DSU and get the same result. 122 00:09:12,130 --> 00:09:18,660 Another thing I want to point out is when you have these single character options you can combine them. 123 00:09:18,760 --> 00:09:21,000 So will do id dash. 124 00:09:21,220 --> 00:09:25,830 And then we can list all the single character options that we need without any for the dashes. 125 00:09:25,840 --> 00:09:31,720 So we'll do dash and you and that will give us our user name or it doesn't matter about the order so 126 00:09:31,720 --> 00:09:37,510 we can do id space dash you and press enter and we get the same result. 127 00:09:37,720 --> 00:09:43,780 For me Id dash you in is a steadily pleasing because it's you are a user and for user name so that makes 128 00:09:43,780 --> 00:09:45,910 sense to me that's the command I'm going to use. 129 00:09:45,910 --> 00:09:51,320 But any of those variations work obviously so if you see them you'll know what's going on. 130 00:09:51,430 --> 00:09:55,730 By the way some of you may be familiar with the Who am I command will execute it now. 131 00:09:56,110 --> 00:09:57,670 And that would be another option. 132 00:09:57,670 --> 00:09:59,920 Let's look at what type of command it is. 133 00:10:01,560 --> 00:10:04,990 It is a program so I'm going to use the man page to learn more about it. 134 00:10:05,840 --> 00:10:06,460 Who am I. 135 00:10:06,460 --> 00:10:08,250 Print effective user ID. 136 00:10:08,250 --> 00:10:08,760 Who am I. 137 00:10:08,760 --> 00:10:11,180 Again you can specify options. 138 00:10:11,190 --> 00:10:15,080 The options are optional and you can specify more than one of them. 139 00:10:15,090 --> 00:10:21,180 The description is prent the user name associated with the current effective user id same as ID dash 140 00:10:21,270 --> 00:10:22,320 you in. 141 00:10:22,320 --> 00:10:24,990 So that's what we're going to use ID dashi when you can use. 142 00:10:24,990 --> 00:10:25,610 Who am I. 143 00:10:25,610 --> 00:10:29,690 If you would like as well hey let's return to editing the script. 144 00:10:32,850 --> 00:10:38,820 Before we display the user name let's assign the user name to a variable we'll call that variable user 145 00:10:38,820 --> 00:10:44,850 underscore name. 146 00:10:44,960 --> 00:10:50,450 If you want to store the output of a command into a variable you use the syntax you see here on your 147 00:10:50,450 --> 00:10:55,920 screen which is a variable name equals dollar sign opening parentheses. 148 00:10:56,000 --> 00:11:01,730 Then the command and the clothing parentheses the value for the variable name user underscore name will 149 00:11:01,730 --> 00:11:04,860 equal the output of ID dash you in. 150 00:11:05,030 --> 00:11:07,470 Let's test this here let's echo it to our screen. 151 00:11:14,670 --> 00:11:16,920 Him to say my changes and execute the script 152 00:11:20,660 --> 00:11:22,860 so it says your ID is 1000. 153 00:11:22,880 --> 00:11:30,840 Your user name is vagrant want to demonstrate an alternative syntax and you'll see sometimes as well. 154 00:11:30,960 --> 00:11:35,060 So let's edit our script let's comment out this line. 155 00:11:35,240 --> 00:11:36,890 I'm going to make a copy of it. 156 00:11:37,110 --> 00:11:44,670 And then instead of dollar sign opening parentheses I'm going to replace that with a tick and the closing 157 00:11:44,670 --> 00:11:50,010 parentheses I'm going to replace that with a tick as well going to execute the script and see if we 158 00:11:50,010 --> 00:11:51,230 get the same result. 159 00:11:51,540 --> 00:11:54,650 And sure enough we do let's get back into here. 160 00:11:54,840 --> 00:11:57,310 So that is an older style syntax. 161 00:11:57,330 --> 00:12:03,730 The tick marks that surround a command to assign that to a variable that's the Oler syntax. 162 00:12:03,900 --> 00:12:08,700 So again I like to show you these things because different people write shell scripts in different ways 163 00:12:08,730 --> 00:12:14,370 and you'll be looking at Shell scripts from perhaps earlier times that were written without these modern 164 00:12:14,370 --> 00:12:20,010 conventions so if you see these back Tick's know that it's the same thing as the dollar sign parentheses 165 00:12:20,010 --> 00:12:21,280 syntax. 166 00:12:21,280 --> 00:12:26,990 So going to remove that line uncomment that other line and we should be back to where we were. 167 00:12:27,360 --> 00:12:30,720 For someone to type out an if statement here and then we'll talk through it. 168 00:12:50,280 --> 00:12:57,990 So an if statement begins with the key word if followed by a space then double opening brackets followed 169 00:12:57,990 --> 00:13:05,670 by a space and then an expression after that expression you'll use a space and then the closing double 170 00:13:05,670 --> 00:13:11,880 brackets on the next line you'll use the keyword then and then whatever follows that will get executed 171 00:13:11,910 --> 00:13:14,560 if the expression evaluates to true. 172 00:13:14,820 --> 00:13:22,320 So in our case if the ID is equal to zero then we'll execute the code that says echo you are root. 173 00:13:22,680 --> 00:13:28,350 If you want to include an else statement then you use the keyword and then the code that follows that 174 00:13:28,350 --> 00:13:29,120 will execute. 175 00:13:29,130 --> 00:13:36,360 If the expression evaluates to false so if ID is not equal to zero then what's going to happen is Echo 176 00:13:36,360 --> 00:13:42,810 will be run with you are not root displaying to the screen you close your if statement with F I which 177 00:13:42,810 --> 00:13:44,400 is the reverse of I-F. 178 00:13:44,610 --> 00:13:48,140 And that is the general syntax for the if statement. 179 00:13:48,240 --> 00:13:50,400 I want to point out a couple of different things here again. 180 00:13:50,400 --> 00:13:53,310 The F word should be followed by a space. 181 00:13:53,430 --> 00:13:58,530 The opening brackets should be followed by a space and the closing double bracket should have a space 182 00:13:58,530 --> 00:13:59,770 preceding it. 183 00:13:59,940 --> 00:14:05,910 When you are using variables inside of these double brackets as a test you always want to enclose them 184 00:14:05,910 --> 00:14:07,310 in quotation marks. 185 00:14:07,590 --> 00:14:11,850 OK let's get back to the shell and learn a little bit more about this statement. 186 00:14:12,360 --> 00:14:13,970 So let's do type dash a. 187 00:14:13,980 --> 00:14:18,950 If it turns out that it is a shell keyword so we can get help on that with help. 188 00:14:19,020 --> 00:14:20,190 I have. 189 00:14:20,360 --> 00:14:26,420 So here it says if commands semi-colon then commands a semi-colon and then as you already know things 190 00:14:26,420 --> 00:14:28,410 that are in brackets are optional. 191 00:14:28,510 --> 00:14:31,340 You can absolutely use Elif or else. 192 00:14:31,460 --> 00:14:34,770 And then what is not optional is the closing f I. 193 00:14:34,790 --> 00:14:42,260 So if you remember I said we were going to do this we're going to use F and then double brackets here 194 00:14:42,320 --> 00:14:57,940 and do a test. 195 00:14:57,970 --> 00:15:04,270 So what I want to point out is in the if description it says if commands a semi-colon then commands 196 00:15:04,270 --> 00:15:06,900 a semi-colon what the semi-colon is. 197 00:15:06,900 --> 00:15:12,520 Is the command separator another command separator is to hit enter or begin on a new line. 198 00:15:12,700 --> 00:15:18,310 So if we bring this last command in our shell history back up I'll just hit the up arrow you can see 199 00:15:18,310 --> 00:15:22,020 that where there were line breaks when I entered them in on the command line. 200 00:15:22,150 --> 00:15:27,790 They are now replaced with semi-colon so again the semi-colon is a command separator so any time you 201 00:15:27,790 --> 00:15:30,370 see a semi-colon in some of these descriptions. 202 00:15:30,400 --> 00:15:36,010 Know that you can use a new line and I like to use the new line so it doesn't run altogether and it 203 00:15:36,010 --> 00:15:39,350 looks visually better to me and it's easier for me to read. 204 00:15:39,640 --> 00:15:45,380 However it is correct to also use a semi-colon and keep it all on one line. 205 00:15:45,760 --> 00:15:50,560 I just want to point out that if your command is not completed when you hit enter you'll get a different 206 00:15:50,560 --> 00:15:53,130 looking prompt here which is the greater than sign. 207 00:15:53,320 --> 00:15:57,310 And then once the command is done then you'll get your normal prompt back. 208 00:15:57,310 --> 00:15:58,950 I just wanted to point that out. 209 00:15:59,350 --> 00:16:01,060 So let's keep reading about the above statement. 210 00:16:01,060 --> 00:16:04,870 The IF command list is executed you can think a list is a list of commands. 211 00:16:04,920 --> 00:16:13,370 If the IF commands list is executed if its exit status is zero then the then commands list is executed. 212 00:16:13,420 --> 00:16:17,360 You can think of an exit status of 0 being true. 213 00:16:17,360 --> 00:16:20,850 So if this is true then DO THAT else. 214 00:16:20,920 --> 00:16:23,200 If it's not true then do something else. 215 00:16:23,350 --> 00:16:26,480 So you have a general idea of how the statement works. 216 00:16:26,680 --> 00:16:33,370 However in the if description it says if commands and if those commands have an exit status zero then 217 00:16:33,370 --> 00:16:34,650 execute some code. 218 00:16:34,870 --> 00:16:37,250 Well where do we get these double brackets. 219 00:16:37,300 --> 00:16:44,310 Well let me show you those double brackets are actually a shell keyword so let's get help on those. 220 00:16:48,620 --> 00:16:54,590 So here it says double brackets expression closing double brackets also says it returns a status of 221 00:16:54,590 --> 00:16:59,260 0 or 1 depending on the evaluation of the conditional expression expression. 222 00:16:59,450 --> 00:17:04,040 Expressions are composed of the same primary is used by the test built in and may be combined using 223 00:17:04,040 --> 00:17:05,570 the following operators. 224 00:17:05,570 --> 00:17:07,360 We're not going to worry about those operators for a moment. 225 00:17:07,370 --> 00:17:12,320 But the thing to keep in mind here is that it says you can use the same things that you can use with 226 00:17:12,320 --> 00:17:15,330 a test built in to generate your expressions. 227 00:17:15,520 --> 00:17:16,050 So 228 00:17:18,670 --> 00:17:22,260 test is indeed a shell built in as well as a command. 229 00:17:22,420 --> 00:17:25,240 We're interested in using the built in so we'll get help with it. 230 00:17:25,240 --> 00:17:31,030 Will do help test and I know from experience that a lot of information is going to scroll off the screen 231 00:17:31,030 --> 00:17:37,050 so one way to prevent that is to use a pager so will use a pipe to pipe the output of help test into 232 00:17:37,060 --> 00:17:40,110 our pager of Lessel Let's hit and and see what happens. 233 00:17:41,020 --> 00:17:44,650 So the test a built in evaluates conditional expressions. 234 00:17:44,650 --> 00:17:48,930 Here again to just reiterate what you already know 0 is true one is false. 235 00:17:49,180 --> 00:17:51,030 So here are all the tests that you can do. 236 00:17:51,030 --> 00:17:57,370 You can do file operators like Dasch a file to test if a file exists Dasch be if its a block special 237 00:17:57,370 --> 00:17:59,760 device and so on Dashti if its a directory. 238 00:17:59,950 --> 00:18:03,250 So there are all these types of tests you can use against files. 239 00:18:03,520 --> 00:18:09,700 Additionally there are string operators so dash Z string returns true if the string is empty or Dasch 240 00:18:09,700 --> 00:18:11,030 in string returns true. 241 00:18:11,050 --> 00:18:12,440 The string is not empty. 242 00:18:12,460 --> 00:18:18,690 You can compare strings with the equal sign or the not equal that less than and greater than etc.. 243 00:18:19,670 --> 00:18:25,120 This is actually what we're doing in our shellscript We're comparing the number of the ID to zero word 244 00:18:25,220 --> 00:18:29,490 saying is the ID dash Q Which stands for equal to zero. 245 00:18:29,960 --> 00:18:35,520 If it is equal then that's true and it returns a zero Q to exit out of the pager. 246 00:18:35,960 --> 00:18:38,950 OK let's execute our script and see what result we get. 247 00:18:42,880 --> 00:18:44,820 Says here you ideas 1000. 248 00:18:44,830 --> 00:18:46,300 Your user name is vagrant. 249 00:18:46,330 --> 00:18:48,490 You are not root. 250 00:18:48,490 --> 00:18:53,980 By the way if I felt the mention of the idea of zero is always assigned to the root user root always 251 00:18:53,980 --> 00:18:58,000 has the ID of 0 on every Linux system that you'll ever encounter. 252 00:18:58,000 --> 00:19:02,590 Every Unix system that you'll ever encounter route is always the idea of zero. 253 00:19:02,590 --> 00:19:05,310 That's why we checked against it because it's a guarantee. 254 00:19:05,350 --> 00:19:10,390 You could in theory test against the username but it's probably a little bit more safe and secure to 255 00:19:10,390 --> 00:19:12,320 test against ID zero. 256 00:19:12,400 --> 00:19:17,050 Because again that is always 100 percent always assigned to the root user. 257 00:19:17,260 --> 00:19:21,040 Speaking a root let's execute this program as the root user. 258 00:19:21,040 --> 00:19:25,270 One way to do that is use the command which stands for super user. 259 00:19:25,300 --> 00:19:27,070 Do so we'll do Sudoku. 260 00:19:27,260 --> 00:19:30,940 No user demo or to SH and hit enter. 261 00:19:31,360 --> 00:19:35,840 So now since we're using the sudo command this command actually gets executed as root. 262 00:19:35,950 --> 00:19:38,280 So it says your ID is 0. 263 00:19:38,290 --> 00:19:41,860 Your user name is root you are root. 264 00:19:42,100 --> 00:19:44,940 We could have also switched to the root user and done this as well. 265 00:19:44,960 --> 00:19:45,810 Can do su. 266 00:19:45,850 --> 00:19:52,730 And by the way the password for the root user on this system is vagrant and then now we can execute 267 00:19:52,730 --> 00:19:57,940 that as the root user and it says here you ID zero your username as root. 268 00:19:57,940 --> 00:19:59,280 You are definitely root. 269 00:19:59,320 --> 00:20:03,190 OK I'm going to exit back out to the vagrant user. 270 00:20:03,190 --> 00:20:04,810 Let's look at the contents of our file. 271 00:20:04,810 --> 00:20:05,890 One more time. 272 00:20:09,830 --> 00:20:17,510 I want to point out that this double bracket syntax we're using with the if statement is a bash specific 273 00:20:17,510 --> 00:20:18,410 thing. 274 00:20:18,410 --> 00:20:21,310 So this may not be portable to all shells. 275 00:20:21,320 --> 00:20:25,030 For example if you use NCSA script that's probably not going to work for you. 276 00:20:25,040 --> 00:20:27,320 But if you're using bash it will work. 277 00:20:27,320 --> 00:20:32,950 Another thing to note about the double bracket syntax is that it really replace the single bracket syntax. 278 00:20:33,050 --> 00:20:37,080 So actually you may see some shell scripts that look like this. 279 00:20:37,150 --> 00:20:42,080 If with a single opening bracket then a conditional expression and a single closing bracket. 280 00:20:42,380 --> 00:20:45,710 So again just know that the double brackets is the new way to do it. 281 00:20:45,710 --> 00:20:49,180 You can also use the single brackets that's an older way to do it. 282 00:20:49,190 --> 00:20:52,690 Let's get out to the shell here and we can actually do this. 283 00:20:53,150 --> 00:21:00,210 The single opening bracket is a shell built in but it's actually also a command on the file system. 284 00:21:00,380 --> 00:21:03,990 So on older Unix systems this is how it would work. 285 00:21:06,250 --> 00:21:09,110 So sure enough that's a file that's located at user bin. 286 00:21:09,120 --> 00:21:13,130 Opening bracket it's a file and you can execute it like that. 287 00:21:13,170 --> 00:21:18,910 So if you're using a really slimmed down shell that doesn't have the opening bracket built in the shell 288 00:21:18,910 --> 00:21:22,410 script will default to using user bin and the opening bracket. 289 00:21:22,410 --> 00:21:24,860 However we're using bash We don't have to worry about that. 290 00:21:24,960 --> 00:21:31,470 And again we're going to use the newer syntax of the double brackets OK let's look at our script one 291 00:21:31,560 --> 00:21:32,760 last time. 292 00:21:32,760 --> 00:21:34,570 Now this last a block of code. 293 00:21:34,570 --> 00:21:35,510 This if statement. 294 00:21:35,520 --> 00:21:38,010 If the UI equals zero then you are root. 295 00:21:38,010 --> 00:21:39,540 Else you are not root. 296 00:21:39,540 --> 00:21:45,810 Some people call this type of checking idiot checking and it's to help people from doing idiotic things 297 00:21:46,560 --> 00:21:49,120 other people call it sanity checking. 298 00:21:49,530 --> 00:21:55,140 And by the way it's a good idea to check for any requirements in your shellscript if you know something 299 00:21:55,140 --> 00:21:58,500 that is required for the script to work properly check for it. 300 00:21:58,500 --> 00:22:02,550 Never assume things are just going to be the way that you want them to be. 301 00:22:02,550 --> 00:22:08,070 For example if a script adds users and adding users requires root privileges then you want to check 302 00:22:08,100 --> 00:22:09,270 for root privileges. 303 00:22:09,270 --> 00:22:13,080 Don't assume that the user knows that they need to execute it with root privileges. 304 00:22:13,080 --> 00:22:15,160 They may executed as their own user. 305 00:22:15,300 --> 00:22:21,060 So you want to do some sanity checking or some idiot checking if you will to make sure that that happens 306 00:22:21,090 --> 00:22:22,310 as needed. 307 00:22:22,560 --> 00:22:27,210 And you want to do this type of sanity checking throughout the script whenever it's needed. 308 00:22:27,210 --> 00:22:31,290 Hey that brings us to the end of this lesson where you learned about some special variables that are 309 00:22:31,290 --> 00:22:32,710 created by the show. 310 00:22:32,730 --> 00:22:36,420 Now you can look up those variables by using man bash. 311 00:22:36,450 --> 00:22:41,640 You also learned how to store the output of a command in a variable using the dollar sign parentheses 312 00:22:41,640 --> 00:22:42,640 syntax. 313 00:22:42,720 --> 00:22:48,630 You also learned about how to use an if statement or and our specific case the if else statement and 314 00:22:48,630 --> 00:22:51,430 we talked a little bit about conditionals. 315 00:22:51,600 --> 00:22:56,520 You also learned that you could use help test from the command line to get some information about possible 316 00:22:56,520 --> 00:23:00,200 tests or expressions or conditions that you can check for. 317 00:23:00,210 --> 00:23:05,670 Finally you learned about some of the older syntax options such as using back text when assigning the 318 00:23:05,670 --> 00:23:11,970 output of a command to a variable and using a single bracket when doing tests versus the double bracket.