Linux Commands for Beginners 18 - Variables — Transcript

Full transcript

1. 0:00[Music]
2. 0:20hey guys welcome to another video in my
3. 0:22Linux commands for beginner series and
4. 0:25this time around we're gonna be taking a
5. 0:28look at variables probably something I
6. 0:31should have gone over earlier but it's
7. 0:33an easy concept I think maybe it's
8. 0:34pretty cool to have a little break and
9. 0:36just do something easy for a minute
10. 0:38before we get into other items on the
11. 0:40series but what I want to show you guys
12. 0:43how to do is how to create a variable
13. 0:45and read from it it's a very important
14. 0:48thing it's more important for scripting
15. 0:50and programming but it is something
16. 0:51you're gonna run into when you're
17. 0:53running commands in Linux so let's go
18. 0:55ahead and take a look back here on my
19. 0:58terminal I'll just give you a quick
20. 0:59refresher we have the echo command which
21. 1:02I mentioned in a previous video anything
22. 1:06you type into echo and the quotes there
23. 1:08is going to be echoed back at you but
24. 1:12the echo command is useful in regards to
25. 1:14seeing what variables contain so what
26. 1:18we're gonna do right now is create a
27. 1:19variable and then we'll use the echo
28. 1:21command to read that back so I'm going
29. 1:25to create a variable called hello msg so
30. 1:30hello message and it's very common to
31. 1:32put variable names and caps but you
32. 1:34don't actually have to do that now if no
33. 1:36space I'm going to do an equal sign and
34. 1:38then in double quotes I'm going to put
35. 1:41hello world so basically I'm creating a
36. 1:44variable called Hello message
37. 1:46abbreviated equal sign so I want to set
38. 1:50it equal to a string hello world and
39. 1:52I'll press Enter
40. 1:54okay so nothing happened well actually
41. 1:56something did happen but you're just not
42. 1:58seeing the output now we do have a new
43. 2:01variable so I'll go ahead and clear the
44. 2:03screen and what I'm actually going to do
45. 2:06is read that variable back so to do so I
46. 2:09will use echo and Hello msg however
47. 2:13that's not gonna work
48. 2:14it's just echoing hello msg it's just
49. 2:18you know the same if I say echo hello or
50. 2:21echo random it's going to echo back
51. 2:24anything I put there but I want the
52. 2:27actual variable I want to see what is
53. 2:29inside
54. 2:30hello msg so when you refer to a
55. 2:34variable on bosch which is basically
56. 2:36what the shell is here you have to put a
57. 2:39dollar sign in front of it that
58. 2:41basically tells the command-line
59. 2:43interpreter that you want to refer to
60. 2:47the contents of a variable when you
61. 2:49declare the variable we don't have the
62. 2:51dollar sign but we do need it when we
63. 2:53want to refer back to it and you can see
64. 2:56there that I get hello world which is
65. 2:59what we would expect to get now when
66. 3:02again when I saved the variable I didn't
67. 3:04have a dollar sign in front of it but to
68. 3:06refer back to it I will need to do that
69. 3:08and I could also echo the dollar sign
70. 3:12question mark get a return code return
71. 3:14code is zero which means that it was
72. 3:16successful it read that variable back so
73. 3:19moving right along let's create another
74. 3:21variable will do my underscore num will
75. 3:25set it to equal three and then I'll do
76. 3:29my num - I'll set that equal to ten so
77. 3:32just like any other variable I can echo
78. 3:34my num the equals three and I could do
79. 3:40the same thing with the other video
80. 3:41variable that equals ten
81. 3:44now one thing I do want to mention
82. 3:45though is that these variables are
83. 3:48specific to my sessions so if I open a
84. 3:51new terminal
85. 3:52I do echo my num nothing and the reason
86. 3:59why is because the variable exists in
87. 4:01this session doesn't exist here so if I
88. 4:03close a terminal I am going to lose
89. 4:05those items now we went over bash RC
90. 4:08briefly in a previous video so I could
91. 4:10put some variables in there if I wanted
92. 4:12to retain them I just wanted to mention
93. 4:14that real quick but anyway getting back
94. 4:16to the topic at hand I created two
95. 4:19variables my num and mind um two that
96. 4:22are equal to three in ten respective so
97. 4:24perhaps a more useful example I could do
98. 4:26my underscore name equals and quotes put
99. 4:31my name there I have that variable so of
100. 4:34course I can't echo that back
101. 4:36in case I forgot my name in which case
102. 4:39I'm just reminded of what that was so I
103. 4:41have a name what can I actually do with
104. 4:43that so what I'm going to do is try an
105. 4:47echo command I'll do echo and then my
106. 4:51name is my name then I'll end the quotes
107. 4:57let's see what happens
108. 4:59says my name is J so what I was able to
109. 5:03do is refer to this variable inside
110. 5:07another command so to see a more
111. 5:09practical example of that do my derp
112. 5:12equals and quotes slash at sea there we
113. 5:17go press Enter
114. 5:18and just to make sure it saved it right
115. 5:23it did so I could do now LS my derp now
116. 5:29we see the contents of the Etsy
117. 5:32directory are printed on the screen now
118. 5:34obviously typing my Durr is more
119. 5:37characters than just simply typing /e TC
120. 5:40but if you had a very long path it might
121. 5:44make more sense to actually have a
122. 5:47variable as a directory so that just
123. 5:49goes to show you that you can save
124. 5:50variables and then refer to them in
125. 5:52commands so that way you don't have to
126. 5:55re-type something over and over again if
127. 5:58it's a long path or something like that
128. 6:01then you can go ahead and save it in a
129. 6:02variable to save typing and this
130. 6:05especially helps when you have scripts
131. 6:08we're not going to get into bash
132. 6:09scripting much in this video if at all I
133. 6:12just wanted you to know that that's
134. 6:13generally where you're going to find
135. 6:15this quite often you're gonna have
136. 6:16variables and that's going to basically
137. 6:19allow you to save some kind of
138. 6:21information so that you can refer to
139. 6:23that later and what's fun is to see what
140. 6:26variables are in your sessions so if I
141. 6:29was to do env and press enter we can
142. 6:33actually see a bunch of variables now
143. 6:34these will need the dollar sign if you
144. 6:37wanted to refer to them so for example
145. 6:39we see I have a variable user it's equal
146. 6:41to me so I could do echo
147. 6:43user but that's not going to work I need
148. 6:45to put the dollar symbol in there that
149. 6:48just goes to show you that we still need
150. 6:50the dollar symbol it's not always going
151. 6:52to be there but basically this env shows
152. 6:54my environment in my logged in session
153. 6:58and all the variables that are included
154. 7:01in that session or in this current
155. 7:03session and there's actually quite a few
156. 7:06so different programs might use this
157. 7:07differently for example the display
158. 7:10variable is one that I'll often use that
159. 7:12refers to the monitor that applications
160. 7:15will show up on by default you can
161. 7:18actually and this beyond the scope of
162. 7:20the series forward your display to
163. 7:22another machine and I can launch a
164. 7:25program on this computer but actually
165. 7:27have the program show up on a completely
166. 7:29different computer basically it's
167. 7:31referring to X forwarding again outside
168. 7:34the scope of this video but just to show
169. 7:36you that this variable is important and
170. 7:38probably something you'll end up using
171. 7:40at some point or another so go ahead and
172. 7:42clear the screen so I'm going to create
173. 7:44a new variable and I'm just going to use
174. 7:46lower case because I'm lazy and I'm
175. 7:49going to make it equal to Linux simple
176. 7:57so let's do a MV let's see if we see
177. 8:02that variable it's lower case so you
178. 8:04know what it really should stand out
179. 8:05because everything here begins with
180. 8:07upper case but if I scroll through here
181. 8:09well I'm not seeing it so I'll go ahead
182. 8:13and clear the screen so what's going on
183. 8:14here why if I have this variable and
184. 8:16it's recognized I can even use the echo
185. 8:18command against it why doesn't it show
186. 8:20up in the environment well it's not
187. 8:22actually part of the environment unless
188. 8:24I make it part of the environment which
189. 8:26means it can exist beyond my shell so
190. 8:29again I created this variable right here
191. 8:31which we know it does indeed work it's
192. 8:33not an illusion it gets a return value
193. 8:35of 0 so we know that it works but it's
194. 8:37not in our environment if I was to open
195. 8:39a new shell to open up a new terminal
196. 8:42window here and if I do the same thing
197. 8:44echo my VAR it doesn't work so let's try
198. 8:48this a bit differently I'm gonna do
199. 8:49export and then I'm going to type my VAR
200. 8:54e
201. 8:55Linux so I'm just basically declaring a
202. 8:59variable called my var setting equal to
203. 9:00Linux by putting X port in front what's
204. 9:02gonna happen so is anything different
205. 9:05here so let's see when echo my var seems
206. 9:08to work fine
207. 9:11echo code seems to return I'm gonna
208. 9:14clear the screen so now let's check out
209. 9:16the environment again I scroll through
210. 9:19and see if our variable is listed and
211. 9:21immediately I see it is listed here
212. 9:24whereas it was not actually listed
213. 9:26before we actually made this variable
214. 9:29part of our environment so that was a
215. 9:32relatively easy video I just showed you
216. 9:33guys how to set variables and read them
217. 9:35back I also showed you that you can use
218. 9:38those variables in Linux commands now
219. 9:40this is a concept that's gonna go you
220. 9:43know above and beyond as the you know as
221. 9:45your knowledge grows and you learn more
222. 9:47things so yeah it's a very simple
223. 9:48example but the possibilities you can do
224. 9:51with variables I think you're gonna find
225. 9:54that you can extend that quite a bit I
226. 9:56mean you can for example you could have
227. 9:58a script that takes arguments maybe have
228. 10:01a directory you give as an argument it
229. 10:03takes that argument as standard input
230. 10:04and then it refers to it in the program
231. 10:08maybe 50 times but you only have to
232. 10:10refer to it that one time is standard
233. 10:12input and since it's a variable you know
234. 10:14you don't have the hard code at that
235. 10:15much time I mean you shouldn't hard code
236. 10:17anything anyway but it just goes to show
237. 10:19you variables are something that are
238. 10:20going to be you know a mainstay in your
239. 10:23tool set especially as your knowledge
240. 10:25grows so I hope that was helpful for you
241. 10:27guys just you know make sure you
242. 10:29understand setting and reading back your
243. 10:31variables just you know give it a shot
244. 10:33play around with it and then once you're
245. 10:35all caught up I will see you in the next
246. 10:37video thank you so much for watching my
247. 10:40video I really appreciate it if you want
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