1
00:00:06,510 --> 00:00:09,540
- In this video, you will
learn about variables.

2
00:00:09,540 --> 00:00:11,160
Now, what is a variable?

3
00:00:11,160 --> 00:00:13,740
A variable is a label to
which the dynamic value

4
00:00:13,740 --> 00:00:15,210
can be assigned.

5
00:00:15,210 --> 00:00:16,230
All right.

6
00:00:16,230 --> 00:00:17,550
How is it useful?

7
00:00:17,550 --> 00:00:20,790
Now, variables are very
convenient for scripting.

8
00:00:20,790 --> 00:00:22,650
You can define the variable once,

9
00:00:22,650 --> 00:00:27,360
and use it in a flexible way
in different environments.

10
00:00:27,360 --> 00:00:32,360
Variables also generically
are used to store information

11
00:00:32,580 --> 00:00:33,993
that is site-specific.

12
00:00:36,120 --> 00:00:38,370
The thing is you can write a script

13
00:00:38,370 --> 00:00:41,100
or write an operating system.

14
00:00:41,100 --> 00:00:43,950
The script or operating
system works with specific

15
00:00:43,950 --> 00:00:46,530
information like your
username, for instance.

16
00:00:46,530 --> 00:00:48,750
And if you want to write a script that is

17
00:00:48,750 --> 00:00:50,730
printing your username,
how are you going to

18
00:00:50,730 --> 00:00:54,300
print a username while you store
the username in a variable?

19
00:00:54,300 --> 00:00:57,420
And then you write a script
that is just referring

20
00:00:57,420 --> 00:00:58,253
to the variable.

21
00:00:58,253 --> 00:01:00,390
And no matter what your username is,

22
00:01:00,390 --> 00:01:03,300
it will always use the right variable.

23
00:01:03,300 --> 00:01:07,040
In programming, we call
that the option to separate

24
00:01:07,040 --> 00:01:10,710
the site specific from
the static information,

25
00:01:10,710 --> 00:01:12,513
also known as decoupling.

26
00:01:13,350 --> 00:01:15,270
In Linux, you have system variables.

27
00:01:15,270 --> 00:01:17,430
System variables contain default settings

28
00:01:17,430 --> 00:01:19,170
that are used by Linux.

29
00:01:19,170 --> 00:01:21,453
A very common variable is PATH.

30
00:01:21,453 --> 00:01:25,020
PATH contains a list of
directories, which are searched

31
00:01:25,020 --> 00:01:27,360
for binaries while executing a command.

32
00:01:27,360 --> 00:01:29,250
There's also a shell,
which is a current shell

33
00:01:29,250 --> 00:01:30,093
that is used.

34
00:01:30,990 --> 00:01:34,470
Environment variables can
be set for application use.

35
00:01:34,470 --> 00:01:39,060
And in order to set them,
you use varname=value.

36
00:01:39,060 --> 00:01:41,580
That is what you do on your prompt.

37
00:01:41,580 --> 00:01:45,870
And if you do that, it's known
to the current shell only.

38
00:01:45,870 --> 00:01:48,180
To refer to the value in the variable,

39
00:01:48,180 --> 00:01:50,040
you use echo, for instance.

40
00:01:50,040 --> 00:01:54,480
Or better you use command dollar
and then the variable name.

41
00:01:54,480 --> 00:01:57,450
So to define a variable,
there's no dollar in front

42
00:01:57,450 --> 00:01:58,950
of the name of the variable.

43
00:01:58,950 --> 00:02:02,160
To use the variable, there's
a dollar in front of the name.

44
00:02:02,160 --> 00:02:03,780
As you can see in these two lines,

45
00:02:03,780 --> 00:02:05,883
varname is value echo $varname.

46
00:02:05,883 --> 00:02:07,233
That is how it works.

47
00:02:08,070 --> 00:02:11,940
But as I mentioned, variables
are by default only known

48
00:02:11,940 --> 00:02:13,020
to the current shell.

49
00:02:13,020 --> 00:02:16,080
And if you want your variables
to be available in all

50
00:02:16,080 --> 00:02:18,840
the sub-shells as well,
that is in all the commands

51
00:02:18,840 --> 00:02:22,800
if you are going to run, from
the shell, you use export.

52
00:02:22,800 --> 00:02:23,943
Let me demonstrate.

53
00:02:27,180 --> 00:02:31,350
So let me start by using echo $COLOR.

54
00:02:31,350 --> 00:02:33,990
I like typing my variables
in uppercase, by the way,

55
00:02:33,990 --> 00:02:35,100
to make it more readable.

56
00:02:35,100 --> 00:02:38,370
You can do it in lowercase as
well, as long as you realize

57
00:02:38,370 --> 00:02:40,380
that it is case sensitive.

58
00:02:40,380 --> 00:02:42,480
I'm using color is red,

59
00:02:42,480 --> 00:02:46,480
then I'm using echo $COLOR

60
00:02:49,320 --> 00:02:50,280
And what do we get?

61
00:02:50,280 --> 00:02:51,870
We get red.

62
00:02:51,870 --> 00:02:55,680
But now this thing about
sub-shells, I am typing bash.

63
00:02:55,680 --> 00:02:56,550
What is happening?

64
00:02:56,550 --> 00:02:58,890
Well, I'm opening a shell within my shell.

65
00:02:58,890 --> 00:03:00,630
So that's a new environment.

66
00:03:00,630 --> 00:03:04,020
And if I'm using echo
$COLOR from the sub-shell,

67
00:03:04,020 --> 00:03:05,400
I get nothing.

68
00:03:05,400 --> 00:03:07,710
I type exit to close my sub-shell.

69
00:03:07,710 --> 00:03:09,930
And I use my echo $COLOR again.

70
00:03:09,930 --> 00:03:13,980
And now we can see that I have
access to my variable again.

71
00:03:13,980 --> 00:03:16,800
If I want my variable
to exist in sub-shells,

72
00:03:16,800 --> 00:03:18,330
I need to use export.

73
00:03:18,330 --> 00:03:21,120
So export color is green, for instance.

74
00:03:21,120 --> 00:03:24,150
echo $COLOR, color is set to green.

75
00:03:24,150 --> 00:03:28,233
And if I type bash echo $COLOR,

76
00:03:30,360 --> 00:03:33,150
color is set to green
in the sub cell as well.

77
00:03:33,150 --> 00:03:34,650
And that is how it is working.

78
00:03:35,850 --> 00:03:38,040
Now, variables can be set by yourself

79
00:03:38,040 --> 00:03:40,590
but more important are the
variables that are a part

80
00:03:40,590 --> 00:03:42,420
of your operating system.

81
00:03:42,420 --> 00:03:45,630
If you use the env command,
env for environment,

82
00:03:45,630 --> 00:03:48,870
then you see these so-called
environment variables.

83
00:03:48,870 --> 00:03:51,930
And these environment variables
are set in different ways.

84
00:03:51,930 --> 00:03:54,840
They can be set by the
commands that you are using.

85
00:03:54,840 --> 00:03:59,400
They can be set by the
startup shells of your shell.

86
00:03:59,400 --> 00:04:01,680
They can be set in different ways.

87
00:04:01,680 --> 00:04:04,950
You might wanna have a
look at env pipeline tools

88
00:04:04,950 --> 00:04:07,830
and you can see all these
different environment variables

89
00:04:07,830 --> 00:04:12,360
that are being set including variables

90
00:04:12,360 --> 00:04:14,850
like the PATH variable.

91
00:04:14,850 --> 00:04:17,880
The PATH variable actually
is a pretty useful variable.

92
00:04:17,880 --> 00:04:19,530
It prints the path.

93
00:04:19,530 --> 00:04:23,040
And this PATH is where
your Linux distribution

94
00:04:23,040 --> 00:04:26,790
is going to look for executable commands.

95
00:04:26,790 --> 00:04:27,630
And you know what?

96
00:04:27,630 --> 00:04:30,990
Let me just give you
a very simple example.

97
00:04:30,990 --> 00:04:33,980
I'm using echo hello world > hello.sh.

98
00:04:39,120 --> 00:04:42,870
hello.sh, the extension
.sh reveals that this is

99
00:04:42,870 --> 00:04:45,030
supposed to be shell script.

100
00:04:45,030 --> 00:04:47,283
Shell scripts need to be executable,

101
00:04:49,489 --> 00:04:53,310
and I am using chmod
+x hello.sh to give it

102
00:04:53,310 --> 00:04:55,080
the execute permission.

103
00:04:55,080 --> 00:04:58,050
We can verify hello.sh.

104
00:04:58,050 --> 00:05:00,420
There we go. It has
the execute permission.

105
00:05:00,420 --> 00:05:03,690
I will tell you all about
this in Lesson 10, by the way.

106
00:05:03,690 --> 00:05:05,562
This is just a quick demo.

107
00:05:05,562 --> 00:05:08,820
What I want you to see
is if I type hello.sh,

108
00:05:08,820 --> 00:05:10,860
I am getting command not found.

109
00:05:10,860 --> 00:05:11,850
Why is it?

110
00:05:11,850 --> 00:05:14,700
Because the current
directory is not in the PATH.

111
00:05:14,700 --> 00:05:16,920
As you can see, I'm in home/student.

112
00:05:16,920 --> 00:05:19,530
And in home/student, though
we have different directories

113
00:05:19,530 --> 00:05:21,750
but home/student itself is not there.

114
00:05:21,750 --> 00:05:25,530
So if I want this to be
executed in a decent way,

115
00:05:25,530 --> 00:05:30,510
I can move hello.sh to the bin directory

116
00:05:30,510 --> 00:05:32,010
that I just created.

117
00:05:32,010 --> 00:05:34,860
And now, if I use hello, there we can see

118
00:05:34,860 --> 00:05:36,513
that completion is working.

119
00:05:37,953 --> 00:05:40,380
And we can also see ...
Oh, command not found.

120
00:05:40,380 --> 00:05:41,640
Why is that?

121
00:05:41,640 --> 00:05:44,880
Because I made a small mistake.

122
00:05:44,880 --> 00:05:49,440
Look, I used echo hello world,
and wrote that to hello.sh.

123
00:05:49,440 --> 00:05:52,800
I should have used echo
hello world to make

124
00:05:52,800 --> 00:05:55,650
that the contents of my hello.sh.

125
00:05:55,650 --> 00:05:58,434
Well, that's an easy fix, of course.

126
00:05:58,434 --> 00:06:01,880
So let me do a vim on bin/hello.sh

127
00:06:02,760 --> 00:06:05,370
And let me enter an echo here.

128
00:06:05,370 --> 00:06:08,790
And then we have our
very simple shell script

129
00:06:08,790 --> 00:06:10,503
that is actually working.

130
00:06:11,580 --> 00:06:15,270
So that is how this PATH variable is used.

131
00:06:15,270 --> 00:06:18,780
Just a quick remark, I
used home/student/bin.

132
00:06:18,780 --> 00:06:20,490
home/student/bin is in the PATH.

133
00:06:20,490 --> 00:06:22,290
That's for the local user only.

134
00:06:22,290 --> 00:06:24,450
If I think that my script is so awesome

135
00:06:24,450 --> 00:06:26,520
that everybody should be using it,

136
00:06:26,520 --> 00:06:29,040
I should put it in usr/local/bin.

137
00:06:29,040 --> 00:06:29,873
So, you know what?

138
00:06:29,873 --> 00:06:34,790
Let me do a pseudo mv to
move it from a bin/hello.sh

139
00:06:35,760 --> 00:06:38,910
to usr/local/bin.

140
00:06:38,910 --> 00:06:40,080
And there we go.

141
00:06:40,080 --> 00:06:42,480
And at this point, well,
for my user account,

142
00:06:42,480 --> 00:06:43,710
nothing is changing.

143
00:06:43,710 --> 00:06:47,280
But now, everybody else
can enjoy the benefits

144
00:06:47,280 --> 00:06:48,993
of my wonderful shell script.