1
00:00:00,280 --> 00:00:03,129
In this video, we will talk about how to

2
00:00:03,129 --> 00:00:05,230
handle command-line arguments with the

3
00:00:05,230 --> 00:00:08,679
clap library. Clap stands for a command

4
00:00:08,679 --> 00:00:10,840
line argument parser, and that is exactly

5
00:00:10,840 --> 00:00:14,139
what we are going to use it for. We are

6
00:00:14,139 --> 00:00:16,449
going to accept three different kinds of

7
00:00:16,449 --> 00:00:19,990
arguments, the first will be in file, an

8
00:00:19,990 --> 00:00:23,740
optional positional argument for a file

9
00:00:23,740 --> 00:00:26,439
to read instead of standard in. The

10
00:00:26,439 --> 00:00:29,740
second will be out file, an option that

11
00:00:29,740 --> 00:00:32,649
requires a value for a filename to

12
00:00:32,649 --> 00:00:35,920
write to instead of standard out. The

13
00:00:35,920 --> 00:00:39,579
third will be silent, which is just a

14
00:00:39,579 --> 00:00:42,520
flag that you can turn on or off, which

15
00:00:42,520 --> 00:00:45,430
has the exact same meaning as our PV

16
00:00:45,430 --> 00:00:47,620
silent environment variable, it will

17
00:00:47,620 --> 00:00:50,620
silence our standard error progress

18
00:00:50,620 --> 00:00:53,230
output. The first thing we need to do is

19
00:00:53,230 --> 00:01:00,949
add clap to our dependencies.

20
00:01:00,949 --> 00:01:03,800
Next, we need to import the items that we

21
00:01:03,800 --> 00:01:08,030
need.

22
00:01:08,030 --> 00:01:11,659
App is where you start with clap, and arg

23
00:01:11,659 --> 00:01:15,300
is used for each of the arguments.

24
00:01:15,300 --> 00:01:17,340
All right, let's make a local variable to

25
00:01:17,340 --> 00:01:19,170
help match us to store stuff in. I'm

26
00:01:19,170 --> 00:01:22,290
going to call app new pipe viewer that's

27
00:01:22,290 --> 00:01:24,300
the name of our program for the

28
00:01:24,300 --> 00:01:26,550
auto-generated help, and then we're using

29
00:01:26,550 --> 00:01:28,470
the Builder pattern here, so I'm going to

30
00:01:28,470 --> 00:01:30,680
give it a bunch of different arguments.

31
00:01:30,680 --> 00:01:32,760
So the first one we're going to give it

32
00:01:32,760 --> 00:01:36,120
is in file. Now this is a positional

33
00:01:36,120 --> 00:01:37,920
argument so all I have to give it is

34
00:01:37,920 --> 00:01:40,590
help, because the default is to get

35
00:01:40,590 --> 00:01:43,140
positional arguments. I want to say read

36
00:01:43,140 --> 00:01:44,970
from a file instead of standard in, and

37
00:01:44,970 --> 00:01:47,910
then continuing our Builder pattern, the

38
00:01:47,910 --> 00:01:51,450
next argument it's going to be out file.

39
00:01:51,450 --> 00:01:54,330
Now out file is going to be an option

40
00:01:54,330 --> 00:01:56,130
that takes a value, so we're gonna

41
00:01:56,130 --> 00:01:58,620
specify the short name for option, which

42
00:01:58,620 --> 00:02:01,200
is going to be oh, it's a long name for

43
00:02:01,200 --> 00:02:03,210
our option which is gonna be out file,

44
00:02:03,210 --> 00:02:06,060
we're gonna say it takes a value so you

45
00:02:06,060 --> 00:02:07,860
can't specify this option without giving

46
00:02:07,860 --> 00:02:09,360
it a value, because we need the file name.

47
00:02:09,360 --> 00:02:11,610
And then of course the help output for

48
00:02:11,610 --> 00:02:14,220
the auto-generated help that clap will

49
00:02:14,220 --> 00:02:17,850
make for us. Then next our third argument

50
00:02:17,850 --> 00:02:21,030
is going to be silent, so once again

51
00:02:21,030 --> 00:02:22,590
we're gonna do Arg

52
00:02:22,590 --> 00:02:25,170
and give us another argument with the

53
00:02:25,170 --> 00:02:27,480
name called silent, the name is what we

54
00:02:27,480 --> 00:02:30,750
retrieve out of the clap library, so when

55
00:02:30,750 --> 00:02:33,480
we want to have that long name that you

56
00:02:33,480 --> 00:02:35,520
can specify we actually have to put the

57
00:02:35,520 --> 00:02:37,680
long name as well. So we did a short name

58
00:02:37,680 --> 00:02:42,060
of s, a long name of silent, and then

59
00:02:42,060 --> 00:02:44,010
that's all the arguments we want, so we

60
00:02:44,010 --> 00:02:46,920
can call get matches, and that's why I

61
00:02:46,920 --> 00:02:49,950
named our local variable matches. Let's

62
00:02:49,950 --> 00:02:52,290
get our three values out of matches, so

63
00:02:52,290 --> 00:02:55,050
starting with in file we call matches,

64
00:02:55,050 --> 00:02:57,480
there's a method called value of that

65
00:02:57,480 --> 00:03:00,270
returns an option that either is none or

66
00:03:00,270 --> 00:03:02,610
it's a string containing the value that

67
00:03:02,610 --> 00:03:05,040
we want. So since it's an option we can

68
00:03:05,040 --> 00:03:07,680
do unwrap or default, like we did with PV

69
00:03:07,680 --> 00:03:10,230
silent, so either it gives us the string

70
00:03:10,230 --> 00:03:12,000
that is there or if it's a none then it

71
00:03:12,000 --> 00:03:13,769
gives us an empty string, the default

72
00:03:13,769 --> 00:03:15,780
value for a string, did the exact same

73
00:03:15,780 --> 00:03:18,340
thing for out file,

74
00:03:18,340 --> 00:03:20,440
then for silent we need to handle both.

75
00:03:20,440 --> 00:03:23,650
So we're gonna use an if expression so

76
00:03:23,650 --> 00:03:26,980
I'm gonna say if matches is present and

77
00:03:26,980 --> 00:03:30,700
then give it silent. So if that is true,

78
00:03:30,700 --> 00:03:33,400
then the value of silent will be true,

79
00:03:33,400 --> 00:03:36,129
otherwise let's go get it from the

80
00:03:36,129 --> 00:03:38,290
environment variable. So the command line

81
00:03:38,290 --> 00:03:39,610
version takes priority.

82
00:03:39,610 --> 00:03:42,310
I left the semicolons off of those if

83
00:03:42,310 --> 00:03:44,500
expression branch arms, so that they get

84
00:03:44,500 --> 00:03:46,599
returned as devalued the if expression.

85
00:03:46,599 --> 00:03:50,980
Okay, let's debug out the values of our

86
00:03:50,980 --> 00:03:52,840
three variables so we can take a look at

87
00:03:52,840 --> 00:03:55,329
them. Here's one thing to remember when

88
00:03:55,329 --> 00:03:56,799
you're dealing with command-line

89
00:03:56,799 --> 00:04:00,310
arguments and Cargo, Cargo accepts its

90
00:04:00,310 --> 00:04:03,730
own options and arguments. So what you

91
00:04:03,730 --> 00:04:07,060
need to do is put - - when you want to

92
00:04:07,060 --> 00:04:09,519
start the arguments and options for your

93
00:04:09,519 --> 00:04:11,709
program, and then Cargo won't interpret

94
00:04:11,709 --> 00:04:14,410
them. So when you pass our program at -

95
00:04:14,410 --> 00:04:18,548
H you see claps built-in help generation,

96
00:04:18,548 --> 00:04:20,560
that's just something you get for free,

97
00:04:20,560 --> 00:04:23,709
there are lots of ways to influence it

98
00:04:23,709 --> 00:04:25,630
and change how it looks I'm not going to

99
00:04:25,630 --> 00:04:26,889
go over those in this video.

100
00:04:26,889 --> 00:04:29,169
Let's go ahead and clear the screen and

101
00:04:29,169 --> 00:04:32,020
try out some of these arguments. So if we

102
00:04:32,020 --> 00:04:34,450
do no arguments we see in file as blank,

103
00:04:34,450 --> 00:04:36,940
out file as blank and silent is false,

104
00:04:36,940 --> 00:04:39,580
okay. So let's run it again and give it

105
00:04:39,580 --> 00:04:44,680
some file, now in file is some file, ok.

106
00:04:44,680 --> 00:04:47,289
Control C out of it again run it again,

107
00:04:47,289 --> 00:04:50,110
this time let's add - - out file and

108
00:04:50,110 --> 00:04:51,010
give it a name

109
00:04:51,010 --> 00:04:55,120
maybe file dot out, and run it again, ok

110
00:04:55,120 --> 00:04:57,490
there we go out file is file that out.

111
00:04:57,490 --> 00:05:01,900
Sontrol C run it again let's do - s, try

112
00:05:01,900 --> 00:05:04,870
out a short argument now silent is true,

113
00:05:04,870 --> 00:05:06,910
ok let's take off the - s and make sure

114
00:05:06,910 --> 00:05:09,430
our environment variable still works set

115
00:05:09,430 --> 00:05:11,200
that to some non blank silent is still

116
00:05:11,200 --> 00:05:13,870
true. There we go we can see that this

117
00:05:13,870 --> 00:05:15,820
all works nicely, so I'm going to go back

118
00:05:15,820 --> 00:05:18,580
and remove our debug statement, and we're

119
00:05:18,580 --> 00:05:22,000
all done. In the next video, we will talk

120
00:05:22,000 --> 00:05:24,639
about reading and writing files, using

121
00:05:24,639 --> 00:05:29,639
buffered I/O and traits.