WEBVTT 00:00.720 --> 00:05.430 We are going to touch only briefly on a parameter expansion in this lecture. 00:05.610 --> 00:07.080 Let's clear the console first. 00:07.080 --> 00:11.100 So but we will be covering it extensively in the later just to mention that. 00:11.100 --> 00:16.410 So it's a feature that is more useful in shell scripts and directly on the command line. 00:16.410 --> 00:23.340 So many of the capabilities have to do with the system's ability to store small chunks of data and to 00:23.340 --> 00:25.380 give each chunk a name. 00:25.620 --> 00:33.090 Many such chunks more properly called R variables are available for your examination. 00:33.090 --> 00:41.460 So, for example, the variable user contains username to invoke parameter expansion and reveal the 00:41.460 --> 00:42.420 contents of user. 00:42.420 --> 00:47.700 You should do this echo dollar sign and user. 00:47.700 --> 00:55.560 And here, as you can see here, I printed my username so to list of the to see a list of available 00:55.560 --> 00:56.040 variables. 00:56.040 --> 01:01.780 You can also try this print any v here and last year we will use this. 01:01.780 --> 01:08.350 And as you can see here we have this system variables and which is called PG. 01:08.440 --> 01:08.950 You can. 01:08.950 --> 01:16.120 And the home directory is this our language log name is iPhone password or. 01:16.150 --> 01:17.680 No, it's actually PWD. 01:18.820 --> 01:25.450 Uh, here, uh, this is the working directory that we are in, and here we have, uh, information 01:25.450 --> 01:26.050 about that. 01:26.050 --> 01:30.310 So let's actually end this here. 01:31.300 --> 01:31.810 Oops. 01:33.200 --> 01:33.800 Here. 01:41.750 --> 01:48.980 Exit this and here with Ctrl Q or for example, and as you can say, press return lock file is already 01:48.980 --> 01:49.400 in use. 01:49.400 --> 01:55.790 So we are use this lock file and here we are going to exit it now. 01:58.530 --> 01:59.530 With controls. 01:59.910 --> 02:06.360 So you may have noticed that with other types of expansion and as you can see, we have our current 02:06.360 --> 02:07.860 working directory is p.w.d.. 02:07.950 --> 02:10.860 So we're going to clear and CD here. 02:10.860 --> 02:16.110 And again, CD, Let's go home here and let's and let's say we are in the home here. 02:16.110 --> 02:23.310 So as I said, if you noticed, with other types of expansion, if you mistype a pattern, the expansion 02:23.340 --> 02:27.960 will not take place and the echo command will simply display the mistyped pattern. 02:27.960 --> 02:33.780 So with parameter expansion, if you misspell the name of variable, the expansion will still take place 02:33.780 --> 02:37.590 but will result in an empty string. 02:37.590 --> 02:39.510 So for example, this. 02:39.510 --> 02:46.020 So as you remember in previous minutes of our lecture, we printed our username with user. 02:46.020 --> 02:51.060 But if we write it like that, Echo Sawyer here or yeah. 02:51.090 --> 02:55.950 Sawyer So actually it's a misspelled type of the user. 02:55.950 --> 02:56.550 WHEN Okay. 02:56.610 --> 03:02.320 And as you can see here, we got nothing as an error and it's just an new line. 03:02.320 --> 03:06.670 So you will also learn about the command substitution in this lecture. 03:06.670 --> 03:12.250 So common substitution allows us to the output of a command as an expansion. 03:12.250 --> 03:14.770 So in order to do that, we will echo. 03:15.430 --> 03:21.550 And here in the inside the braces we will use LHS and here. 03:21.550 --> 03:24.010 And as you can see, we got this command here. 03:24.010 --> 03:35.260 So and one of my favorites is this here which and CP and as you can see here, we got this. 03:35.260 --> 03:44.530 So here we pass the results of which CP as an argument to the command, thereby getting the list of 03:44.790 --> 03:48.670 a list of the CP program without to know its full path name. 03:48.670 --> 03:52.510 So we are not limited, just a simple commands entire pipe. 03:52.510 --> 03:57.310 Pipelines can be used for example, and working with it. 03:57.310 --> 03:59.380 We also have the cool thing here. 03:59.380 --> 04:05.680 So now that we have seen how many ways the Shell can perform expansions, it's time to learn how we 04:05.680 --> 04:06.730 can control it. 04:06.730 --> 04:09.790 So take for example, this echo. 04:09.820 --> 04:14.800 This is Oxley and oops, not like this. 04:14.830 --> 04:17.620 Oh, this is Oxley Academy. 04:17.620 --> 04:21.160 And as you can see here, this ignored all of these spaces here. 04:21.160 --> 04:26.290 And you can also do, for example, echo the total. 04:27.130 --> 04:28.270 Total. 04:29.050 --> 04:30.100 Price. 04:30.460 --> 04:31.050 Price. 04:32.730 --> 04:36.270 Price is, for example, $100. 04:36.270 --> 04:40.810 And here and as you can see here, the total price is zero. 04:40.830 --> 04:50.550 So in this first example with this word splitting by the shell, removed extra whitespace from the Echo 04:50.550 --> 04:53.370 command and the echo commands list of arguments. 04:53.370 --> 05:01.380 So in the second example, parameter expansion substitute an empty string for the value of $101 because 05:01.380 --> 05:03.510 it was an undefined variable. 05:03.510 --> 05:10.650 So the shell provides a mechanism called quoting to selectively suppress unwanted expressions. 05:10.650 --> 05:15.180 Here we also have the double quotes here, double quotes here. 05:15.180 --> 05:21.480 So the first type of quoting we will look at is double quotes. 05:22.480 --> 05:29.590 If a place text inside the double quotes, all the special characters used by the shell lose their special 05:29.590 --> 05:32.590 meaning and are treated as an ordinary characters. 05:32.590 --> 05:40.150 So the expressions are the dollar sign and backslash and backtick here. 05:40.180 --> 05:41.100 Backtick. 05:41.110 --> 05:47.530 So this means that the word splitting pathname expansion, titled expansion and brace expansions are 05:47.530 --> 05:48.280 suppressed. 05:48.280 --> 05:53.920 In this case, however, parameter expansion, arithmetic expansion and common substitution are still 05:53.920 --> 06:00.110 carried out Using double quotes, we can cope with file names containing embedded spaces. 06:00.130 --> 06:04.740 Let's say we were the unfortunate victim of a file called this. 06:04.750 --> 06:09.460 Two words dot txt here. 06:11.080 --> 06:19.650 If we try to use this on the command line, we are word splitting wood lines called the two words txt. 06:19.660 --> 06:25.450 And if we try to use this on a command, word splitting will cause this to be treated as a two arguments 06:25.450 --> 06:28.390 rather than the desired single argument. 06:28.390 --> 06:32.140 So for example, I want to create a new file, for example. 06:32.590 --> 06:37.780 Torch torch my file without question mark or something. 06:37.780 --> 06:44.050 And here and as you can see, we created my file, which now if we write it like that, for example, 06:44.080 --> 06:49.150 ls l my file my file dot txt. 06:49.300 --> 06:56.140 This will take as a two arguments, but in order to do access this my file we will use double quotes 06:56.140 --> 06:58.030 for accessing this here. 06:58.030 --> 07:02.350 And as you can see here we access to my file here. 07:04.000 --> 07:04.900 So there. 07:05.170 --> 07:09.570 And as you can see now, we don't have to keep tying those pesky double quotes. 07:09.570 --> 07:15.970 So remember, parameter expansions, arithmetic expansions and common substitution still take place 07:15.970 --> 07:17.180 within the double quotes. 07:17.180 --> 07:18.880 So for example, look at this. 07:18.880 --> 07:22.990 So echo and here we will do double quotes. 07:22.990 --> 07:25.690 So inside this double quotes we will user. 07:27.020 --> 07:29.900 And dollar sign to. 07:31.020 --> 07:31.980 The last two. 07:32.850 --> 07:33.930 Two plus two. 07:34.440 --> 07:37.590 And after that we will use a dollar sign. 07:37.590 --> 07:39.390 And after that we will call calendar. 07:39.390 --> 07:45.330 So this call is actually calendar, which we will show us and calendar is actually not installed. 07:45.330 --> 07:47.580 So let's actually install calendar. 07:47.580 --> 07:57.900 So app sudo apt apt, install ankle and I don't think it will take much time because it's I think it's 07:57.900 --> 07:58.140 is just. 07:59.280 --> 07:59.820 Here. 08:00.780 --> 08:01.680 And here. 08:01.680 --> 08:06.450 This is just a 69kB of file size, and it's already installed here. 08:06.450 --> 08:09.950 So now let's clear our files. 08:09.960 --> 08:12.630 Clear and use previous here. 08:12.630 --> 08:16.950 And as you can see here, we got calendar on it on the screen. 08:16.950 --> 08:22.440 So we shall take a moment to look at the effect of double quotes on common substitutions. 08:22.440 --> 08:27.780 So first, let's look a little deeper at words, how words splitting works. 08:27.780 --> 08:35.730 In earlier lectures, we saw how words splitting appears to remove extra spaces of in our text. 08:35.730 --> 08:42.090 For example, this is this is Oxley and Here Academy. 08:42.090 --> 08:44.760 And as you can see here, it just ignored our text. 08:44.760 --> 08:51.090 So by default, the word splitting looks for the presence of a space tabs and newlines in line characters 08:51.090 --> 08:56.130 line feed characters and actually and treats them as delimiters between words. 08:56.130 --> 09:02.950 So this means unquoted space tabs and newlines are not considered to be part of the text, so they serve 09:02.950 --> 09:07.360 only as separators because they separate the words into different arguments. 09:07.390 --> 09:12.430 Our example command line contains a comment followed by for district arguments. 09:12.460 --> 09:20.710 If we add like this, for example, add the quotation marks in the start of the end line and as you 09:20.710 --> 09:23.740 can see here, it is not ignoring our. 09:24.680 --> 09:32.490 The new lines or spaces as it did in our previous, like previous example.