[Music] Welcome to the session on command line editors. In this session, we will work with text files. When we have situations like we have logged into a supercomputer or a remote computer with the poor bandwidth, we often have a requirement to be able to edit some files for submitting our job for execution. And within a terminal to be able to do that editing is going to be very convenient because often the file transfer will also take more than one hop to go into a system such as well within the network of an organization where such facilities are made available to you. So the ability to use a command line editor is going to be important for you to make the best use of such an environment. So when it comes to editors, we have various classes of editors. So we have got this so-called line editors, that is editors which work in a terminal environment and help you edit one line at a time. And the old editors such as the ed, ed, or ex, the x editor which is a slightly improved version of the ed editor. These come from the days

of Unix. They are almost always there for any flavor of Unix or Linux. Terminal editors, which are visual in nature, are more popular because of the experience of the user in editing the file. There is an editor which came along with a popular email application called Pine, which is called Pico. It could also be launched as a standalone editor, and additional features were added to this editor. It is now called as a Nano editor. It is one of the most popular, simple, and yet very user-friendly text editor within the terminal environment that we can find today. The Vi editor is perhaps the most popular and also the most complex editor for the terminal environment, and Emacs is an alternative to the Vi editor. One normally says that among programmers, there are two kinds: the ones who are fanatical about Vi editor and the rest who are fanatical about Emacs. Very rare to find someone who likes both. And as for me, I am a Vi editor fan. There are graphical user interface editors also available, and these are different for different user interfaces, such as the KDE.

environment would have k or k write which are like the notepad of windows operating system. In the gnome environment, it is a g edit which would provide you a gui editor for pure text files. There are editors which open a window in the gui environment and allow you to edit files with lot of features. For programmers, these are quite popular tools such as sublime or atom. Atom editor is quite popular among the github users. Then an editor called brackets which is also popular among those who write html code. There are so-called the integrated development environments which come with an editor, compiler, debugger, and so on. We have the popular ides on linux such as eclipse, bluefish, or netbeans from apache. So while you have a very wide range of editors that are there, it is very useful for us to know one or two editors that would work without any additional installation that is required. In that sense, va editor and nano editor would fit the bill because these come automatically bundled with every linux operated system. What are the kind of features that you

must keep in mind while selecting an editor. You must be able to scroll through the file. You must be able to change the way you view the file, that is, page by page, to be able to look at different portions of the text file that you are editing. You should also be able to see where you are in the current position, which line number you are within the file, and so on. Navigating within that text file, either character by character, or word by word, or line by line, or by matching a pattern using regular expressions, would be very useful when you are looking for a particular keyword that you would like to change and perform some code editing. The requirement to insert text, replace some text, or delete some text, is a very common requirement for all of us. Cut, copy, paste is also required for us because we are going to move text around within our files. We should be able to search and replace a portion of the text, either in the entire text file or in a portion of the text file. It would be good if the editor is

language aware so that it can provide us a syntax highlighting. So that minus syntactical errors can be avoided while we write the code in the editor itself. And added benefits would be when we are able to do some key mapping so that certain combinations of keys can perform slightly elaborate operations within the editor. And if the editor can start with the init script where such key mappings could be made available then that would be good. And if you are able to record a macro and play it so that we perform some repetitive actions on multiple text files that also would be an added benefit. And the capabilities of the editor can be extended by plugins. And so those editors that support the concept of plugins would also be a good choice. And as you could expect both ebi editor as well as the emacs editor would satisfy all these requirements. So we will actually start off learning how to edit line by line using the add editor. I don't think you will actually be using it in your daily life but the commands that you will be learning

through this will be used as part of va editor and therefore it is worth seeing from where these commands have originated and also in an emergency situation where there is absolutely no editor then add will always be there with every unix or linux variant and therefore you still at least know how to go about editing some files if other tools are not available here is a list of features for the add editor which is same as for the x editor except for the x editor is slightly more user friendly so by default it has no prompt and if you press the capital p the prompt will be shown and most of the commands have to be given a certain address or a range of lines on which it has to act so the command format would take the starting address ending address followed by the command and then parameters for that command to act on and to locate uh the cursor on any particular line of the file you could use the line number itself like for example if you press 2 it means that you are in the second line

of the text file. If you press dot, it means that you are referring to the current line that the cursor is on. A dollar sign would refer to the last line. A percentage sign would refer to all the lines, so that any action that you are doing will be applicable to all the lines. Plus and minus are used for the line that is after the cursor or before the cursor, respectively. Comma is also going to refer to the entire buffer, that is, the whole of the text file. The semicolon is used to refer from the current position in the text file until the end of the text file. A regular expression given with the two forward slashes can also be used to refer to a location where that particular regular expression has been matched. And then there are a bunch of commands for editing. So these single letters do very powerful actions, and we will come to that when we come to the demo. You could also execute a command within the add editor by using the bang in front of the command that you want to do, so the command will be passed on to

the shell, and it will be executed there, and the output will be shown on the screen. You can edit a particular file by using the file name. You could read the contents of any file into the buffer of the file that you are editing currently by using the r command, and you could also read the output of any command into the buffer of the file that you are executing by using the combination of r and the exclamation mark preceding the command. You could save whatever you have done to the file by using the w command, and then once you have saved, you can come out of it using the queue command. So, ed editor that way is fairly comprehensive when it comes to simple code editing that is required, but it is quite painful because you don't get to see the whole file, and you also cannot move around the file visually. So it is not a visual editor, it's a line by line editor. Let us go ahead and explore some of those features using demo. Let us look at the illustration of ed as an editor for

line editing. It's only to learn how these shortcuts for the various actions are coming, which will be also repeated in the va editor. We need a file to play around, so we do not want to touch important files in our home directory. So, we will create a test file that we could edit. So, I would use the output of echo command along with some line number indicators for us to understand which line we are talking about. So, the very first line would be hello world, and I would like to copywrite this to test.txt. And in the second line, I would like to append it, so you have got the second line here, and there I would write it as welcome to line editor. And then the third line, I would write something else, ed is perhaps the oldest editor out there. And the fourth line, I will have end of file. Okay, so look at the file, and you can see that there are four lines now. Now we would like to edit this file to make some changes. So, that is where we are going to have the add editor coming.

up so to know more about the end editor so you can press man add and you see that there is some manual page that is available for the editor and it doesn't give you much about the commands etc. so that you could do from info. so info ed would open the info page for the ad editor and you can go through the overview then scroll through and you will go to the second section on introduction to line editing and there are some illustrations of how to go about and then you go on to the third session where the format of various commands is discussed and so on. i come out of this help now and we start using the ad editor so the way to invoke it is just add and then the file name and you see that when it opens it's a little bit confusing because we do not know what it is going to do so it is actually showing you some number which indicates how many bytes it has read into the memory so let us go and confirm that so we have to come out of

it by using `q` and `ls -l test.txt` and you will see that 117 is a number of bytes. So it is reading the entire file into the memory and then it is going to then show you line by line something that you can edit. So let us go ahead and start again so there is no prompt at all and sometimes it is useful to know that there is a prompt for you to type commands. So press capital `p` and it would use the prompt which is `*`. You can change it if you wish but for now we will go ahead with this. Now there are some commands that we need to practice. So let us say we go to the first line and it shows you what is in the first line. And if you want to go to the last line press `$` and you are in the last line. You could actually put a comma and a small `p` to know what are the contents of the entire buffer. So all the four lines are shown. You could also give a range so you can say `2,2`.

3 then p so the second to third line are getting printed. So you can see that the display is actually scrolling and the star is actually telling you a prompt that you would like to then add some commands after that. But it's not showing you the file in a visual manner so there is a whole idea of a line by line editing concept using the line editor. Add can we then also look at which line we will be if you were to search for a pattern. So let us say i search for hello and let us see it is there in the first line. I search for oldest and it is saying that you are in the line three so it would search for the pattern and bring you to the first occurrence of that particular pattern. And we could also uh go to a specific line and then when you press a plus it will go to the next line. When you press minus it go to the previous line so you can scroll up and down by looking at one line at a time. You could also go from the specific line.

to the end of the buffer like this. So let us say you are going from three and from there if you want to go to the end of it end of the buffer. So you see from the third line till the end of the buffer, which is third and fourth lines, are being displayed when you use a semicolon followed by p for printing. Okay, now the percentage is also a short form for the entire set of lines. So I want to see all of them. So I am also able to use the percentage. So these are all the various ways of locating which line you are going to operate on. So dot actually says which is the current line right now. So we are actually on the fourth line. Okay, and now again we are back on the first line. So we can actually always see where we are and therefore the command that we are going to run is going to be affected on that particular line. As I mentioned, you could also run some commands here. So I put an exclamation mark and then put date. So the command has been run and the display

is done, but it doesn't affect your buffer yet. So you could see that the four lines are still there. So if I want to read the output of the command into the buffer, and I want to specify where I want to read it, let me go to the fourth line. And then here I want to say, read the output of the date command so it comes into the buffer at the bottom. And you can then ask it to show all the lines so you can see that it's added at the bottom of the file, but it's not yet written. And I can press w, and then it writes the file. So now the file is saved, okay? And once you have saved, if you wish, you can come out of the file. So q will get you out of the editor. So I'm back to the prompt. This prompt is something you are already familiar with. This is the prompt of the shell, okay? And let us look at the occupation of test dot text. It is 156 bytes. So it means it has grown. And sure enough, it is because we have

added a line at the end of the file, so you can see that we actually successfully were able to scroll through the file and add something at the bottom. So, we can try out a few more features now. So now we have got five lines. So I want to go to the last line. So I put a dollar. So now I am in the last line, and I want to say that on the current line, do the action, which is deleting it. So it has done that action. And to help myself, I put capital P for the prompt. And then I want to see the buffer. You see that the buffer has been reduced. It's only four lines now. And if I press w, the file is saved, come out of it, and cat test dot text. And you see that the last line has been deleted. So which command has deleted the last line? It is this command dot d that is in the current position delete the line. And what is the current position? Dollar says that you are going to the last line of the file. And therefore, the command together when you read it says:

that you should delete the last line. Now we can actually go ahead and insert something in between. Also, look at the lines. So, I want to go to the first line and there I want to append some text. So, if I want to append some text, I press a and then when I enter, it is now asking for the text that I want to append. So, we can say appended this line after the first line. Okay, now once you are done, you should come out of this appending action. So, on an empty line, if you press dot and then enter, then the ed editor will know that you have finished giving the text and then it will bring the prompt back to you. Now I go and look at the contents of the file and you can see that what I wrote here is then appended after the first line because my position of the line is known here as one. So, after that, the appending has taken place and now you can see that the text has been appended up to the first line. Okay, now we could also do editing of this line so.

this editing can be using some principles of regular expressions that we have learnt so let me go to that line that we are talking about. So that's the second line. I want to change the case of the first word. So what I do is that I search for the word and I replace it with something. So the second part that comes after the forward slash would be the replacement string. So I would say replacement is by using the capital A, okay, and I close the path. So now you can see that there are two pairs of regular expressions that are given. The first one is for the matching, so this part is for the matching part, and this is for the replacement, okay. And on which line is it going to be active on the line that you have just now mentioned. So we do that and then let us say I want to print the second line. So you can see that the second line has been modified. The first word has been edited. So we successfully edited one line by identifying the line number and doing some action. Now the editing is done.

actually by regular expression and replacement, but not in a visual sense. So this is something very different way of editing, and this is how people used to do in the distant past. So we could also do some more editing. So let us say we go to the third line, and we want to change something here. So substitute welcome and I want to have all capitals, okay? And I want to print that line and see that I have changed that line. Now we have done a couple of changes, so we go ahead and write the buffer and then quit, okay? And I clean the screen and cat test dot text, and you see that we have done the changes. So we have edited the file so that the second line has a capital A and there is a line also after the first line, and the third line has a capitalized word. So we have successfully added some text, and we have also changed the letters. So we have been able to edit the file, but of course it's not very optimal in terms of experience. But in olden days when you had a very slow speed of

computers. So, if you are able to change one line at a time, that is already good enough for you to write a program because those computers were very slow compared to what we are using today. Now, let us look at a couple of more features so that we understand the commands, so the prompt and what file am I working on. The f command would show that, so this command is going to be there also in VA editor. So, f shows the name of the file that is being edited, and p would show the contents of the current line. So, right now we are at the end of the file, so the last line is being shown, and a is for appending at the current line. So, I am at the last line, so I append okay and I write something here. So, this line is appended at the end of the file, and then dot and enter. So, the line is done. You can see all the lines now. So, you could see that I have appended a line at the bottom of the file. Now, you could also do some joining here.

so i would go and try to join the last line to the fourth line. Four, so i go to five line 4 and the syntax would be i want to join 5 and 6 with a j. And then you can look at the current line and you can see that these last two lines have been joined. So line 4 end of file. And then without anything between that and the next line, this line is appended at the end of the file. So you can see that the two lines have been joined. So 5, comma, 6 j means join the fifth and the 6th lines. Now you could also move a particular line to a particular position. So right now where are we. So we can see that we are at this line. So we can say move this line to the first position. Okay. And then let us go and see what happened. So you can see that the last line has come just below the first line. So you could actually now print the second line and you see that it has come.

and which means that I have successfully been able to move the last line to that. Now if I want to make this as a first line, so first let me check I am in the line two so I can say move this line to the zero position. And then I can ask it to print all the lines and you can see that that particular line I'm working on, line 4 end of file etc., is now at the first position. So m0 would mean go to the very beginning of the file, m1 means put it after the first line and so on. So you can actually give a number after m to say to which line you want to move the current line and that way you can move the lines also up and down. Okay, and whatever we have done we don't want that change, so I want to undo. So I press u and press and enter and you can see again all the lines and see that the change I have made is reverted back. So you can see that here the line one and then line four are one.

after another then when i moved. Then i have got the line four and line one are like this. And then after i did undo, then you can see that they are back to the original form. So you can also do some reversal of the actions that you have taken. And at any time you want to write the file to the disk, and then you can come out and cat to see that whatever changes you have done have been affected to the file. This is how the ed editor is actually going to help the people to go around changing the text. Now we will just illustrate one more feature of the add editor where the regular expression power can be illustrated. So look at all the lines. So let us say i would like to move all the lines a bit side by putting something in the front. So what i would do is i should say the entire range i want to apply. So percentage will say that all the lines are the address range, and i want to search for something that is basically any character that can be matched. So now

you see that we have grouped dot star, which means that all the characters that can be matched are grouped like that. And I want to replace them by writing something in front. What do I want to write in front? I would say prefix and then a space, and then backslash one. Backslash one means whatever we have grouped just now is going to be printed in that position. So essentially what we are expecting is every line will be now prefixed with the word prefix and space, okay. And I press a forward slash to complete the replacement expression and then enter. So now this action has been taken place on all the lines, and I can now print all the lines to see what happened. So prefix word has come in front of all the lines. So you can see that the regular expressions that we have learnt in the previous class and also the back substitution can also be used for editing action straight away, and it will apply to all the lines or any specific line. So let us say I want to reverse some of it and make it

applicable only for specific lines. So that I would do let us say from the third to fifth lines. I want to substitute prefix word with a smaller version. And now you can see that the text has been modified. So from the third onwards to fifth. So third, fourth, fifth lines have changed. The first word where with a regular expression, I am substituting capital prefix with a small prefix. So you can see that we are actually using the regular expressions and replacements to change one word at a time in a given line. And we can move the lines up and down. We can add a line. We can delete a line. And we can also append something at the bottom of the file. And we can also put some things in front of the line or at the end of the line using the regular expressions. And therefore we are able to edit the lines. So this is a very powerful feature of the editor. And it is pretty good for us for a daily live editing of source code. But today we have a much more powerful and visual experience with

the vi editor so ed editor that way is not so popular today, but it continues to be there in the system packaged so that you can always try it out if you are interested in some historically useful editors that people have used in the yesteryears. So let me write the buffer and queue, okay, and cat. What did we do the testing? You can see that we have edited the file significantly, all using the line editor called ed. Here is a summary of commands that we have explored in the demonstration for both the editor's ed as well as x, and each of these commands is a one-letter command. And these letters are the same in va editor also, so it would be good to also associate these one-letter commands with their meaning. And I have highlighted them in blue color to hint that one can actually try to remember these already.

Command line editors - Part 01

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