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Unit 02 - Using Linux

18. BASH Proficiency

Now, there are many tools and tricks that will help make your use of these shells a lot easier. And basically the idea is, how do I navigate back and forth? For instance, using the shift key to move the page up or the shift key to move the page down can take you to the top or bottom of a current page. and that's especially helpful when you're trying to look at your recent output. So, if I issued some sort of LS command and saw the listings of a certain directory, but those listings have now moved to the top of the screen due to other commands, I can page up or page down if you want to go to the top. CTRL t bam takes you right to the top of that list. It's a lot easier than trying to use the uparrow and scroll all the way up, or, depending on the shell, using the mouse pointer to try to drag that little bar along the vertical bar up and down to move up and down the page. It's very simple, very straightforward. People say this is faster. And the reason they say it's faster is I'm not taking my hands off the keyboard to go get a mouse, move the mouse, and do all this stuff. I'm typing along. I can shift page up, shift page down. CTRL T is very easy to get from one to the other without moving my hands off the keyboard. One of the reasons why people will tell you that they can always outperform you with a command line over your windows is because they have a command line. GUI because they don't leave the keyboard; they don't have to move things around. They don't have to wait for the next page; read the next question. They're just entering these commands at a slow pace, provided they know the commands. That's the other part of the trick. If they don't know the commands, they might be slower than you because they're going to have to look that stuff up anyway. There are numerous ways to make using a shell more convenient for you.

19. Command History

Another thing you can do is look at your recent command history. So if, let's say, you recently typed a clear command and you want to type it again, if you just recently—like, the last two or three commands typed clear—you could hit the up arrow two or three times, and you would see the clear command, and you would just hit Enter, and it would clear the screen just like you did before. It's your history. The up and down arrows show you the previous commands. If you used many pre-up arrows to see many previous commands, the downarrow will show you the commands that followed. Now, be careful. By the way, you can also use a history command, which will, depending on how you've set up the parameters, show you the last ten or however many commands you have actually typed in. But here's what I want to be careful of. I would suggest that you be cautious with your use of the up and down arrows. For example, if the command clear five letters was the one I wanted to repeat and I typed it 20 commands ago, you hit that up arrow 20 times; that's 20 keystrokes, as opposed to just typing the command five letters, five keystrokes. So it's not always to your best advantage, but if you aren't sure about what you've typed or you want to verify some of the commands that you've typed, that's where I think this is a perfect representation of using up and down arrows and going through there. Now, if you have a long command that you're typing in, and I mean a long command that may be scrolling off the screen due to its length—it's longer than the 80-character limit—you can use Control B and CTRL F to go back and forward. Now, in some cases, the right and left arrows will move you just like Control B and Control F. But that kind of takes us back to the types of emulators we used to have, where basically an emulator was emulating a command line or a shell in a Windows environment. And so we often couldn't get those back arrows and forward arrows to translate correctly into that environment using the emulator. So we were stuck with what we often call an eMac's style of abbreviations. So Control B and Control F would move back a letter. Control for forward control D would delete a character. Control A would take you to the front of the command. Control E would take you to the very end of the command. I mean, there are a lot of different options that you have that would help in editing the commands that you're typing in. This is especially useful if you made a mistake or need to go back and forth without having to press the back or front arrows or do anything else. So the more of those you got used to, the faster you were running or working with the command line.

20. Shell Variables

Now, inside of each shell is a reference to what we call variables. Variables are just that; they are placeholders of data or information, and often they make references to system information that you can relate to inside your shell. Now, most variables are going to almost always be all capital letters, and almost all of them start with a dollar sign. So if you wanted to know, for instance, what's the name of the host of the actual operating system, the dollar sign host name would tell you that information. You have a lot of these variables that, again, are usually system-generated information available to you within your shell and programmatically available to you, as we'll talk about later on. You can pipe information from these variables into other types of commands and vice versa, or send them off to printers or whatever you want to do. So a shell variable is generally system information, but it doesn't mean that you can't make your own. Now, keep in mind that if you have multiple command lines open, we'd call them multiple shells. These variables that you are using that are system-wide are available to everyone. If you create a variable inside of your shell, it is ignored, or, I should say, invisible to any other shell. Everything you do is contained within that one screen or window, and it doesn't transpose from one to the other. So be careful about that. And working with these variables, remember which shell you put your own variable in and which one it's not in.

21. Shell Metacharacters

So, you know, the bad thing about talking about the shell and all these little commands is that you just can't do justice to all the commands. You've got to play with it. Read some blogs, find some other things where people say, "Oh, I tried this; it was really cool." For example, you can make a series of commands executed in order look like a single command by separating them with a colon. So you can say, "Do this command, echo colon, and do some other work." You can group your commands together inside these braces or brackets, and they'll be executed as far as the exit when it leaves this grouping, as though it was a single command. A single ampersand can say "run this command in the background. Don't annoy me. Don't make me sit there waiting for all this output to show up on my screen. I want to type some more stuff. Double ampersands, logical endings, double what we call pipe characters That's a vertical line that's up and down on your keyboard. If you haven't seen it, some keyboards make it look like two little dashes that are vertical above each other. Anyway, those are the pipes. Double pipes would be logical or So, for example, you could say CD bin.That means change your directory to the bin directory, followed by an ampersand and a LS, which means list the file contents. And what happens is that if both of those are true, that you have permission to go to the bin directory and run the LS command, then it will run. If you didn't have permission to go to the bin directory, it wouldn't run the LS because logic dictates that you must be able to do both or nothing happens. Logical. And if this is true and this is true, then the result is true. Logical or simply means that one of the two sides must be true in order for the command to be executed successfully. So again, there's just a lot of really cool stuff you can get into with all of these little metacaracters and working with the shells.

22. Tab Completion

Now, many of you don't want to remember all of the typing and all of the commands, at least as far as the exact spelling. Don't blame you. Command lines don't have spell checks. If you make a mistake, it will be the first one to tell you, "Hey, you did. What is this? I don't know. You made a mistake. It reminds me of working with Cisco routers. Not only does it make you realise that you made a mistake, it puts a little carrot underneath where you screwed up the spelling so that everybody can see it. Tab completion helps you out with that by not making you have to remember the entire command. So you type in the first couple of letters of a command, hit the tab key, and it finishes the command for you. Now, assuming you've typed enough of the commands to make them distinct, in other words, if you type just the letter C, I'm not sure if you mean cat or clear. Maybe you mean one of the two. But if you hit the tab, I can't finish it because you didn't give me enough. If you type CL now, I've got enough to know that it's clear because no other command starts with the letter CL. That's what we mean by unique enough. So you have to put enough of that in and then hit the tab to complete it. By using tab, by the way, ithelps you be faster at typing. You don't have to memorise spellings, especially if you're not sure how some things are supposed to be spelled. You can also then use the tab-tab double-tab to see potential matches. So if you type C and you're not sure what outthere, you can tap tab and you should be able tosee a list of all the commands that start with theletter C so that it helps you get unique enough.