{"id":242,"date":"2012-09-07T01:00:00","date_gmt":"2012-09-07T00:00:00","guid":{"rendered":"https:\/\/www.fussylogic.co.uk\/blog\/?p=242"},"modified":"2012-09-07T00:23:21","modified_gmt":"2012-09-06T23:23:21","slug":"bashing-at-the-keyboard-part-i-2","status":"publish","type":"post","link":"https:\/\/www.fussylogic.co.uk\/blog\/?p=242","title":{"rendered":"Bashing at the Keyboard (part I)"},"content":{"rendered":"

The command line. The feared enemy of anyone who started using a computer after 1995 (ish). It\u00e2\u20ac\u2122s got a bad reputation, and for day to day computer tasks it\u00e2\u20ac\u2122s certainly not necessary. However, if you are a programmer or a systems administrator then you\u00e2\u20ac\u2122ll benefit enormously from knowing how to get things done on a computer using only a keyboard and a terminal.<\/p>\n

I\u00e2\u20ac\u2122d better take a step back at this point, and define my terms. When a Linux system boots, you (probably) see a text mode screen showing lots of gobbledygook (people peering over my shoulder usually wittily opine \u00e2\u20ac\u0153it\u00e2\u20ac\u2122s like the Matrix\u00e2\u20ac\u009d). That gobbledygook does have useful information, but I don\u00e2\u20ac\u2122t want to burden you with that at present. Suffice it to say: what you\u00e2\u20ac\u2122re seeing is all the background plumbing of your computer starting up. The same sort of thing happens on Windows, but you don\u00e2\u20ac\u2122t see it happening, you just see a progress bar or some kind of animated spinning arrow.<\/p>\n

This text appears on what is called the console<\/em>. It\u00e2\u20ac\u2122s a very simple screen and keyboard implementation provided to get you access to your system when all else fails. Once the boot is completed, a login prompt gets shown on the console, and you could login with your normal user and password. However, on a modern system what probably happens, before you get time to look at the login prompt, or type anything, is that the graphics environment starts up and shows you a graphical login prompt. Should you wish, you can return to the text mode console screen by pressing Ctrl-Alt-F1. To get back to the graphical login you can press Ctrl-Alt-F7 (sometimes F8, depending on your distribution; and actually the Ctrl is unnecessary when going from text to graphics, but don\u00e2\u20ac\u2122t worry too much about that).<\/p>\n

So; we can choose to log in graphically, or textually. Let\u00e2\u20ac\u2122s do textually to start, just so you\u00e2\u20ac\u2122ve got an idea of what you would see (this is what I see, yours will be similar, with the differences depending on what distribution you use).<\/p>\n

Debian GNU\/Linux squeeze\/sid host tty1\n\nhost login: user\nPassword:\n\nNo mail.\nuser@host:~$\n<\/code><\/pre>\n
Your password doesn\u00e2\u20ac\u2122t get echoed to the screen, for obvious reasons, and you get very little noise after logging in, you are simply given a command prompt<\/em>, in this case \u00e2\u20ac\u0153user@host:~$<\/code>\u00e2\u20ac\u009d. Remember that for now, and we\u00e2\u20ac\u2122ll go back to the graphical login with Ctrl-Alt-F7<\/code>.<\/p>\n
Login as normal. Now, find in your application menu something like \u00e2\u20ac\u0153Terminal\u00e2\u20ac\u009d, \u00e2\u20ac\u0153Console\u00e2\u20ac\u009d, \u00e2\u20ac\u0153gterm\u00e2\u20ac\u009d, \u00e2\u20ac\u0153xterm\u00e2\u20ac\u009d, \u00e2\u20ac\u0153rxvt\u00e2\u20ac\u009d, \u00e2\u20ac\u0153Konsole\u00e2\u20ac\u009d \u00e2\u20ac\u201d exactly what it\u00e2\u20ac\u2122s called is dependent (as usual) on your distribution and your chosen graphical environment (GNOME or KDE probably) \u00e2\u20ac\u201d I like xterm<\/code> myself (and I\u00e2\u20ac\u2122ll say xterm<\/code> from now on, but whichever you pick is fine). These are all so-called \u00e2\u20ac\u0153terminal\u00e2\u20ac\u009d programs. What they do is provide a graphical equivalent of the console we\u00e2\u20ac\u2122ve just seen. The name \u00e2\u20ac\u0153terminal\u00e2\u20ac\u009d is a holdover from the days when computing was done with one huge single computer, with a dumb terminal (basically a screen and keyboard with a long wire connecting them to the computer) on the users desk. These terminal programs are the modern day equivalent, you open one and it provides a screen and keyboard in a window. What you\u00e2\u20ac\u2122ll see when you run it is a window with the following in it.<\/p>\n
user@host:~$\n<\/code><\/pre>\n
This is, for all intents, exactly the same as the command prompt you got when you logged in on a console. Anything you can do in text mode, you can do in a graphical terminal supplied by xterm (et al<\/em>).<\/p>\n
It\u00e2\u20ac\u2122s worth pointing out here that it is not xterm that is providing this prompt \u00e2\u20ac\u201d it\u00e2\u20ac\u2122s merely supplying a screen and keyboard for another program. Terminal programs, once they have started, then proceed to run another program that has its input and output redirected to use the window that the terminal program is providing. By default, they run a program called bash<\/code>. It is of a class of programs called shells<\/em> and is the most commonly used. bash<\/code> is the program that has output this command prompt \u00e2\u20ac\u0153user@host:~$<\/code>\u00e2\u20ac\u009d. In exactly the same way, when we logged in on the console we were logging in using a program called (unsurprisingly) login<\/code>; which upon accepting the login credentials you give it runs a copy of bash<\/code> as well. It should be no surprise then to see that we get an identical prompt in both cases.<\/p>\n
Shells are programs that are used to run other programs. At first hearing, it sounds like a silly thing to want. However, it\u00e2\u20ac\u2122s use is so implicit that often people don\u00e2\u20ac\u2122t even notice that they\u00e2\u20ac\u2122ve been using it. As an example: many people think that the task bar, explorer and start menu in Windows are<\/em> Windows \u00e2\u20ac\u201d when actually they are just a graphical shell, and a surprisingly small part of the operating system. Whatever graphical system you just used to find xterm<\/code> is a graphical shell is simply one program that is used to start another. bash<\/code> is a textual shell, but has the same job \u00e2\u20ac\u201d it lets you start other programs on the system. Let\u00e2\u20ac\u2122s run a few programs now in our shell to get the idea.<\/p>\n
user@host:~$ ls -a\n.  ..  .bash_history  .bash_profile  .bashrc\nuser@host:~$ cat .bash_profile\n# include .bashrc if it exists\nif [ -f ~\/.bashrc ]; then\n    . ~\/.bashrc\nfi\n\n# set PATH so it includes user's private bin if it exists\nif [ -d ~\/bin ] ; then\n    PATH=~\/bin:"${PATH}"\nfi\n<\/code><\/pre>\n
We ran a program called ls<\/code>, which lists the contents of the current directory (the -a<\/code> means list all<\/em> files, including hidden files). Then we ran a program called cat<\/code>, which reads a file and outputs it to the terminal \u00e2\u20ac\u201d in this case, one of the bash startup files, .bash_profile<\/code>. The startup files for bash<\/code> are a list of commands that are run automatically when bash<\/code> starts. They are pretty much entirely devoted to setting up a workable command line, and configuring bash<\/code> to suit the user (the default versions are fine for you for now).<\/p>\n
Notice what I said about that first command, that it lists the contents of the current<\/em> directory. bash<\/code> maintains the idea of a current directory, and whenever you run another program with it, it passes that current directory to it. If you\u00e2\u20ac\u2122re here from an entirely Windows-based computer upbringing, the terms \u00e2\u20ac\u0153directory\u00e2\u20ac\u009d and \u00e2\u20ac\u0153folder\u00e2\u20ac\u009d are entirely synonymous. Let\u00e2\u20ac\u2122s look again at the command prompt that bash<\/code> has been giving us.<\/p>\n
user@host:~$\n<\/code><\/pre>\n
This prompt is configurable, and your distribution might have picked something different, but this is a common default form. There are three parts to this, separated by three punctuation characters.<\/p>\n