Linux/C2/The-Linux-Environment/English-timed

Time	Narration
00:00	Welcome to this spoken tutorial on The Linux Environment and ways to manipulate it.
00:07	A working Linux system, preferably Ubuntu will be required to try out the examples illustrated in this tutorial.
00:13	We assume that you know how to get started with the Linux operating system and have some basic idea about commands, file systems and shell.
00:22	If you are interested or need to brush these concepts up, please feel free to do so through another spoken tutorial available on our website.
00:32	Please note that Ubuntu 10.10 was used for recording this tutorial.
00:36	Please also note that Linux is case sensitive and all the commands used in this tutorial are in lower case unless otherwise mentioned.
00:46	The Linux environment determines how the operating system behaves with you, how it responds to your commands, how it interprets your actions and so on.
00:55	Linux can be highly customized by changing the settings of the shell.
00:58	Let us understand how all this can be done.
00:59	The behavior of the shell is generally determined by the shell variables.
01:04	There are mainly two kinds of shell variables: Environment Variables and Local Variables.
01:12	Environment variables, named so because they are available entirely in the user's total environment.
01:19	These are also available in sub-shells spawned by the shell like the ones for running shell scripts.
01:24	Local Variables, which as the name suggests, have a more restricted or limited availability.
01:31	These are not available in the sub-shells spawned by the shell.
01:36	While in this tutorial, we will mainly talk about environment variables. Let us first see how the value of these shell variables can be seen.
01:48	To see all the variables available in the current shell, we run the command set.
01:53	Type at the terminal: set space pipeline character more and press Enter.
02:00	We can see all the current shell variables.
02:04	For example: take a look at the HOME environment variable, also notice the value assigned to it.
02:15	Press Enter to move through the list and in order to come out press q.
02:21	Here, the output from set was pipelined to more in order to display a more systematic multipage output of the variable list.
02:38	To see only the environment variables, run the command env.
02:45	Type at the terminal:env space 'vertical-bar' more and press Enter.
02:52	For example, notice the SHELL variable whose value is slash bin slash bash.
03:00	Again, you may press q to come out of the list.
03:07	Now let us discuss some of the more important environment variables in Linux.
03:11	We would be using the bash shell for all our demonstrations here.
03:15	Different shells are customized in slightly different ways.
03:19	To see what a variable actually stores, we have to prefix a dollar sign to the name of that variable and use the echo command along with it.
03:30	The first environment variable that we would see is the SHELL variable.
03:35	It stores the name of the current shell.
03:37	To see what is the value of the SHELL variable, type at the terminal: echo space dollar S-H-E-L-L in capital and press Enter.
03:55	Here, slash bin slash bash is the shell where we are currently operating.
04:02	The next variable is HOME.
04:05	When we login into Linux, it normally places us in a directory named after our user name.
04:11	This directory is called the 'home directory' and this is exactly what is available in 'HOME' variable.
04:17	To see the value, type at the terminal: echo space dollar H-O-M-E in capital and press Enter.
04:29	The next environment variable is PATH.
04:32	The PATH variable contains the absolute paths of the directories that the shell is supposed to search for locating any executable command.
04:40	Let's see the value of the path variable.
04:43	Again, type at the terminal: echo space dollar P-A-T-H in capitals and press Enter.
04:51	On my computer it shows slash user slash local slash sbin slash user slash local slash bin slash user slash sbin slash user slash bin etc.
05:04	This may slightly vary from one system to another.
05:07	It is actually a list of directories separated by the :(colon) delimiter, that the shell would search in this order for finding an executable command.
05:18	We can also add our own directory to this list so that our directory is also searched by the shell.
05:25	In order to add our own directory type at the terminal:
05:29	P-A-T-H in capital 'equal-to' dollar P-A-T-H again in capital colon slash home slash the name of my own home directory and press Enter.
05:54	Now if we echo the value of PATH,
06:04	our added directory will also be a part of the PATH variable.
06:10	See the directory is now present here.
06:16	Another interesting variable is the LOGNAME.
06:20	It stores the username of the currently active user.
06:24	In order to see the value, type: echo space dollar LOGNAME and press Enter.
06:35	When we open the terminal we can see the dollar sign which is the prompt at which we enter all our commands.
06:42	This is the primary prompt string represented by the environment variable 'PS1'.
06:47	There is a secondary prompt string also.
06:50	If our command is long and it spans for more than one line then from the second line onwards we can see a greater than sign “>” as the prompt.
07:00	This is the secondary prompt string represented by the environment variable 'PS2'.
07:05	To see the value of the secondary command prompt, type at the terminal: echo space dollar PS2 and press Enter.
07:20	We may change our primary prompt string to say “at the rate” <@> at the prompt.
07:28	In order to get this done, type PS1 'equal-to' now within quotes 'at the rate' “@"and press Enter.
07:41	Now instead of the dollar sign we can see the at the rate sign as the prompt.
07:50	We may do something more interesting. Like we may display our username at the prompt.
07:56	Just type: PS1 in capital 'equal-to' within quotes dollar LOGNAME and press Enter.
08:12	Now my username is my prompt.
08:16	To revert back, type: PS1 'equal-to' dollar within quotes and press Enter.
08:28	We have assigned values to many of the environment variables.
08:32	But, remember one thing that these modifications are only applicable for the current session.
08:37	Like we had just added our directory to the PATH variable.
08:40	If we close the terminal and open it again or open a new terminal altogether and check the path variable by echoing its value,
09:00	we will be surprised to see that our modifications are no longer present.
09:05	The way by which we can make these modifications permanent will be covered in some advanced tutorial.
09:13	Often we want to re-execute a command that we had executed in the recent past. What do we do? Do we have to type the entire command again?
9:22	No, there are a number of solutions.
09:26	First, normally if you press the 'up key' on your keyboard then it will show the last command that you typed.
09:33	Keep pressing and it will keep scrolling through the previous commands.
09:37	To go back press the down key.
09:42	But when you have to scroll through many commands this becomes a little clumsy and tedious. A better way is to use the history command.
09:52	Type at the prompt history.
09:58	and press Enter, see a list of previously executed commands appear.
10:04	If instead of the large list you want to see only the last ten,
10:08	type history space 10' and press Enter.
10:20	Notice, in this list there is a number assigned to each of the previously executed commands.
10:27	In order to repeat a particular command,
10:32	just type exclamation mark followed by the number of the command for example 442 in my case would execute echo space dollar PATH
10:51	If you need to re-execute the last command simply type exclamation mark twice and press Enter.
11:03	The next thing we would see is called 'tilde' substitution. The tilde(~) character is a shorthand for the home directory.
11:12	So, say you have a directory with name testtree in your home directory. You can move to it by typing: cd space ~(tilde) slash testtree.
11:25	One may also toggle between the current working directory and the last directory used by giving the command cd '~(tilde)' minus or only cd minus
11:35	Like now that we are in the testtree directory, the last directory we visited was the home directory.
11:41	So if we run cd space minus and press Enter, it will go to the home directory.
11:47	Run it again and it will take us back to the testtree directory.
11:55	The last but quite important command we will see is the alias command.
11:59	It may happen that you have a large command that needs to be run again and again.
12:04	In this case we can give it a short alias name and use the alias name instead, to invoke it.
12:11	Assuming that you have such a long directory hierarchy that you frequently visit for music, you may create an alias for it like this.
12:20	Type: alias space cdMusic 'equal-to' within double quotes cd space slash home slash arc slash files slash entertainment slash music and press Enter.
12:47	Now every time you need to switch to this directory simply write cdMusic and press Enter.
12:55	See, we are in the music directory now.
12:58	Now, you may type cd space minus at the prompt to go back to the previous working directory.
13:08	To unset an alias, simply write unalias space cdMusic and press Enter.
13:20	Now again if you fire cdMusic from the terminal, you will get an error stating that the command was not found.
13:30	Suppose we have two files, test1 and test2 in our present working directory
13:38	and if we fire rm test1, test1 is silently deleted.
13:45	We know that hyphen i option of the rm command makes the removal process interactive.
13:52	So we may set an alias like, alias rm equal-to, now within quotes rm space hyphen i
14:03	Now when we run rm ,rm hyphen iwill actually be run.
14:13	So we saw that while test1 was silently deleted, system asked before deleting test2.
14:20	So, in this tutorial, you have learned about environment variables, history and aliasing.
14:25	This brings me to the end of this tutorial.
14:28	Spoken Tutorials are a part of the Talk to a Teacher project, supported by the National Mission on Education through ICT.
14:36	More information on the same is available from our website.
14:39	The Script for this tutorial was created by Anirban.
14:42	This script has been contributed by ----------------------(name of the translator) and this is -----------------------(name of the recorder) from --------------------------(name of the place)signing off.

Contributors and Content Editors

Minal, PoojaMoolya, Pratik kamble, Sandhya.np14