Difference between revisions of "BOSS-Linux/C2/The-Linux-Environment/English-timed"
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| − | |A working Linux system will be required to try out the examples illustrated in this tutorial. | + | |A working '''Linux system''' will be required to try out the examples illustrated in this tutorial. |
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| − | |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. | + | |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. |
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| − | |Please | + | |Please note that Linux is '''case sensitive''' and all the commands used in this tutorial are in lower case unless otherwise mentioned. |
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| − | |The behavior of the '''shell''' is generally determined by the shell variables. | + | |The behavior of the '''shell''' is generally determined by the '''shell variables'''. |
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| − | |To see what is the value of the '''SHELL''' variable, type at the terminal '''echo''' space '''dollar S-H-E-L-L''' in capitals and press '''Enter.''' | + | |To see what is the value of the '''SHELL''' variable, type at the terminal: '''echo''' space '''dollar S-H-E-L-L''' in capitals and press '''Enter.''' |
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| − | |This directory is called the '''home''' directory and this is exactly what is available in 'HOME' variable. | + | |This directory is called the '''home''' directory and this is exactly what is available in the 'HOME' variable. |
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| − | |On my computer, it shows '''slash user slash local slash | + | |On my computer, it shows '''slash user slash local slash bin slash user slash bin''' etc. |
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| − | |To see the value of the secondary command prompt, type at the terminal: '''echo''' space '''dollar PS2''' and press '''Enter.''' | + | |To see the value of the 'secondary command prompt', type at the terminal: '''echo''' space '''dollar PS2''' and press '''Enter.''' |
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|07:13 | |07:13 | ||
| − | |We may change our primary prompt string to say “at the rate” (@) at the prompt. | + | |We may change our 'primary prompt string' to say “at the rate” (@) at the prompt. |
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| − | |But, remember one thing that these modifications are only applicable for the current session | + | |But, remember one thing that these modifications are only applicable for the current session |
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|08:30 | |08:30 | ||
| − | | | + | |like we had just added our directory to the '''PATH''' variable. |
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| − | |First, normally if you press the up-key on your keyboard then it will show the last command that you typed. | + | |First, normally if you press the up-key on your keyboard then it will show the last command that you have typed. |
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|09:30 | |09:30 | ||
| − | |To go back press the down-key. | + | |To go back, press the down-key. |
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|09:36 | |09:36 | ||
| − | |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. | + | |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. |
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| − | |and press Enter | + | |and press Enter; see a list of previously executed commands appears. |
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|09:57 | |09:57 | ||
| − | |If instead of the large list you | + | |If instead of the large list, you wanted to see only the last ten, |
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| − | |just type exclamation mark followed by the number of the command. For example- 442 in my case would execute '''echo space dollar PATH'''. | + | |just type an exclamation mark followed by the number of the command. For example- 442 in my case, would execute '''echo space dollar PATH'''. |
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|10:44 | |10:44 | ||
| − | |If you need to re execute the last command simply type exclamation mark twice and press Enter. | + | |If you need to re-execute the last command, simply type exclamation mark twice and press Enter. |
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| − | |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'''. | + | |So, say you have a directory with the name 'testtree' in your home directory. You can move to it by typing: '''cd''' space '''~(tilde)''' slash '''testtree'''. |
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| − | |It may happen that you have a large command that needs to be run again and again. | + | |It may quite happen that you have a large command that needs to be run again and again. |
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| − | |So, we may set an '''alias''' like, '''alias rm''' equal-to, now within quotes '''rm space hyphen i''' | + | |So, we may set an '''alias''' like, '''alias rm''' equal-to, now within quotes '''rm space hyphen i'''. |
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|13:56 | |13:56 | ||
| − | |Now, when we run '''rm''',''' rm hyphen i'''will actually be '''run'''. | + | |Now, when we run '''rm''', '''rm hyphen i''' will actually be '''run'''. |
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|14:05 | |14:05 | ||
Revision as of 17:08, 12 August 2015
| Time | Narration |
| 00:01 | Welcome to this spoken tutorial on the Linux environment and ways to manipulate it. |
| 00:07 | A working Linux system will be required to try out the examples illustrated in this tutorial. |
| 00:12 | 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. |
| 00:20 | 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. |
| 00:30 | Please note that Linux is case sensitive and all the commands used in this tutorial are in lower case unless otherwise mentioned. |
| 00:40 | 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. |
| 00:48 | Linux can be highly customized by changing the settings of the shell. |
| 00:51 | Let us understand how all this can be done. |
| 00:54 | The behavior of the shell is generally determined by the shell variables. |
| 00:58 | There are mainly two kinds of shell variables: Environment Variables and Local Variables. |
| 01:06 | Environment variables, named so because they are available entirely in the user's total environment. |
| 01:12 | These are also available in sub-shells spawned by the shell like the ones for running shell scripts. |
| 01:18 | Local Variables which as the name suggests have a more restricted or limited availability. |
| 01:24 | These are not available in the sub-shells spawned by the shell. |
| 01:28 | 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. |
| 01:41 | To see all the variables available in the current shell, we run the command 'set'. |
| 01:46 | Type at the terminal: set space pipeline character more and press Enter. |
| 01:53 | We can see all the current shell variables. , |
| 01:58 | For example- take a look at the HOME environment variable; also notice the value assigned to it. |
| 02:07 | Press Enter to move through the list and in order to come out, press q. |
| 02:14 | Here, the output from set was pipe-lined to more in order to display a more systematic multi-page output of the variable list. |
| 02:31 | To see only the environment variables, run the command 'env'. |
| 02:38 | Type at the terminal: env space 'vertical-bar' more and press Enter. |
| 02:46 | For example, notice the SHELL variable whose value is slash bin slash bash. |
| 02:53 | Again, you may press q to come out of the list. |
| 03:00 | Now, let us discuss some of the more important environment variables in Linux. |
| 03:05 | We would be using bash shell for all our demonstrations here. |
| 03:09 | Different shells are customized in slightly different ways. |
| 03:12 | 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. |
| 03:24 | The first environment variable that we would see is the SHELL variable. |
| 03:28 | It stores the name of the current shell. |
| 03:31 | To see what is the value of the SHELL variable, type at the terminal: echo space dollar S-H-E-L-L in capitals and press Enter. |
| 03:48 | Here, slash bin slash bash is the shell where we are currently operating. |
| 03:56 | The next variable is HOME. |
| 03:58 | When we login into Linux, it normally places us in a directory named after our user name. |
| 04:04 | This directory is called the home directory and this is exactly what is available in the 'HOME' variable. |
| 04:10 | To see the value, type at the terminal: echo space dollar H-O-M-E in capital and press Enter. |
| 04:22 | The next environment variable is PATH. |
| 04:26 | The PATH variable contains the absolute paths of the directories that the shell is supposed to search for locating any executable command. |
| 04:33 | Let's see the value of the path variable. |
| 04:36 | Again, type at the terminal: echo space dollar P-A-T-H in capitals and press Enter. |
| 04:45 | On my computer, it shows slash user slash local slash bin slash user slash bin etc. |
| 04:57 | This may slightly vary from one system to another. |
| 05:00 | 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. |
| 05:11 | We can also add our own directory to this list so that our directory is also searched by the shell. |
| 05:18 | In order to add our own directory, type at the terminal: |
| 05:22 | P-A-T-H in capitals 'equal-to' dollar P-A-T-H again in capital colon slash home slash the name of my own home directory and press Enter. |
| 05:47 | Now, if we echo the value of PATH, |
| 05:57 | our added directory will also be a part of the PATH variable. |
| 06:03 | See, the directory is now present here. |
| 06:09 | Another interesting variable is the LOGNAME. |
| 06:12 | It stores the username of the currently active user. |
| 06:16 | In order to see the value, type: echo space dollar LOGNAME and press Enter. |
| 06:28 | When we open the terminal, we can see the dollar sign which is the prompt at which we enter all our commands. |
| 06:35 | This is the primary prompt string represented by the environment variable 'PS1'. |
| 06:40 | There is a secondary prompt string also. |
| 06:43 | 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. |
| 06:53 | This is the secondary prompt string represented by the environment variable 'PS2'. |
| 06:58 | To see the value of the 'secondary command prompt', type at the terminal: echo space dollar PS2 and press Enter. |
| 07:13 | We may change our 'primary prompt string' to say “at the rate” (@) at the prompt. |
| 07:20 | In order to get this done, type: PS1 'equal-to' now within quotes 'at the rate(@)' “ and press Enter. |
| 07:34 | Now, instead of the dollar sign we can see the at the rate sign as the prompt. |
| 07:43 | We may do something more interesting like- we may display our username at the prompt. |
| 07:49 | Just type: PS1 in capital 'equal-to' within quotes dollar LOGNAME and press Enter. |
| 08:05 | Now, my username is my prompt. |
| 08:09 | To revert back, type: PS1 'equal-to' dollar ($) within quotes and press Enter. |
| 08:21 | We have assigned values to many of the environment variables. |
| 08:25 | But, remember one thing that these modifications are only applicable for the current session |
| 08:30 | like we had just added our directory to the PATH variable. |
| 08:34 | If we close the terminal and open it again or open a new terminal altogether and check the 'path' variable by echoing its value, |
| 08:53 | we will be surprised to see that our modifications are no longer present. |
| 08:59 | The way by which we can make these modifications permanent will be covered in some advanced tutorial. |
| 09:06 | 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? |
| 09:15 | No, there are a number of solutions. |
| 09:19 | First, normally if you press the up-key on your keyboard then it will show the last command that you have typed. |
| 09:26 | Keep pressing it and it will keep scrolling through the previous commands. |
| 09:30 | To go back, press the down-key. |
| 09:36 | 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. |
| 09:45 | Type at the prompt history |
| 09:51 | and press Enter; see a list of previously executed commands appears. |
| 09:57 | If instead of the large list, you wanted to see only the last ten, |
| 10:02 | type: history space 10' and press Enter. |
| 10:13 | Notice, in this list, there is a number assigned to each of the previously executed commands. |
| 10:20 | In order to repeat a particular command, |
| 10:24 | just type an exclamation mark followed by the number of the command. For example- 442 in my case, would execute echo space dollar PATH. |
| 10:44 | If you need to re-execute the last command, simply type exclamation mark twice and press Enter. |
| 10:56 | The next thing we would see is called tilde substitution. The tilde(~) character is a shorthand for the home directory. |
| 11:05 | So, say you have a directory with the name 'testtree' in your home directory. You can move to it by typing: cd space ~(tilde) slash testtree. |
| 11:18 | 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 |
| 11:28 | Like now that we are in the testtree directory, the last directory we visited was the home directory. |
| 11:34 | So, if we run cd space minus and press Enter, it will go to the home directory. |
| 11:40 | Run it again and it will take us back to the testtree directory. |
| 11:47 | The last but quite important command we will see is the alias command. |
| 11:51 | It may quite happen that you have a large command that needs to be run again and again. |
| 11:57 | In this case, we can give it a short alias name and use the alias name instead to invoke it. |
| 12:03 | Assuming that you have such a long directory hierarchy that you frequently visit for music, you may create an alias for it like this. |
| 12:13 | Type: alias space cdMusic 'equal-to' within double quotes cd space slash home slash the name of my own home directory slash music and press Enter. |
| 12:39 | Now, every time you need to switch to this directory, simply write cdMusic and press Enter. |
| 12:47 | See, we are in the music directory now. |
| 12:52 | Now, you may type cd space minus
and the prompt to go back to the previous working directory. |
| 13:01 | To unset an alias, simply write unalias space cdMusic and press Enter. |
| 13:13 | Now again if you fire cdMusic from the terminal, you will get an error stating that the command was not found. |
| 13:22 | Suppose, we have two files- test1 and test2 in our present working directory |
| 13:31 | and if we fire rm test1, test1 is silently deleted. |
| 13:38 | We know that hyphen i option of the rm command makes the removal process interactive. |
| 13:45 | So, we may set an alias like, alias rm equal-to, now within quotes rm space hyphen i. |
| 13:56 | Now, when we run rm, rm hyphen i will actually be run. |
| 14:05 | So, we saw that while test1 was silently deleted, system asked before deleting test2. |
| 14:13 | So, in this tutorial, we have learned about environment variables, history and aliasing. |
| 14:18 | This brings me to the end of this tutorial. |
| 14:21 | Spoken Tutorials are a part of the Talk to a Teacher project, supported by the National Mission on Education through ICT. |
| 14:29 | More information on the same is available from our website. |
| 14:32 | The Script for this tutorial was created by Anirban. |
| 14:36 | This Anuvrath Parashar from MIT university, signing off. |
