Difference between revisions of "Linux/C2/The-Linux-Environment/English-timed"
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|Linux can be highly customized by changing the settings of the '''shell'''. | |Linux can be highly customized by changing the settings of the '''shell'''. | ||
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|00:58 | |00:58 | ||
| − | |Let us understand how all this can be done. | + | |Let us understand how all this can be done.The behavior of the shell is generally determined by the '''shell variables'''. |
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| − | + | ||
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|01:04 | |01:04 | ||
| − | |There are mainly two kinds of shell variables: | + | |There are mainly two kinds of shell variables: '''Environment Variables''' and '''Local Variables'''. |
| − | '''Environment Variables''' and | + | |
| − | '''Local Variables'''. | + | |
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|01:12 | |01:12 | ||
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|02:52 | |02:52 | ||
| − | |For example, | + | |For example, notice the '''SHELL''' variable whose value is '''slash bin slash bash'''. |
| − | notice the '''SHELL''' variable whose value is '''slash bin slash bash'''. | + | |
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|03:00 | |03:00 | ||
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|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? | |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? | ||
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| − | | | + | |09:22 |
| No, there are a number of solutions. | | No, there are a number of solutions. | ||
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|11:25 | |11:25 | ||
| − | |One may also toggle between the current working directory and the last directory used by giving the command | + | |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''' |
| − | '''cd '~(tilde)' minus''' or only '''cd minus''' | + | |
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|11:35 | |11:35 | ||
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|12:58 | |12:58 | ||
| − | |Now, you may type '''cd space minus | + | |Now, you may type '''cd space minus ''' at the prompt to go back to the previous working directory. |
| − | ''' at the prompt to go back to the previous working directory. | + | |
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|13:08 | |13:08 | ||
Latest revision as of 17:31, 23 March 2017
| 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.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? |
| 09:22 | 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. |
