Difference between revisions of "BOSS-Linux/C2/The-Linux-Environment/English-timed"

|Welcome to this spoken tutorial on the Linux environment and ways to manupulate it.

|A working linux system, will be required to try out the examples illustrated in this tutorial.

|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 .

|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.

 

|Please also note that Linux is case sensitive and all the commands used in this tutorial are in lower case unless otherwise mentioned.

|Linux can be highly customized by changing the settings of the shell.  

|The behaviour of the shell is generally determined by the shell variables.

|There are mainly two kinds of shell variables:

 

Environment Variables and

 

Local Variables

|Environment variables, named so because they are available entirely in the user's total environment.  

|These are also available in subshells spawned by the shell like the ones for running shell scripts.  

|Local Variables , which as the name suggests have a more restricted or limited availability.  

|These are not available in the subshells spawned by the shell.  

|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.

|To see all the variables available in the current shell , we run the command set.

| Type at the terminal '''set''' space pipeline character '''more''' and press enter.

|We can see all the current shell variables ,  

|For example : take a look at the '''HOME''' environment variable also notice the value assigned to it.

|press '''Enter''' to move through the list and in order to come out press '''q'''

|Here the output from set was pipelined to more in order to display a more systematic multipage output of the variable list.

|To see only the environment variables run the command '''env'''

|Type at the terminal'''env'''  space 'vertical-bar' '''more''' and press enter

|For example, Notice the '''SHELL''' variable whose value is '''slash bin slash bash'''  

|Now let us discuss some of the more important environment variables in linux.  

|We would be using bash shell for all our demonstrations here.  

|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.

|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.'''

|Here '''slash bin slash bash''' is the shell where we are currently operating.

|This directory is called the home directory and this is exactly what is available in HOME variable.

|To see the value, type at the terminal '''echo''' space '''dollar H-O-M-E''' in capital and press '''enter'''

|The '''PATH''' variable contains the absolute paths of the directories that the shell is supposed to search for locating any executable command.

|Lets see the value of the path variable.

|Again, type at the terminal '''echo''' space '''dollar P-A-T-H ''' in capitals and press enter

|On my computer it shows '''slash user slash local slash sbin slash user slash bin etc'''.

|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.

|In order to add our own directory type at the terminal  

|'''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.

|Now if we '''echo''' the value of '''PATH''',  

|Our added directory will also be a part of the '''PATH''' variable.  

|See the directory is now present here.

|In order to see the value type '''echo''' space '''dollar LOGNAME''' and press '''enter.'''

|When we open the terminal we can see the dollar sign, which is the prompt at which we enter all our commands.  

|This is the primary prompt string represented by the environment variable PS1.  

|There is a secondary prompt string also

|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

|This is the secondary prompt string represented by the environment variable PS2.

|To see the value of the secondary command prompt, type at the terminal '''echo''' space '''dollar PS2'''  and press '''enter.'''

|We may change our primary prompt string to say “at the rate” (@) at the prompt.

|In order to get this done Type ''' PS1'''  'equal-to' now within quotes 'at the rate(@)' “ and press enter.

|Now instead of the '''dollar''' sign we can see the '''at the rate''' sign as the prompt.

|We may do something more interesting. Like we may display our username at the prompt.  

|Just type '''PS1''' in capital 'equal-to' within quotes '''dollar LOGNAME''' and press '''enter'''

|Now my username is my prompt.

|To revert back type '''PS1''' 'equal-to' dollar within quotes and press enter."

|But, remember one thing that these modifications are only applicable for the current session.

|Like we had just added our directory to the PATH variable.  

|If we close the terminal and open it again or open a new terminal altogether and check the path variable by echoing its value

|We will be surprised to see that our modifications are no longer present.

|First, normally if you press the up key on your keyboard then it will show the last command that you typed.  

|To go back press the down key.  

|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.

|and press enter, see a list of previously executed commands appears

|If instead of the large list you want to see, only the last ten  

|Type '''history space 10''' and press ''enter'''

|In order to repeat a particular command.

|Just type exclamation mark followed by the number of the command for example 442 in my case would execute '''echo space dollar PATH'''  

|If you need to re execute the last command simply type exclamation mark twice and press enter.

|The next thing we would see is called tilde substitution. The tilde(~) character is a shorthand for the home directory.

|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'''.  

|One may also toggle between the current working directory and the last directory used by giving the command

|Like now that we are in the '''testtree''' directory, the last directory we visited was the home directory.   

|So if we run '''cd space minus''' and press enter. It will go to the home directory.  

|Run it again and it will take us back to the '''testtree''' directory.

|It may happen that you have a large command that needs to be run again and again.  

|In this case we can give it a short '''alias''' name and use the '''alias''' name instead ,to invoke it.

|Assuming that you have such a long directory hierarchy that you frequently visit for music, you may create an alias for it like this

|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

|Now every time you need to switch to this directory simply write '''cdMusic''' and press enter.  

|Now, you may type '''cd space minus

''' and the prompt to go back to the previous working directory.  

|To unset an '''alias''' simply write '''unalias space cdMusic''' and press enter

|Now again if you fire '''cdMusic''' from the terminal, you will get an error stating that the command was not found.

|Suppose we have two files, '''test1''' and '''test2''' in our present working directory

|So we may set an '''alias''' like, '''alias rm''' equal-to, now within quotes '''rm space hyphen i'''  

|Now when we run '''rm''' ,''' rm hyphen i'''will actually be run.

|So we saw that while '''test1''' was silently deleted, system asked before deleting '''test2.'''

|So, in this tutorial, we have learned about environment variables, history and aliasing.

|Spoken Tutorials are a part of the Talk to a Teacher project, supported by the National Mission on Education through ICT.  

|The Script for this tutorial was created by Anirban

|This Anugrath Parashah from MIT university signing off.