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Environment variables. What are

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the environment variables?

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A regular definition or off the book definition of

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environment variable is, an environment variable is a

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dynamic-named value that can affect the way running

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process will behave on a computer.

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They are part of the environment

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in which a process runs.

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So in simple words, environment variables are a set

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of defined rules and values to build an environment.

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So to make it more simpler I will

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give you an example of a house.

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We have our house, by the way, it's just a tall

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house that's the only house that I could pick out from

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one of the search engine that would depict the dining room

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and bedroom, anyway, so we have our house, just take an

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example of our own house, the house that you live in.

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That house has its own environment.

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What it means is, it has a bedroom where you sleep,

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it has a dining room where you eat, sorry it has

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a kitchen where you cook, and the dining room where you

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eat, and it has a playroom where you play, and

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so many different rooms for other purposes.

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Now those are defined a set of rules or someone would

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say an unwritten rules. You cannot really sleep in the kitchen,

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or you cannot really cook in the bedroom, right?

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Because those places are built for that specific

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needs, so that's the environment of your house.

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Similarly, in Linux, we have environment. Environment meaning

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when a user logs in the user is

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given an environment by saying, hey,

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your home directory is this, your process or your shell where you're going

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to run the scripts those paths are these, these are the

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coloring you will get for all directory and listing, and

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these are a few set of rules that you will

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get so they are inherited automatically through the global environment.

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Those environments by the user can also be

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changed by a specific user as well.

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So to view all environment variables in Linux environment

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you have to run the command printenv

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sorry by the way it's not evn, it's env.

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I should fix that and I will definitely will, or

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you could just simply type env. So both commands are

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the same, we'll give you the same results,

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if you wanted to list what is in your environment.

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To view one specific environment variable, you could run a

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command like echo shell, so $SHELL, if you wanted

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to find out what is your shell. Usually the shell

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in Linux is given as bash, bin bash, that's a

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bash shell, corn shell, korn shell, whichever your shell is.

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If you want to find out your

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shell, you will specifically do echo $SHELL.

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If you want to find out specifically

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your path, you'll do echo $PATH.

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Anyway, I'll show you in a

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few minutes what I'm talking about.

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To set the environment variables on the fly for

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example you could do export TEST equal one and

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then do echo $TEST, then when you type echo

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$TEST you will get a result as one.

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If you want to set the environment variable permanently then

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you will have to modify that .bashrc file, and

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there you will have to put in TEST equals single quote, 123, single quote.

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Why 123, why not 1? It's just numbers,

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I should have picked 1 anywhere to be

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consistent, and then you type export TEST.

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But if you wanted to set the environment globally and

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you want to have it permanently you have to modify

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the profile and bashrc that are inside of etc.

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Now let me log in to my Linux machine and

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I will show you exactly what the shells are.

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So I am logged in and I'm logged in as myself,

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iafzal, and if I wanted to list the environment

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I will type printenv, hit enter, and as you see it's

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showing me a lot of stuff, and it's going straight up.

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I could do printenv, and pipe it to more

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so I could view it one page at a time.

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And you will see it will tell you my terminal,

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meaning the terminal that will be used to log in,

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it would be like an xterm.

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My hostname is myfirstlinuxvm.

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This is my SESSION ID,

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which will automatically be assigned.

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This is my SHELL.

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Whenever I log in a shell is given to me and

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that shell is /bin/bash. The history size, which is whenever

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I run certain commands those commands will be kept in history

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when you type history command. By the

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way, history command is something that I

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have covered in the later lectures.

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So history command, that is telling you it

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has to record up to 1000 line.

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So this is the environment.

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What is the SSH_TTY terminal that will be given?

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What is the USER,

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what comes in of course is my user.

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What are the different colors?

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You see LS_COLORS. When you run ls -l,

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you see a different color for directory, different color for

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file, those are the colors you could change.

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Like every time you put in a gif file, it will

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show up in the color code 01;35. What it

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refers to, I'm not sure, you can google it,

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you could find out. Then going

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forward where my MAIL will go.

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Whenever someone sends me a mail, where it will

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go, it will go to /var/spool/mail/iafzal.

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So these are the set rules, set-defined rules.

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Where is my PATH?

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Every time I run a command, the operating system will

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go in and find the location of that command, and

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if that location is not listed in one of these

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path, it will not find that command.

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My own bin directory will be /home/iafzal/bin.

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My home directory,

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if I run pwd, would be /home/iafzal.

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The LANG that is set to UTF-8.

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Some HISTCONTROL, my HOME directory, my LOGNAME,

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my DATA path, SSH_CONNECTION, and a couple of other

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things which I could go over in detail, but I

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don't want to take up a lot of your time.

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So these are the set variables,

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set environment that is given to you.

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If you want to check which environment variable, a

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specific environment that you wanted to check, you will

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do echo and you will do for example, $MAIL.

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You always have to start with dollar sign by

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the way, because this way it will understand oh,

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this is a variable that I'm talking about.

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So all I'm trying to find out what is

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the path that is defined for my mail.

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Right here you see it says MAIL

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equals /var/spool/mail/iafzal.

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So if my shell is reading the correct

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environment variable, I should get this back.

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So now if I do echo $MAIL, /var/spool/mail/iafzal. Perfect.

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Yes, I am getting that.

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Now if I want to do path, I

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will do echo $PATH, and it will

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list every directory where my path has defined.

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Just like that, the path is defined here.

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If I wanted to do my home, I will do echo $HOME,

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and you will see this is my home directory, right here HOME.

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This is my home directory.

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Also, sometimes what people do if they are in

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one directory, let's say if you are in at etc directory,

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you do pwd, you could simply do

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cd $HOME, and it will take you home.

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That's how your environment is defined as well.

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So if you want to set an environment

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variable while you're logged into the system. Let's

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say you wanted to do TEST equal 1.

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So you would type export TEST,

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which is variable equal one.

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And then when you do echo $TEST, OR

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$TEST, you will see the value one.

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But of course this is not permanently defined.

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If you log out of your system right

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now, this environment variable will go away.

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So if you want to set this environment

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variable permanently in your operating system, then you

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would have to actually, vi your bashrc file.

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So you are in your home directory,

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first make sure, then I will recommend that you copy

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your bashrc file, which is .bashrc

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and make it bashrc.orig.

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So this way if you make a mistake,

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you can always revert back to this.

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Now you vi your bashrc, and hit enter.

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Now come all the way down and define your variable.

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So I wanted to define my variable as TEST equal,

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now this time I just want to do 123.

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When you define a variable inside of the

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bashrc, you put a quote, single quote.

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Then you hit enter, and then type export TEST.

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Go ahead and save your file.

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Now if you do test again,

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echo $TEST, you'll get one.

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Why are you getting one?

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Because it is still reading that export TEST equal one

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and then it is giving you that same value.

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You have to log out and log back in.

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So for me I could just go here

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and click on this little icon up here

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and type in Duplicate Session from my PuTTY.

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I will do iafzal, login, hit enter,

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and now when I do echo TEST, you will

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see it is giving me 123.

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Because this session I have a new session,

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and when I logged into a new session, every time I

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log into a new session, my login will read .bashrc file,

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this file, and when you do ls -l, and by the way

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should go up, you will see .bashrc is not

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showing up, because it is a hidden file.

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Anytime you need to view a hidden file, you

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have to do ls -la, hit enter.

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And now you could go up, and you

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will see your .bashrc file right here.

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This is the one I modified.

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If you have modified, you could modify this file to

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set your environment and then make sure you log out

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and log back in or start a new session,

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if you want to have your

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environment variable take effect right away.

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And the last one I have right here, I'll

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go ahead and close the session behind it, is

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if I want to set globally environment permanently.

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Meaning if I wanted to set these environment not

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just for myself, I wanted to set it for

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everyone else who logs into that system,

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make sure you know what you're doing first of all.

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So I will not recommend you that you

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do it right now in your lab.

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However, I will explain you the files

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that you would need to modify.

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Those files are at /etc/profile.

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This is the file that you would have to

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modify or you have to modify /etc/bashrc.

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So you can modify these files and you will put in

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exactly the same way you put it in your .bashrc.

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But again, as I said, please do not modify

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this because this is lab environment, I don't want you

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to modify globally, and it will cause problem.

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And then you will know the root cannot

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log in or yourself cannot log in.

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But if you want to do it for your

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production environment or your corporate environment where you're working,

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I would recommend you to do it in development

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first and then you try it in production. All right, good luck.

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