Compunet Corporation Introduction to Unix (CA263) Your Environment By Tariq Ibn Aziz Dammam Community College.

Compunet Corporation

Introduction to Unix (CA263)

Your Environment

By

Tariq Ibn AzizDammam Community College


Objectives

• In this lecture you will learn – Local variable

– Subshells

– Exported Variables

– PS1 and PS2

– Your PATH


Objectives

• In this lecture you will learn – Your Current Directory

– More on Subshells

– Your .profile File

– The TERM Variable


Local Variables[1]

• The vartest consists of echo command that display value of x, surrounded by colons

$ cat vartestecho :$x:$ x=100$vartest::$• vartest doesn’t know about the value of x.

Therefore its value is null.


Local Variables[2]

$ cat vartest2x=50echo :$x:$ x=100$vartest2:50:$• Now question is what is the value of x?$ echo $x100$


Subshells

• The behavior exhibited by vartest and vartest2 is due to the fact that these two program are run as subshells by your login shell.

• When you ask your login shell to execute the vartest, it startup a new shell to execute the program.

• Whenever new shell runs it runs in its own environment with its own set of local variables.

• A subshells has no knowledge of local variables that were assigned values by the login shell.

• A subshell cannot change the value of a variable in the parent shell.


Exported Variables

• There is a way to make the value of a variable known to a subshell, and that’s by exporting it with the export command.

• Here is a program vartest3$ cat vartest3echo x = $xecho y = $y$

$ x=100$ y=10$ vartest3x =y =$ export y$ vartest3x =y = 10$


Exported Variables

• Change the value of x and y in vartest4.$ cat vartest4x=50y=5$vartest4$echo $x $y100 10$


Exported Variables

$ cat vartest4x=50y=5z=1export zVartest5$$cat vartest5echo x = $xecho y = $yecho z = $z$

$ vartest4x=y=10z=1$• When vartest4 get

executed, the exported variable y will be copied into the subshell’s environment.

• vartest4 set the value of x, y and z. The it export z.

• This makes the value of z accessible to any subshell.


Exported Variables Example

$ cat vartest4x=50y=5z=1export y zvartest5$$cat vartest5echo x = $xecho y = $yecho z = $z$

• If you change the vartest4 to explicitly export the value of y, then changed value of y will get exported down to vartest5.

$ vartest5x=y=5z=1$


export with No argument

• If you just type export, not followed by any argument, you will get a list of the exported variables that are exported by your shell.

export LOGNAMEexport PATHexport TIMEOUTexport TZexport y


PS1 and PS2

• The characters that the shell display as your command prompt are stored in the variable PS1. You can change this variable to be anything you like.

• $ echo :$PS1::$ :$ PS1="==>“==>


PS1 and PS2

• Your secondary command prompt, normally > is kept in the variable PS2, where you can change it as you like.

$ echo :$PS2::> :$ PS2="=====>”$

$ for x in 1 2 3=====> do=====> echo $x=====> done123$


HOME variable

• Your home directory is where you are placed whenever you log onto the system.

• A special variable HOME is also automatically set to this directory when you log on.

$ echo $HOME/usr/steve$ HOME=/usr/steve/book$ pwd/usr/steve$ cd$ pwd/usr/steve/book


Your PATH

• Whenever you type in the name of a program to be executed, the shell searches a list of directories until it find the program.

• The list of directories is contained in a special variable called PATH$ echo $PATH/bin:/usr/bin:

• You can always override the path variable by specifying the path to the file to be executed.


Example PATH

• The shell will go directly to /bin to execute date.$ /bin/date

Or./rolo

• You can execute rolo without specifying path if the directory of rolo is specified in PATH variable.


Your Current Directory

$ cat cdtestcd /usr/steve/binpwd$ pwd/usr/steve$ cdtestusr/steve/bin

$ pwd/usr/steve• Why we are in /usr/steve where as we changed the directory in cdtest program?


CDPATH Variable

• The CDPATH variable works like the PATH variable.

• It specifies a list of directories to be searched by the shell whenever you execute the cd command.

• This search is done only if the specified directory is not given by a full path name and if CDPATH is not null.


CDPATH Variable Example

• If you type cd /usr/steve then the shell changes your directory directly to /usr/steve

• But if you type cd memos, then shell will look at your CDPATH variable to find the memo directoy. And if the CDPATH looks like this$ echo $CDPATH:/usr/steve:/usr/steve/docs$ cd /usr/steve$ cd memos/usr/steve/docs/memos$ cd bin/usr/steve/bin


More on Subshells

$ cat varsBOOK=/usr/steve/bookUUPUB=/usr/spool/uucppublicDOCS=/usr/steve/docs/memosDB=/usr2/data$$ vars$ echo $BOOK

$


The . Command

• There is a shell built-in command called . (dot)

. file• Whose purpose is to execute the content of file in

the current shell.

$ . vars$ echo $BOOK/usr/steve/BOOK$


The .profile Command

• Your login shell executes two special files on the system.

• The first is /etc/profile. This file is set up by the two system administrator and usually does things like checking to see if you have mail.

• The second file that get automatically executed is .profile in your home directory. Your system administrator may have given you a default .profile file when you got your account.


The .profile Command

$ cat $HOME/.profilePATH="/bin:/usr/bin:/usr/lbin::"export PATH$


The TERM Variable

• If you tend to use more than one type of terminal, then the .profile is a good place to put some code to prompt for the terminal type and then set the TERM variable accordingly.

• A sample code from .profile file on next slide.


The TERM Variable

echo "what terminal are you using(hp-2621 is default ?\c"

read TERMif [ -z "$TERM" ]then

TERM=2621fiexport TERM

Compunet Corporation Introduction to Unix (CA263) Your Environment By Tariq Ibn Aziz Dammam Community College.

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Compunet Corporation Introduction to Unix (CA263) Your Environment By Tariq Ibn Aziz Dammam Community College.