Linux Dummies 9th

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Part IV: Junior Administrator Boot Camp
Take a look at a different kind of string that the shell interprets differently: an
interpolated string. An interpolated value is one in which the shell interprets
special characters before processing the value. Rather than using single
quotes, this time you use the same example with double quotes:
echo “Hello, my name is $USER”
Notice what the output is this time:
Hello, my name is rich
Instead of displaying the exact text you provided, the shell replaces the variable
name, designated with a dollar sign, with the actual value stored in that
variable.
Why did you use single quotes in the first example but double quotes with the
second one? The items with the backslashes (\) are interpreted one way or
another. However, if you use double quotes, they’re interpreted only once, so
the item that lists what directory you’re in changes only the first time. If you
use single quotes, the variables are interpreted every time you do something.
If you’re going to play around with environment variables, start by using
the methods we discuss in this section. After you’re comfortable with any
changes you have made, you can make your changes permanent by opening
the ~/.bashrc file and adding the same text there. The next time you log in,
the changes go into effect. You can make changes for all your users’ profiles
in /etc/profile as well.
If you experiment heavily with these files, create a separate user account so
that you can do whatever you want without messing up your own login. This
advice especially goes for /etc/profile. You can damage everyone’s logins
with this one! To create a separate /etc/profile, you can make a backup
by typing cp /etc/profile /etc/profile.original. Then edit /etc/
profile all you like, knowing that you can always delete it with the rm command
and use the mv command to rename /etc/profile.original to
/etc/profile.
To create an environment variable from scratch, you typically name it with
all capital letters and then you have to export it — like this, for example:
CUSTOMVAR=”new variable”
export CUSTOMVAR
Don’t be too discouraged if you don’t understand all this variable stuff right
now. As you become more proficient with Linux, you should explore shell
scripting. Shell scripting is the art of creating computer programs with just
the shell. Most Linux and UNIX administrators speak shell-script language as
easily as you and I speak our native tongues.