Bourne tutorial

executes the command. Because the variable is "passed" to the command by the environment, the shell

simply sets the standard output to the appropriate file, and then processes the line for variables, and lastly

executing the command on the line. More on this later.

The set command

If you with to examine the values of all of your current variables, use the command "set:"

$ set

DISPLAY=:0.0

HOME=/home/barnett

IFS=

LD_LIBRARY_PATH=/usr/openwin/lib:/usr/openwin/lib/server

LOGNAME=barnett

MAILCHECK=600

OPENWINHOME=/usr/openwin

PATH=/home/barnett/bin:/usr/ucb:/usr/bin

PS1=$

PS2=>

PWD=/home/barnett

SHELL=/bin/csh

TERM=vt100

USER=barnett

$

Notice the alphabetical order of the variables, and the equals character between the variable and the value.

The "set" command is one way to determine which shell you are currently using. (The C shell puts spaces

between the variable and the value.) Also note the assortment of variables already defined. These are

environment variables.

Environment Variables

UNIX provides a mechanism to pass information to all processes created by a parent process by using

environment variables. When you log onto a system, you are given a small number of variables, predefined.

You can add to this list in your shell start-up files. Every program you execute will inherit these variables. But

the information flow is one-way. New UNIX users find this confusing, and cannot understand why a shell

script can't change their current directory. Picture it this way: suppose you executed hundreds of programs,

and they all wanted to change their environment to a different value. It should be obvious that they can't all

control the same variable. Imagine hundreds of programs trying to change the directory you are currently

using! Perhaps these variables ought to be called hereditary, and not environmental. Children processes

inherit these values form the parents, but can never change how the parents were created. That would require

time-travel, a feature not currentable available in commercial UNIX systems.

As I mentioned before, the shell command "export" is used to update environment variables. The command

export a b c

Bourne Shell Tutorial

http://www.grymoire.com/Unix/Sh.html

15 of 66

11/21/2011 12:03 PM

---

marks the variables "a" "b" and "c," and all child processes will inherit the current value of the variable. With

no arguments, it lists those variables so marked. The command "export" is necessary. Changing the value of

an environment variable does not mean this change will be inherited. Example:

HOME=/

myprogram

When "myprogram" executes, the value of the "HOME" variable is not changed. However, in this example:

HOME=/

export HOME

myprogram

the program does get the modified value. Another way to test this is to start a new copy of the shell, and

execute the "export" command. No variables are reported.

The command marks the variable. It does not copy the current value into a special location in memory. A

variable can be marked for export before it is changed. That is,

export HOME

HOME=/

myprogram

works fine.

Special Environment Variables

There are several special variables the shell uses, and there are special variables the system defined for each

user. SunOS and Solaris systems use different environment variables. If in doubt, check the manual pages. I'll

describe some the important Solaris variables.

PATH - Sets searchpath

The "PATH" environment variable lists directories that contain commands. When you type an arbitrary

command, the directories listed are searched in the order specified. The colon is used to separate directory

names. An empty string corresponds to the current directory. Therefore the searchpath

:/usr/bin:/usr/ucb

contains three directories, with the current directory being searched first. This is dangerous, as someone can

create a program called "ls" and if you change your current directory to the one that contains this program,

you will execute this trojan horse. If you must include the current directory, place it last in the searchpath.

/usr/bin:/usr/ucb:

HOME - Your home directory

The "HOME" variable defines where the "cd" goes when it is executed without any arguments. The HOME

Bourne Shell Tutorial

http://www.grymoire.com/Unix/Sh.html

16 of 66

11/21/2011 12:03 PM

---

evvironment variable is set by the login process.

CDPATH - cd searchpath

When you execute the "cd" command, and specify a directory, the shell searches for that directory inside the

current working directory. You can add additional directories to this list. If the shell can't find the directory in

the current directory, it will look in the list of directories inside this variable. Adding the home directory, and

the directory above the current directory is useful:

CDPATH=$HOME:..

export CDPATH

IFS - Internal Field Seperator

The "IFS" variable lists the characters used to terminate a word. I discussed this briefly earlier. Normally,

whitespace separates words, and this variable contains a space, a tab and a new line. Hackers find this

variable interesting, because it can be used to break into computer systems. A poorly written program may

carelessly execute "/bin/ps." A hacker may redefine the PATH variable, and define IFS to be "/." When the

program executes "/bin/ps," the shell will treat this as "bin ps." In other words, the program "bin" is executed

with "ps" as an argument. If the hacker has placed a program called "bin" in the searchpath, then the hacker

gains privileged access.

PS1 - Normal Prompt

The "PS1" variable specifies the prompt printed before each command. It is normally "$ ." The current

directory cannot be placed inside this prompt. Well, some people make a joke, and tell a new user to place a

period inside this variable. A "." does signifies the current directory, however, most users prefer the actual

name.

PS2 - Secondary Prompt

The "PS2" environment variable defines the secondary prompt, This is the prompt you see when you execute

a multi-line command, such as "for" or "if." You also see it when you forget to terminate a quote. The default

value is "> ."

MAIL - Incoming mail

The "MAIL" variable specifies where your mailbox is located. It is set by the login process.

MAILCHECK - How often to check for mail

The "MAILCHECK" variable specifies how often to check for mail, in seconds. The default value is 600

seconds (10 minutes). If you set it to zero, every time the shell types a prompt, it will check for mail.

Bourne Shell Tutorial

http://www.grymoire.com/Unix/Sh.html

17 of 66

11/21/2011 12:03 PM

---