Section 11: The Execution Environment
CONCEPT: The exact behavior of commands issued in the shell
depends upon the execution environment provided by the shell.
The Unix shell maintains a set of environment variables that
are used to provide information, like the current working directory,
and the type of terminal being used, to the programs you run. The
environment variables are passed to all programs that are not built in
to the shell, and may be consulted, or modified, by the program. By
convention, environment variables are given in upper case letters.
To view all the environment variables, use the command
You can also view a particular
environment variable using the echo command:
The above command echos the value of the TERM
environment variable to the standard output.
The creation of the execution environment
When you log in, a sequence of events establishes the execution
environment. The exact sequence of events depends on the particular
flavor of Unix, and also depends upon the default shell for your
account. The following is a description of the login process for the
HP-UX operating system. Other operating systems may differ.
- The getty process
- The getty process provides the login: prompt that you see
on the terminal screen. The getty process reads your username, and
invokes the login program.
- The login program
- The login program receives the username from getty, and prompts
you for your password. Login then consults the system database
/etc/passwd to verify your password. (Note that login will request
your password even if there is no entry in /etc/passwd for the
username you've given. That prevents someone from finding valid
usernames by trial and error.) Login turns off terminal echo so that
your password is not displayed on the screen.
Having verified your password, login then uses information in
/etc/passwd to invoke your default shell. If no default shell is
specified in the /etc/passwd entry, login starts the Bourne shell
- Shell startup: System login scripts
- When the shell program starts, it reads configuration files called
login scripts to configure the execution environment. On
HP-UX, the file /etc/profile provides initialization parameters for
ksh and sh, while the file /etc/csh.login is used for csh. After the
system login scripts are read, the shell looks for user-specified
- Shell startup: User login scripts
- After the system login scripts are read, the shell reads user
login scripts. User login scripts are kept in one's home directory,
and are the means by which one can customize the shell environment.
Sh and ksh look for a file called .profile. Ksh also reads a file
defined in the environment variable ENV. Csh reads a file called
.cshrc, and (if it is the login shell), the file .login.
Important environment variables
Here are descriptions of some of the most important environment
variables, and examples of how some variables can affect the execution
- The TERM environment variable defines the type of terminal that
you are using. Most Unix systems have a database of terminal types,
and the capabilities of each terminal type.
- The PATH variable contains the names of directories to be searched
for programs that correspond to command names. When you issue a
command to the shell, the shell searches sequentially through each
directory in the PATH list until it finds an executable program with
the command name you typed.
- The USER variable contains your username. Any time you access a
file or directory, the access permissions are checked against the
value of USER.
- The HOME variable contains the name of your home directory. When
you issue the cd command with no directory argument, you will be
placed in the directory defined in the HOME environment variable. The
HOME variable is also where the shell will look for the user login
- The MAIL variable contains the name of the directory where your
incoming mail is stored. When you start a mail program, the program
will look in the directory stored in the MAIL environment variable for
your incoming mail messages.
- The EDITOR variable is used by programs that must invoke a text
editor to provide the ability to edit or compose documents. One
example is the elm program, which is used to read and send electronic
mail. If you elect to compose a new mail message while in elm, the
elm program will check the contents of the EDITOR variable to
determine which editor to invoke.
- The HOST environment variable contains the name of the host
machine that is running your shell program. When you connect to a
remote host through telnet or ftp, the name of your host is relayed to
the remote machine, so the administrators of the remote machine can
keep track of who is connecting, and from where.
Setting environment and shell variables
The exact mechanism for setting the environment and shell variables
depends upon the type of shell you're using.
sh, or ksh
To set an environment variable in sh or ksh, use
the syntax VAR=value;export VAR, where VAR is the name of the
environment variable and value is the value you wish to assign. Do
not put spaces on either side of the equals sign. The export command
instructs the shell to propagate the value of the variable to all
programs that are run by the shell. If an environment variable is
reset, but not exported, the change will only apply to the shell
itself. To set the EDITOR variable to the value emacs in ksh or sh,
use the command:
- EDITOR=emacs;export EDITOR
It is also possible to unset environment variables, with the unset
command. Unsetting an environment variable removes the definition of
To set an environment variable in csh, use the setenv
command. The command has the syntax: setenv VARIABLE value. To set
the EDITOR variable to the value emacs in csh, use the command:
- setenv EDITOR emacs
For more information about the shell environment, consult the manual
page for the shell you're using.