# UNIX / Linux Environment - Unix/Linux

## What are UNIX / Linux Environment?

In this division, we will discuss in detail about the UNIX environment. A main UNIX model is the environment, which is defined by environment variables. Some are set by the system, others by you, yet others by the shell, or any program that loads another program.

A variable is a character string to which we allocate a value. The value assigned could be a number, text, filename, device, or any other kind of data.

For instance, first we set a variable TEST and then we contact its value using the echo command –

It produces the following result.

Note that the environment variables are set without using the $sign but while access them we use the$ sign as prefix. These variables retain their values until we come out of the shell.

When you log in to the structure, the shell undergoes a phase called initialization to set up the environment. This is generally a two-step process that involves the shell understanding the following files −

• /etc/profile
• profile

The development is as follows −

• The shell checks to see whether the file /etc/profile exists.
• If it exists, the shell reads it. Otherwise, this file is skipped. No error message is displayed.
• The shell checks to see whether the file .profile exists in your home directory. Your home directory is the directory that you start out in after you log in.
• If it exists, the shell reads it; otherwise, the shell skips it. No error message is displayed.

As soon as both of these files have been read, the shell displays a prompt –

This is the prompt where you can enter commands in order to have them execute.

Note − the shell initialization process complete here applies to all Bourne type shells, but some extra files are used by bash and kasha.

### The .profile File

The file /etc/profile is maintain by the scheme administrator of your UNIX device and contains shell initialization information required by all users on a system.

The file .profile is under your control. You can add as much shell customization information as you want to this file. The minimum set of information that you need to configure includes −

• The type of terminal you are using.
• A list of directories in which to locate the commands.
• A list of variables affecting the look and feel of your terminal.

You can check your .profile accessible in your home directory. Open it using the VI editor and check all the variables set for your environment.

### Setting the Terminal Type

Usually, the kind of terminal you are using is automatically configured by either the login or Getty programs. Sometimes, the auto arrangement process guesses your terminal wrongly.

If your workstation is set wrongly, the output of the commands might look strange, or you might not be able to interact with the shell correctly.
To make sure that this is not the case, most users set their fatal to the lowest common denominator in the following way –

## Setting the PATH

When you form any command on the command prompt, the shell has to locate the command before it can be executed.

The PATH variable specifies the location in which the shell should look for commands. Usually the Path variable is set as follows –

Here, each of the individual entry separated by the colon character (:) are directories. If you request the shell to execute a command and it cannot find it in any of the directories given in the PATH variable, a message parallel to the following appears –

There are variables like PS1 and PS2 which are discuss in the next section.

### PS1 and PS2 Variables

The characters that the shell displays as your command prompt are stored in the variable PS1. You can modify this variable to be anything you want. As soon as you alter it, it'll be used by the shell from that point on.

For instance, if you issued the command –

Your prompt will become =>. To set the value of PS1 so that it shows the working directory, issue the command –

The result of this command is that the prompt display the user's username, the equipment name (hostname), and the effective directory.

There are fairly a few escape sequences that can be used as value influence for PS1; try to limit yourself to the most critical so that the prompt does not overpower you with information.

S.No. Escape Sequence & Description
1
\t
Current time, expressed as HH:MM:SS
2
\d
Current date, expressed as Weekday Month Date
3
\n
Newline
4
\s
Current shell environment
5
\W
Working directory
6
\w
Full path of the working directory
7
\u
8
\h
Hostname of the current machine
9
\#
Command number of the current command. Increases when a new command is entered
10
\$If the effective UID is 0 (that is, if you are logged in as root), end the prompt with the # character; otherwise, use the$ sign

You can construct the modify yourself every time you log in, or you can have the change made automatically in PS1 by adding it to your .profile file.

When you issue a charge that is incomplete, the shell will show a secondary prompt and wait for you to complete the command and hit Enter once more.

The default secondary prompt is > (the greater than sign), but can be transformed by re-defining the PS2 shell variable −

Following is the instance which uses the defaulting secondary prompt –

The instance given below re-defines PS2 with a modified prompt –

### Environment Variables

Following is the partial list of important environment variables. These variables are located and accessed as mention below −

S.No. Variable & Description
1
DISPLAY
Contains the identifier for the display thatX11programs should use by default.
2
HOME
Indicates the home directory of the current user: the default argument for the cdbuilt-incommand.
3
IFS
Indicates theInternal Field Separatorthat is used by the parser for word splitting after expansion.
4
LANG
LANG expands to the default system locale; LC_ALL can be used to override this. For example, if its value ispt_BR, then the language is set to (Brazilian) Portuguese and the locale to Brazil.
5
LD_LIBRARY_PATH
A Unix system with a dynamic linker, contains a colonseparated list of directories that the dynamic linker should search for shared objects when building a process image after exec, before searching in any other directories.
6
PATH
Indicates the search path for commands. It is a colon-separated list of directories in which the shell looks for commands.
7
PWD
Indicates the current working directory as set by the cd command.
8
RANDOM
Generates a random integer between 0 and 32,767 each time it is referenced.
9
SHLVL
Increments by one each time an instance of bash is started. This variable is useful for determining whether the built-in exit command ends the current session.
10
TERM
Refers to the display type.
11
TZ
Refers to Time zone. It can take values like GMT, AST, etc.
12
UID
Expands to the numeric user ID of the current user, initialized at the shell startup.

Following is the sample instance presentation few environment variables –