Variables are named with the usual naming constructs. When an application is executing, it will be passed a set of variables called environment variables. To view all the environment variables related to a terminal, issue the env command. For every process, environment variables in its runtime can be viewed by:

cat /proc/$PID/environ

Set PID with a process ID of the process (PID always takes an integer value).

For example, assume that an application called gedit is running. We can obtain the process ID of gedit with the pgrep command as follows:

$ pgrep gedit
12501

You can obtain the environment variables associated with the process by executing the following command:

$ cat /proc/12501/environ
GDM_KEYBOARD_LAYOUT=usGNOME_KEYRING_PID=1560USER=slynuxHOME=/home/slynux

The aforementioned command returns a list of environment variables and their values. Each variable is represented as a name=value pair and are separated by a null character (\0). If you can substitute the \0 character with \n, you can reformat the output to show each variable=value pair in each line. Substitution can be made using the tr command as follows:

$ cat /proc/12501/environ  | tr '\0' '\n'

Now, let us see how to assign and manipulate variables and environment variables.

A variable can be assigned as follows:

var=value

var is the name of a variable and value is the value to be assigned. If value does not contain any space character (such as space), it need not be enclosed in quotes, Otherwise it is to be enclosed in single or double quotes.

Note that var = value and var=value are different. It is a usual mistake to write var =value instead of var=value. The later one is the assignment operation, whereas the earlier one is an equality operation.

Printing contents of a variable is done using by prefixing $ with the variable name as follows:

var="value" #Assignment of value to variable var.

echo $var

Or:

echo ${var}

We will receive an output as follows:

value

We can use variable values inside printf or echo in double quotes:

#!/bin/bash
#Filename :variables.sh
fruit=apple
count=5
echo "We have $count ${fruit}(s)"

The output will be as follows:

We have 5 apple(s)

Environment variables are variables that are not defined in the current process, but are received from the parent processes. For example, HTTP_PROXY is an environment variable. This variable defines which proxy server should be used for an Internet connection.

Usually, it is set as:

HTTP_PROXY=192.168.1.23:3128
export HTTP_PROXY

The export command is used to set the env variable. Now any application, executed from the current shell script, will receive this variable. We can export custom variables for our own purposes in an application or shell script that is executed. There are many standard environment variables that are available for the shell by default.

For example, PATH. A typical PATH variable will contain:

$ echo $PATH
/home/slynux/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games

When given a command for execution, the shell automatically searches for the executable in the list of directories in the PATH environment variable (directory paths are delimited by the ":" character). Usually, $PATH is defined in /etc/environment or /etc/profile or ~/.bashrc. When we need to add a new path to the PATH environment, we use:

export PATH="$PATH:/home/user/bin"

Or, alternately, we can use:

$ PATH="$PATH:/home/user/bin"
$ export PATH

$ echo $PATH
/home/slynux/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/home/user/bin

Here we have added /home/user/bin to PATH.

Some of the well-known environment variables are HOME, PWD, USER, UID, SHELL, and so on.

$ echo '$var' will print $var

Let us see more tips associated with standard and environment variables.

length=${#var}

$ var=12345678901234567890$
echo ${#var}
20

echo $SHELL

echo $0

$ echo $SHELL
/bin/bash

$ echo $0
/bin/bash

If [ $UID -ne 0 ]; then
    echo Non root user. Please run as root.
else
    echo Root user
fi