<code>alias clearfile=”cat /dev/null &gt;”</code>

Shell variables a classified in 2 groups:

Variables can be read/write or read-only. Name of a variable can be any sequence of letters and numbers, but it must not start with a number.

Shell environment variables shape the working environment whenever you are logged in. Common shell variables include:

The <code>set</code> command will display all available variables

* <code>PATH</code> is an environment variable present in Unix/Linux operating systems, listing directories where executable programs are located

* Use <code>./</code> prefix or modify the <code>PATH</code> as needed

Syntax: <code>name=value</code>

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* Including the keyword <code>readonly</code> before the command assignment prevents you from changing the variable afterwards. For example: <code>readonly phone=“123-4567”</code>

* If no variable name is supplied a list of defined read only variables will be displayed

<code>variable=</code>

Read-only variables cannot be removed, you must log out for them to be cleared.

Whenever you wish to read a variable (its contents), use the variable name preceded by a dollar sign ($). This is commonly called ''variable substitution''.

* <code>$#</code> represents the number of parameters (not including the script name)

* Displays messages to the terminal followed by a newline. Use the <code>-n</code> option to suppress the default newline.

* Arguments are usually double quoted.

* The <code>read</code> command allows obtaining user input and storing it in a variable. Everything is captured until the Enter key is pressed.

echo Hello $name

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The purpose of the if statement is execute a command or commands based on a condition. The condition is evaluated by a test command, represented below by a pair of square brackets

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The <code>test</code> command can be used in two ways:

Check <code>man test</code> for more details

A for loop is a very effective way to repeat the same command(s) for several arguments such as file names. The syntax and example of for command is shown below: