ULI101: INTRODUCTION TO UNIX / LINUX AND THE INTERNET

WEEK 10 LESSON 1

INTRODUCTION TO SHELL SCRIPTING /CREATING SHELL SCRIPTSSHELL VARIABLES

PHOTOS AND ICONS USED IN THIS SLIDE SHOW ARE LICENSED UNDER CC BY-SA

LESSON 1 TOPICS

Shell Scripts• Purpose

• Considerations When Creating Shell Scripts

• Creating Shell Scripts

Shell Variables

• Purpose

• Environment Variables

• User Defined Variables

Perform Week 10 Tutorial• Investigation 1

• Review Questions (Questions xx - xx)

SHELL SCRIPTING

Definition

A shell script is a computer program designed to be run by the Unix shell, a command-line interpreter. The various dialects of shell scripts are considered to be scripting languages.

Typical operations performed by shell scripts include file manipulation, program execution, and printing text.

Reference: https://en.wikipedia.org/wiki/Shell_script

SHELL SCRIPTING

Considerations When Creating Shell Scripts

The reason to create shell scripts is to automate the execution of commonly issued Linux commands and tasks.

Prior to simply placing Linux commands in a file to run, you should plan-out the purpose of the shell script and list the steps that you want to accomplish. This will help to lists the sequence of steps required to ensure that your shell script is successful

SHELL SCRIPTING

Creating a Shell Script

Once you have planned your shell script by listing the sequence of steps in your script, you need to create a file that will contain your Linux commands.

NOTE: Avoid using filenames of already existing Linux Commands to avoid confusion. Using shell script filenames that include the file extension of the shell that the script will run within is recommended.

Examples:

clean-directory.bashcopy-directory-structure.sh

SHELL SCRIPTING

Creating a Shell Script

The Shebang Line

If you are learning Bash scripting by reading other people’s code you might have noticed that the first line in the scripts starts with the #! characters and the path to the Bash interpreter.

This sequence of characters (#!) is called shebang and is used to tell the operating system which interpreter to use to parse the rest of the file.

Reference: https://linuxize.com/post/bash-shebang/

SHELL SCRIPTING

Creating a Shell Script

Examples of Shebang Line#!/bin/bash#!/bin/csh

You can use the which command to determine the full pathname of the shell. For example: which bash

The shebang line must appear on the first line and at the beginning of the shell script, otherwise, it will be treated as a regular comment and ignored.

SHELL SCRIPTING

Creating a Shell Script

Displaying Text: The echo Statement

The echo command is used to display text.

Example:

echo This is some text

SHELL SCRIPTING

Creating a Shell Script

Prompting User Input: The read Statement

The read command pauses and waits for a user to enter data and then stores into a variable when the user presses the ENTER key.

Example:

read –p “Enter your name: ” ageecho “Your age is $age”

Note: Placing a dollar sign ($) in front of variable name expands the value stored in the variable.

SHELL SCRIPTING

Creating a Shell Script

Setting Script Permissions & Running Script

To run your shell script by name, you need to assign execute permissions for the user. To run the shell script, you canexecute it using a relative, absolute, or relative-to-home pathname

Example:

chmod u+x myscript.bash./myscript.bash/home/username/myscript.bash~/myscript.bash

You can run a shell script that does not have permissions by issuing the shell command with the shell script as an argument.

Example:

bash ./myscript.bash

You can add the directory that contains the shell script so it can be issued by just name and not pathname by adding the directory pathname into the PATH environment variable

Example:

PATH=$PATH:directory-pathname

SHELL SCRIPTING

Creating Shell ScriptsInstructor Demonstration

Task1:

Create a Bash Shell script to clear the screen and then display all users that are logged onto the system.

Task2:

Create a Bash Shell script to clear the screen, prompt the user for their full name, and then display the text message:

Hello, your name is (your full name)

SHELL SCRIPTING

Using Variables

Variables are used to store information to be referenced and manipulated in a computer program. They also provide a way of labeling data with a descriptive name, so our programs can be understood more clearly by the reader and ourselves.

It is helpful to think of variables as containers that hold information. Their sole purpose is to label and store data in memory. This data can then be used throughout your program.

Reference: https://launchschool.com/books/ruby/read/variables

SHELL SCRIPTING

Using Variables

Shell variables are classified in 2 groups:

System (shell) variables: describing the working environment

User-created variables: associated with scripts

Variables can be read/write or read-only

The name of a variable can be any sequence of letters and numbers, but it must NOT start with a number

SHELL SCRIPTING

Environment Variables

Shell environment variables shape the working environment whenever you are logged in Common shell. Some of these variables are displayed in the table below (you can issue the pipeline command set | more to view all variables)

Variable Name Purpose

PS1 Primary shell prompt

PWD Absolute path of present working directory

HOME Absolute path to user's home

PATH List of directories where commands / programs are located

HOST Host name of the computer

USER Name of the user logged in

SHELL Name (type) of current shell used

SHELL SCRIPTING

Environment Variables

Placing a dollar sign ($) prior to the variable name will cause the variable to expand to the value contained in the variable.

Examples:echo “My current location is: $PWD”who | grep $USERecho $HOST

SHELL SCRIPTING

The PATH Environment Variable

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

Multiple entries are separated by a colon (:)

The shell searches these directories whenever a command is invoked in the sequence listed. In case of multiple matches use the which utility to determine which match has a precedence

On some systems the present working directory may not be included in the PATH by default

To run shell scripts the user can issue using the ./ prefix or the user can modify the PATH variable if they want to issue the shell script without the ./ prefix.

SHELL SCRIPTING

User Defined (Created) Variables

User-defined variables are variables which can be created by the user and exist in the session.

This means that no one can access user-defined variables that have been set by another user, and when the session is closed these variables expire.

Reference: https://mariadb.com/kb/en/user-defined-variables/

SHELL SCRIPTING

User Defined Variables

Assigning a Value to a User Defined Variable:

name=value

If a variable’s value contain spaces or tabs, they should be surrounded by quotes

Example:

phone="1 800 123-4567"

SHELL SCRIPTING

User Defined Variables

Removing a Value from a User Defined Variable:

name=

OR

unset name

Examples:

customerName=unset userAge

SHELL SCRIPTING

User Defined (Created) Variables

Including the keyword readonly before the command assignment prevents you from changing the variable afterwards

Examples:

name=“Murray Saul”readonly namereadonly phone="123-4567"

If no variable name is supplied a list of defined read only variableswill be displayed.

NOTE: Read-only variables cannot be removed – you must log out for them to be cleared

SHELL SCRIPTING

Getting Practice

To get practice to help perform assignment #3, perform Week 10 Tutorial:

• INVESTIGATION 1: CREATING A SHELL SCRIPT

• INVESTIGATION 2: USING VARIABLES IN SHELL SCRIPTS

• LINUX PRACTICE QUESTIONS (Part A 1 – 3 , Part B Walk-Thru #1)

ULI101: INTRODUCTION TO UNIX / LINUX AND THE INTERNET

WEEK 10: LESSON 2

POSITIONAL PARAMETERSTEST STATEMENT / CONTROL FLOW STATEMENTS (LOGIC / LOOPS)

PHOTOS AND ICONS USED IN THIS SLIDE SHOW ARE LICENSED UNDER CC BY-SA

LESSON 2 TOPICS

Positional Parameters• Purpose

• Usage

Control Flow Statements

• Purpose

• Exit Status $? / Using the test statement

• Control Flow Statements (Conditionals, Loops)

Perform Week 10 Tutorial• Investigation 2

• Review Questions (Questions xx – xx)

POSITIONAL PARAMETERS

A positional parameter is a variable within a shell program; its value is set from an argument specified on the command line that invokes the program.

Positional parameters are numbered and are referred to with a preceding ``$'': $1, $2, $3, and so on.

Reference: http://osr600doc.xinuos.com/en/SDK_tools/_Positional_Parameters.html

arg1 arg2 arg3 … argN

POSITIONAL PARAMETERS

Assigning Values as Positional Parameters

There are a few ways to assign values as positional parameters:

• Use the set command with the values as argument after the set command

• Run a shell script containing arguments

arg1 arg2 arg3 … argN

POSITIONAL PARAMETERS

Using the set command:

set apples oranges bananas

You place a dollar sign ($) prior to the number corresponding to the position of the argument

Examples:

echo $1echo $2echo $3

arg1 arg2 arg3 … argN

POSITIONAL PARAMETERS

Running a Shell Script with Arguments:

You would use positional parameters in your shell script that would expand the positional parament with its stored value.

Here are the contents of the shell script called myScript.bash:

#!/bin/bash

echo “First argument is $1”echo “Second argument is $2”

You would then issue the myScript.bash shell script with arguments that would be used within the shell script. For Example:

./mySript.bash My-First-Argument My-Second-Argument

arg1 arg2 arg3 … argN

POSITIONAL PARAMETERS

Command line parameters are referred to as $0…$9

The positional parameter $0 refers to either the name of shell where command was issued, or name of shell script being executed.

If using positional parameters greater than 9, then you need to include number within braces.

Examples:

echo ${10}ls ${23}

arg1 arg2 arg3 … argN

POSITIONAL PARAMETERS

The shift command can be used with positional parameters to shift positional parameters to the left by one or more positions.

Examples:

shiftshift 2

arg1 arg2 arg3 … argN

SPECIAL PARAMETERS

There are a group of special parameters that can be used for shell scripting. A few of these special parameters and their purpose are displayed in the table below.

$* $# $?

Parameter Purpose$* Display all positional parameters.

“$*” Containing values of all arguments separated by a single space

“$@” Multiple double-quoted strings, each containing the value of one argument

$# Represents the number of parameters (not including the script name)

$? Exit Status of previous command (discussed in next lesson)

POSITIONAL AND SPECIAL PARAMETERS

Positional and Special ParametersInstructor Demonstration

CONTROL FLOW STATEMENTS

So far, we have created Bash Shell Scripts that executed Linux commands in a fixed sequence.

Although those type of scripts can be useful, we can usecontrol flow statements that will control the sequence ofthe running script based on various situations or conditions.

Control Flow Statement are used to make your shell scripts more flexible and can adapt to changing situations.

CONTROL FLOW STATEMENTS

The “$?” Special Parameter

The special parameter $? Is used to determine the exit status of the previously issued Linux command.

The exit status will either display a zero (representing TRUE) or a non-zero number (representing FALSE). This can be used to determined if a Linux command was correctly or incorrectly executed.

Examples:

PWDecho $?pwdecho $?

CONTROL FLOW STATEMENTS

The test Linux Command

The test Linux command is used to test conditions to see if they are TRUE (i.e. value zero) or FALSE (i.e. value non-zero) so they can be used with control flow statements to control the sequence of a shell script.

Examples:

name=“Murray”test $name = “Murray”echo $?test $name = “David”echo $?

CONTROL FLOW STATEMENTS

The test Linux Command – Numerical Comparison

You CANNOT use the > or < symbols when using the test command since these are redirection symbols. Instead, you need to use options when performing numerical comparisons.

Refer to the table below for test options and their purposes.

Option Purpose-eq Equal to

-ne Not equal to

-lt , -le Less than, Less than or equal to

-gt, -ge Greater than, greater than or equal to

CONTROL FLOW STATEMENTS

The test Linux Command – Additional Options

There are other comparison options that can be used with the test command such as testing to see if a regular file or directory pathname exists, or if the regular file pathname is –non-empty.

Below are some of those options.

Option Purpose-f file_pathname Regular filename exists

-d file_pathname Directory filename exists

-s file_pathname Regular filename is non-empty

-w file_pathname file exists / write permission is granted

CONTROL FLOW STATEMENTS

Logic Statements

A logic statement is used to determine which Linux commands to be executed based on the result of a condition (i.e. TRUE (zero value) or FALSE (non-zero value)).

There are several logic statements, but we will just concentrate on the if statement.

if test condition then

command(s) fi

CONTROL FLOW STATEMENTS

Logic Statements

If the test condition returns a TRUE value, then the Linux Commands between then and fi statements are executed.

If the test returns a FALSE value, then the if statement is bypassed.

Example:

num1=5num2=10if test $num1 –lt $num2then

echo “Less Than” fi

CONTROL FLOW STATEMENTS

Logic Statements

The set of square brackets [ ] can be used to represent the test command.

NOTE: There must be spaces between the square brackets and the test condition.

Example:

num1=5num2=10if [ $num1 –lt $num2 ]then

echo “Less Than” fi

CONTROL FLOW STATEMENTS

Loop Statements

A loop statement is a series of steps or sequence of statements executedrepeatedly zero or more times satisfying the given condition is satisfied.

Reference: https://www.chegg.com/homework-help/definitions/loop-statement-3

CONTROL FLOW STATEMENTS

There are several loops, but we will look at the for loop using a list.

for item in list

do

command(s)

done

Variable "item" will hold one item from the list every time the loop iterates (repeats).

A list can consist of a series of arguments (separated by spaces) or supplied by a command

CONTROL FLOW STATEMENTS

Example:

for x in apples oranges bananas

do

echo “The item is: $x”

done

CONTROL FLOW STATEMENTS

Instructor Demonstration

Your instructor will demonstrate examples of using control flow statements.

REGULAR EXPRESSIONS

Getting Practice

To get practice to help perform assignment #2, perform Week 10 Tutorial:

• INVESTIGATION 3: USING CONTROL FLOW STATEMENTS IN SHELL SCRIPTS

• LINUX PRACTICE QUESTIONS (Part A 4 , Part B Walk-Thru #2)