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Learning Linux Shell Scripting

Chapter 1. Getting Started and Working with Shell Scripting

Whoever works with Linux will come across shell as the first program to work with. The Graphical user interface (GUI) usage has become very popular due to the ease of use. Those who want to take advantage of the power of Linux will use the shell program by default.

Shell is a program which provides the user direct interaction with the operating system. Let's understand the stages in the evolution of the Linux operating system. Linux was developed as a free and open source substitute for UNIX OS. The chronology can be as follows:

The UNIX operating system was developed by Ken Thomson and Dennis Ritchie in 1969. It was released in 1970. They rewrote the UNIX using C language in 1972.
In 1991, Linus Torvalds developed the Linux kernel for the free operating system.

In this chapter, we will cover the following topics:

Comparison of shells
Working in shell
Learning basic Linux commands
Our first script—Hello World
Compiler and interpreter—difference in process
When not to use scripts
Various directories
Working more effectively with shell—basic commands
Working with permissions

Comparison of shells

Initially, the UNIX OS used a shell program called Bourne Shell. Then eventually, many more shell programs were developed for different flavors of UNIX. The following is brief information about different shells:

Sh—Bourne Shell
Csh—C Shell
Ksh—Korn Shell
Tcsh—enhanced C Shell
Bash—GNU Bourne Again Shell
Zsh—extension to Bash, Ksh, and Tcsh
Pdksh—extension to KSH

A brief comparison of various shells is presented in the following table:

Feature	Bourne	C	TC	Korn	Bash
Aliases	no	yes	yes	yes	yes
Command-line editing	no	no	yes	yes	yes
Advanced pattern matching	no	no	no	yes	yes
Filename completion	no	yes	yes	yes	yes
Directory stacks (pushd and popd)	no	yes	yes	no	yes
History	no	yes	yes	yes	yes
Functions	yes	no	no	yes	yes
Key binding	no	no	yes	no	yes
Job control	no	yes	yes	yes	yes
Spelling correction	no	no	yes	no	yes
Prompt formatting	no	no	yes	no	yes

What we see here is that, generally, the syntax of all these shells is 95% similar. In this book, we are going to follow Bash shell programming.

Learning basic Linux commands

The following table lists a few basic Linux commands:

Command	Description
`$ ls`	This command is used to check the contents of the directory.
`$ pwd`	This command is used to check the present working directory.
`$ mkdir work`	We will work in a separate directory called `work` in our `home` directory. Use this command to create a new directory called `work` in the current folder.
`$ cd work`	This command will change our working directory to the newly created directory `work`.
`$ pwd`	This command can be used to verify whether we moved to the expected directory.
`$ touch hello.sh`	This command is used to create a new empty file called `hello.sh` in the current folder.
`$ cp hello.sh bye.sh`	This command is used to copy one file into another file. This will copy `hello.sh` as `bye.sh`.
`$ mv bye.sh welcome.sh`	This command is used to rename a file. This will rename `bye.sh` as `welcome.sh`.
`$ ll`	This command will display detailed information about files.
`$ mv welcome.sh .welcome.sh` `$ ls`	Let's see some magic. Rename the file using the `mv` command and the run the `ls` command. Now, the `ls` command will not display our file `.welcome.sh`. That file gets hidden. Any filename or directory name starting with "`.`" (dot) becomes hidden.
`$ ls -a`	This command is used to see hidden files.
`$ rm .welcolme.sh`	This command is used to delete the file.

Note

If we delete any file from GUI such as Graphical User Interface, then it will be moved to the /home/user/.local/share/Trash/files/ all deleted files folder.

Our first script – Hello World

Since we learned basic commands to use Linux OS, we will now write our first Shell script called hello.sh. You can use any editor of your choice such as vi, gedit, nano, and other similar editors. I prefer to use the vi editor.

Create a new hello.sh file as follows:

#!/bin/bash
# This is comment line
echo "Hello World"
ls
date

Save the newly created file.

The #!/bin/bash line is called the shebang line. The combination of the characters # and ! is called the magic number. The shell uses this to call the intended shell such as /bin/bash in this case. This should always be the first line in a Shell script.

The next few lines in the Shell script are self explanatory.

Any line starting with #, will be treated as a comment line. An exception to this would be the first line with #!/bin/bash
The echo command will print Hello World on the screen
The ls command will display directory content on the console
The date command will show the current date and time

We can execute the newly created file by the following commands:

Technique one:
```
$ bash hello.sh
```
Technique two:
```
$ chmod +x hello.sh
```

By running any of the preceding commands, we are adding executable permission to our newly created file. You will learn more about file permissions in later in this same chapter.

$ ./hello.sh

By running the preceding command, we are executing hello.sh as the executable file. By technique one, we passed filename as an argument to Bash shell.

The output of executing hello.sh will be as follows:

Hello World
hello.sh
Sun Jan 18 22:53:06 IST 2015

Since we have successfully executed our first script, we will proceed to develop a more advanced script, hello1.sh. Please create the new script hello.sh as follows:

#!/bin/bash
# This is the first Bash shell
# Scriptname : Hello1.sh
# Written by:  Ganesh Naik
echo "Hello $LOGNAME, Have a nice day !"
echo "Your are working in directory `pwd`."
echo "You are working on a machine called `uname -n`."
echo "List of files in your directory is."
ls      # List files in the present working directory
echo  "Bye for now $LOGNAME. The time is `date +%T`!"

The output of executing hello.sh will be as follows:

Hello student, Have a nice day !.
Your are working in directory /home/student/work.
You are working on a machine called ubuntu.
List of files in your directory is.
hello1.sh  hello.sh
Bye for now student. The time is 22:59:03!

You will learn about the LOGNAME, uname, and other similar commands as we go on with the book.

Working more effectively with shell – basic commands

Let us learn a few commands, which are required very often, such as man, echo, cat and similar:

Enter the following command. It will show the various types of manual pages displayed by the man command:

$ man man

From the following table, you can get an idea about various types of man pages for the same command:

Section number	Subject area
1	User commands
2	System calls
3	Library calls
4	Special files
5	File formats
6	Games
7	Miscellaneous
8	System admin
9	Kernel routines

We can enter the man command to display corresponding manual pages as follows:
```
$ man 1 command
$ man 5 command
```
Suppose we need to know more about the passwd command, which is used for changing the current password of a user, you can type the command as follows:
```
$ man command
  man -k passwd   // show all pages with keyword
  man –K passwd  // will search all manual pages for pattern
$ man passwd
```
This will show information about the passwd command:
```
$ man 5 passwd
```
The preceding command will give information about the file passwd, which is stored in /etc /passwd.

We can get brief information about the command as follows:

$ whatis passwd

Output:

passwd (1ssl)        - compute password hashes
passwd (1)           - change user password
passwd (5)           - the password file

Every command we type in the terminal has an executable binary program file associated with it. We can check the location of a binary file as follows:
```
$ which passwd
/usr/bin/passwd
```
The preceding line tells us that the binary file of the passwd command is located in the /usr/bin/passwd folder.

We can get complete information about the binary file location as well as manual page location of any command by following:

$ whereis passwd

The output will be as follows:

passwd: /usr/bin/passwd /etc/passwd /usr/bin/X11/passwd /usr/share/man/man1/passwd.1.gz /usr/share/man/man1/passwd.1ssl.gz /usr/share/man/man5/passwd.5.gz

Change the user login and effective user name:
```
$ whoami
```
This command displays the user name of the logged in user:
```
$ su
```
The su command (switch user) will make the user as the administrator; but, you should know the administrators, password. The sudo command (superuser do) will run the command with administrator's privileges. It is necessary that the user should have been added in the sudoers list.
```
# who am i
```
This command will show the effective user who is working at that moment.
```
# exit
```
Many a times, you might need to create new commands from existing commands. Sometimes, existing commands have complex options to remember. In such cases, we can create new commands as follows:
```
$ alias ll='ls –l'
$ alias copy='cp –rf'
```
To list all declared aliases, use the following command:
```
$ alias
```
To remove an alias, use the following command:
```
$ unalias copy
```
We can check about the operating system details such as UNIX/Linux or the distribution that is installed by the following command:
```
$ uname
```
Output:
```
Linux
```
This will display the basic OS information (UNIX name)
Linux kernel version information will be displayed by the following:
```
$ uname –r
```
Output:
```
3.13.0-32-generic
```

To get all the information about a Linux machine, use the following command:

$ uname –a

Output:

Linux ubuntu 3.13.0-32-generic #57~precise1-Ubuntu SMP Tue Jul 15 03:50:54 UTC 2014 i686 i686 i386 GNU/Linux

The following commands will give you more information about the distribution of Linux:
```
$ cat /proc/version   // detailed info about distribution
$ cat /etc/*release
# lsb_release -a   // will tell distribution info for Ubuntu
```
The command cat is used for reading files and displayed on the standard output.
Sometimes, we need to copy a file or directory in many places. In such situations, instead of copying the original file or directory again and again, we can create soft links. In Windows, a similar feature is called as creating a shortcut.
```
$ ln  -s  file file_link
```
To learn about the type of file, you can use the command file. In Linux, various types of files exist. Some examples are as follows:
- Regular file (-)
- Directory (d)
- Soft link (l)
- Character device driver (c)
- Block device driver (b)
- Pipe file (p)
- Socket file (s)
We can get information about a file using the following command:
```
$ file   fil_name  // show type of file
```
Printing some text on the screen for showing results to the user or to ask details is an essential activity.
- The following command will create a new file called file_name using the cat command:
```
$ cat > file_name
line 1
line 2
line 3
< Cntrl + D will save the file    >
```
But this is very rarely used, as many powerful editors are already existing, such as vi or gedit.
- The following command will print Hello World on the console. The echo command is very useful for Shell script writers:
```
$ echo "Hello World"
```
- The following command will copy the string Hello World to the hello.c file:
```
$ echo "Hello World" >   hello.c
```
  The command echo with > overwrites the content of the file. If content already exists in the file, it will be deleted and new content will be added in the file. In a situation, when we need to append the text to the file, then we can use the echo command as follows:
```
$ echo  "Hello World" >> hello.c  will append the text
```
- The following command will display the content of the file on screen:
```
$ cat hello.c
```

Working with permissions

The following are the types of permissions:

Read permission: The user can read or check the content of the file
Write permission: The user can edit or modify the file
Execute permission: The user can execute the file

Changing file permissions

The following are the commands for changing the file permissions:

To check the file permission, give the following command:

$ ll file_name

The details of file permissions are as seen in the following image:

In the preceding diagram, as we can see, permissions are grouped in owner-user and group and other users' permissions. Permissions are of three types such as read, write, and execute permissions. As per the requirement, we may need to change permissions of the various files.

Command chmod

We can change the file or directory permissions by the following two ways:

Technique one – the symbolic method

The following command will add the read/write and execute permissions to the file wherein, u is for user, g is for group, and o is for others:

$ chmod ugo+rwx file_name

Alternatively, you can use the following command:

$ chmod +rwx file_name

Technique two – the numeric method

The following command will change the file permissions using the octal technique:

$ chmod +rwx file_name

The file permission 777 can be understood as 111 111 111, which corresponds to the rwx.rwx.rwx permissions.

Setting umask

We will see how Linux decides the default permissions of the newly created file or folder:

$ umask
0002

The meaning of the preceding output is that, if we create a new directory, then from the permissions of +rwx, the permission 0002 will be subtracted. This means that for a newly created directory, the permissions will be 775 or rwx rwx r-x. For a newly created file, the file permissions will be rw- rw- r--. By default, for any newly created text file, the execute bit will never be set. Therefore, the newly created text file and directory will have different permissions even though the umask is same.

Setuid

Another very interesting functionality is the setuid feature. If the setuid bit is set for a script, then the script will always run with the owner's privileges irrespective of which user is running the script. If the administrator wants to run script written by him by other users, then he can set this bit.

Consider either of the following situations:

$ chmod u+s file_name
$ chmod 4777 file

The file permissions after any of the preceding two commands will be drwsrwxrwx.

Setgid

Similar to setuid, the setgid functionality gives the user the ability to run scripts with group owner's privileges, even if it is executed by any other user.

$ chmod g+s filename

Alternatively, you can use the following command:

$ chmod 2777 filename

File permissions after any of the preceding two commands will be drwxrwsrwtx.

Sticky bit

Sticky bit is a very interesting functionality. Let's say, in the administration department there are 10 users. If one folder has been set with sticky bit, then all other users can copy files to that folder. All users can read the files, but only the owner of the respective file can edit or delete the file. Other user can only read but not edit or modify the files if the sticky bit is set.

$ chmod +t filename

Alternatively, you can use the following command:

$ chmod 1777

File permissions after any of the preceding two commands will be drwxrwxrwt.

Key benefits

Learn how to efficiently and effectively build shell scripts and develop advanced applications with this handy book

Develop high quality and efficient solutions by writing professional and real-world scripts, and debug scripts by checking and shell tracing

A step-by-step tutorial to automate routine tasks by developing scripts from a basic level to very advanced functionality

Description

Linux is the one of the most powerful and universally adopted OSes. Shell is a program that gives the user direct interaction with the operating system. Scripts are collections of commands that are stored in a file. The shell can read this file and act on the commands as if they were typed on the keyboard. Shell scripting is used to automate day-to-day administration, and for testing or product development tasks. This book covers Bash, GNU Bourne Again SHell, preparing you to work in the exciting world of Linux shell scripting. We start with an introduction to the Shell environment and explain basic commands used in Shell. Next we move on to check, kill, and control the execution of processes in Linux OS. Further, we teach you about the filter tools available in Linux and explain standard output and standard errors devices. Then we will ensure you understand Shell’s interpretation of commands and get a firmer grasp so you use them in practice. Next, you’ll experience some real-world essentials such as debugging and perform Shell arithmetic fluently. Then you’ll take a step ahead and learn new and advanced topics in Shell scripting, such as starting up a system and customizing a Linux system. Finally, you’ll get to understand the capabilities of scripting and learn about Grep, Stream Editor, and Awk.

What you will learn

Familiarize yourself with the various text filtering tools available in Linux

Combine the fundamental text and file processing commands to process data and automate repetitive tasks

Understand expressions and variables and how to use them practically

Automate decision-making and save a lot of time and effort of revisiting code

Get to grips with advanced functionality such as using traps and signals and using dialogs to develop screens

Start up a system and customize a Linux system

Take an in-depth look at regular expressions and pattern matching to understand the capabilities of scripting

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Andras Pasztirak Feb 02, 2016

One of the better, more comprehensive guides on the topic of shell scripting I've ever seen. It goes through all the necessary basics and dwells into the more advanced topics but in a way that the pacing is great, easy to follow, not jarring. The chapters are built up logically for the most part, the one tiny gripe I'd have is that regular expressions are only brought up in the very last chapter, I feel they should be featured a lot earlier in the book as they are an integral part of scripts and command line work. Still, I don't think that's enough reason to deprive it of 5 stars.I'd highly recommend this ebook for either a complete beginner in shell scripting or someone who has experience in the field but is looking to refresh their memory, perhaps after not touching the shell after a long time.

Amazon Verified review

Laurent Jan 06, 2016

This book is well structured about shell scripting used to manage daily tasks on a Linux computer.Of course it is more for people likes terminal.A very good book to grow up your knowledge of this intimacy with the essential tools of shell.

Andrea Jan 31, 2016

For sure is not the only book about shell scripting (books on this topic have started since '70 years), but it's really good and with a step by step approach.You start learning some common Unix commands (included some Unix/Linux concepts), the job control, the text filtering features.With all this basis you start build some scripts using variables, flows, conditions.Several tricks are included and also several examples.Finally it's included also a part to build text based window interface with dialog.Considering that shell scripting is still used (also if languages like Perl, Ptyton, Ruby can totally replace them), this book is really useful for Linux/Unix system administrator and people that need to learn on how programming with shell script language.

CC Dec 03, 2017

I LOVE this book!!! Ganesh takes the time to explain and give examples on almost everything. Give it a read and you will not regret it. Thank you to Ganesh and all who have contributed to a totally AWESOME book! Please keep writing!!!

adnan baloch Jan 15, 2016

This book is borne out of the author's own experiences over the years solving day to day problems in his work. That should give anyone confidence that the examples in this book are as real-world as they can be. Veterans from the DOS days will fondly remember creating batch files to accomplish a myriad of tasks. If you are one of them and switched to an OS like Unix or Linux, this is the book to get. Everything from file permissions, process management, task scheduling, text processing to interactive shell scripting is covered here. Want to backup a folder in the middle of the night and want its status emailed to you? Check. Have a complex operation in mind involving loops, file handling, arrays, functions and message dialogs? The author arms you with the necessary information to achieve that. The final chapters take the readers through understanding the Linux startup process and customizing the system environment as well as using regular expressions for complex pattern matching. Apart from networking, this book is all you need to handle common Linux system admin duties.

Learning Linux Shell Scripting: Unleash the power of shell scripts to solve real-world problems by breaking through the practice of writing tedious code

What do you get with Print?

Learning Linux Shell Scripting

Chapter 1. Getting Started and Working with Shell Scripting

Comparison of shells

Tasks done by shell

Working in shell

Learning basic Linux commands

Note

Our first script – Hello World

Compiler and interpreter – difference in process

When not to use scripts

Various directories

Working more effectively with shell – basic commands

Working with permissions

Changing file permissions

Command chmod

Technique one – the symbolic method

Technique two – the numeric method

Setting umask

Setuid

Setgid

Sticky bit

Summary

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Technique one – the symbolic method

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