Learning Linux Shell Scripting: Leverage the power of shell scripts to solve real-world problems , Second Edition

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

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

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.

Whenever we type any text in the shell Terminal, it is the responsibility of the shell (/bin/bash) to execute the command properly. The activities done by the shell are as follows:

We will discuss the preceding topics in the subsequent chapters.

Let's get started by opening the Terminal, and we will familiarize ourselves with the bash shell environment:

$ echo $SHELL/bin/bash

$ bash -versionGNU bash, version 4.3.48(1)-release (x86_64-pc-linux-gnu)Copyright (C) 2013 Free Software Foundation, Inc.License GPLv3+: GNU GPL version 3 or later http://gnu.org/licenses/gpl.html

This is free software; you are free to change and redistribute it.There is NO WARRANTY, to the extent permitted by law.

Hereafter, we will use the word Shell to signify the Bash shell only. If we intend to use any other shell, then it will be specifically mentioned by name, such as KORN and other similar shells.

In Linux, filenames in lowercase and uppercase are different; for example, the files Hello and hello are two distinct files. This is unlike Windows, where case does not matter.

As far as possible, avoid using spaces in filenames or directory names such as:

This will make certain utilities or commands fail or not work as expected, for example, the make utility.

While typing in filenames or directory names of the existing files or folders, use the tab completion feature of Linux. This will make working with Linux faster.

The following table lists a few basic Linux commands:

Since we have learned basic commands in the 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, emacs, geany, and other similar editors. I prefer to use the vi editor:

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

The #!/bin/bash line is called the shebang line. The combination of the characters # and ! is called the magic sequence. 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:

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

$ bash hello.sh

$ chmod +x hello.sh

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

$ ./hello.sh

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

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

Hello Worldhello.shSun 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 hello.sh script 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 "You are working in directory `pwd`."
    echo "You are working on a machine called `uname -o`."
    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 GNU/Linux.List of files in your directory is :hello1.sh  hello.shBye for now student. The time is 22:59:03!

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

In any program development, the following are the two options:

When we use a compiler-based language, we compile the complete source code and, as a result of compilation, we get a binary executable file. We then execute the binary to check the performance of our program.

On the other hand, when we develop the shell script, such as an interpreter-based program, every line of the program is input to the Bash shell. The lines of shell script are executed one by one sequentially. Even if the second line of a script has an error, the first line will be executed by the shell interpreter.

Shell scripts have certain advantages over compiler-based programs, such as C or C++ language. However, shell scripting has certain limitations as well.

The following are the advantages:

The following are the limitations of shell scripts:

We cannot use shell scripts in the following situations:

We will explore the directory structure in Linux so that it will be useful later on:

Let's learn a few commands that are required very often, such as man, echo, cat, and similar:

$ man man

$ man 1 command$ man 5 command

$ man command  man -k passwd   // show all pages with keyword  man -K passwd  // will search all manual pages content for pattern "passwd"$ man passwd

$ man 5 passwd

$ whatis passwd

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

$ which passwd/usr/bin/passwd

$ whereis passwd

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

$ whoami

$ su

# who am i

# exit

$ alias ll='ls -l'$ alias copy='cp -rf'

$ alias

$ unalias copy

$ uname

Linux

This will display the basic OS information (Unix name)

$ uname -r

   3.13.0-32-generic
  

$ uname -a

Linux localhost.localdomain 3.10.0-693.el7.x86_64 #1 SMP Tue Aug 22 21:09:27 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

$ cat /proc/version   // detailed info about distribution$ cat /etc/*release# lsb_release -a.

$ ln -s file file_link

$ file file_name

    $ cat > file_name
    line 1
    line 2
    line 3
    < Cntrl + D will save the file    >
  

$ echo "Hello World"

$ echo "Hello World" > hello.sh

$ echo "Hello World" >> hello.sh  will append the text

    $ cat hello.sh
  

The following are the types of permissions:

Changing file permissions

The following are the commands for changing file permissions:

To check the file permission, enter the following command:

$ ll file_name

The file permission details are as seen in the following diagram:

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

$ chmod ugo+rwx file_name

$ chmod +rwx file_name

$ chmod 777 file_name

$ umask0002

$ chmod u+s file_name$ chmod 4777 file

$ chmod g+s filename

$ chmod 2777 filename

$ chmod +t filename

$ chmod 1777

In this chapter, you learned different ways to write and run shell scripts. You also learned ways to handle files and directories, as well as work with permissions.

In the next chapter, you will learn about process management, job control, and automation.