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:

• 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:

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:

• Reading text and parsing the entered command
• Evaluating meta-characters, such as wildcards, special characters,
  or history characters
• Process io-redirection, pipes, and background processing
• Signal handling
• Initializing programs for execution

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:

1. Open the Linux Terminal and type in:

    $ echo $SHELL
    /bin/bash
  

2. The preceding output in the Terminal says that the current shell is /bin/bash, such as the Bash shell:

    $ bash -version
    GNU 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:

• Wrong filename—Hello World.txt
• Correct filename—Hello_World.txt or HelloWorld.txt

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:

1. Create a new hello.sh file as follows:

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

2. Save the newly created file.

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:

• 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 in the console
• The date command will show the current date and time

We can execute the newly created file with 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 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 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 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.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 through the book.

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

• Compilation: Using a compiler-based language, such as C, C++, Java, and other similar languages
• Interpreter: Using interpreter-based languages, such as Bash shell scripting.

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:

• Scripts are easy to write
• Scripts are quick to start and easy for debugging
• They save time in development
• Tasks of administration are automated
• No additional setup or tools are required for developing or testing
  shell scripts

The following are the limitations of shell scripts:

• Every line in shell script creates a new process in the operating system. When we execute the compiled program, such as a C program, it runs as a single process for the complete program.
• Since every command creates a new process, shell scripts are slow compared to compiled programs.
• Shell scripts are not suitable if heavy math operations are involved.
• There are problems with cross-platform portability.

We cannot use shell scripts in the following situations:

• Where extensive file operations are required
• Where we need data structures, such as linked lists or trees
• Where we need to generate or manipulate graphics or GUIs
• Where we need direct access to system hardware
• Where we need a port or socket I/O
• Where we need to use libraries or interface with legacy code
• Where proprietary, closed source applications are used (shell scripts put the source code right out in the open for the entire world to see)

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

• /bin/: This contains commands used by a regular user.
• /boot/: The files required for the operating system startup are stored here.
• /cdrom/: When a CD-ROM is mounted, the CD-ROM files are accessible here.
• /dev/: The device driver files are stored in this folder. These device driver files will point to hardware-related programs running in the kernel.
• /etc/: This folder contains configuration files and startup scripts.
• /home/: This folder contains a home folder of all users, except the administrator.
• /lib/: The library files are stored in this folder.
• /media/: External media, such as a USB pen drive, are mounted in this folder.
• /opt/: The optional packages are installed in this folder.
• /proc/: This contains files that give information about the kernel and every process running in the OS.
• /root/: This is the administrator's home folder.
• /sbin/: This contains commands used by the administrator or root user.
• /usr/: This contains secondary programs, libraries, and documentation about user-related programs.
• /var/: This contains variable data, such as HTTP, TFTP, logs, and others.
• /sys/: This dynamically creates the sys files.

Let's learn a few commands that 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:

• We can enter the man command to display the 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 content for pattern "passwd"
    $ 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 the /etc/ folder, such as /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 the manual page location of any command, with the 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 username:

    $ whoami

• This command displays the username of the logged in user:

    $ su

• The su (switch user) command will make the user the administrator but you should know the administrator's password. The sudo (superuser do) command will run the command with administrator privileges. The user should have been added to 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 operating system details, such as UNIX/Linux or the distribution that is installed with 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 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
    
  

• The following commands will give you more information about the Linux distribution:

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

• The cat command is used for reading files and is displayed on the standard output.

• Sometimes, we need to copy a file or directory to many places. In such situations, instead of copying the original file or directory again and again, we can create soft links. In Windows, it is a similar feature to 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 file 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 file_name

• Printing some text on screen for showing results to the user, or to ask for 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 already exist, 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"
  

    $ echo "Hello World" > hello.sh

• The echo command with > overwrites the content of the file. If the content already exists in the file, it will be deleted and new content added. In situations where we need to append the text to the file, then we can use the echo command as follows:

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

• The following command will copy the Hello World string to the hello.sh file:
• The following command will display the content of the file on screen:

    $ cat hello.sh
  

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

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.

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

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
  

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

    $ chmod 777 file_name

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

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 the directory will have different permissions, even though umask is the same.

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 a 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.

Similar to setuid, the setgid functionality gives the user the ability to run scripts with a 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.

The 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 users 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.

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.