Unix, Unix-like, or Linux-based OS provide a lot of powerful features to work upon. Among them, the most powerful and important feature is executing a wide range of commands to perform a task quickly and easily; for example, ls, cat, sort, grep, and so on. We will come to know about a subset of commands and usages throughout this book. In order to run a command, we need an interface that is widely known as shell.

Shell is a program that acts as an interface between the users (we) and the OS kernel (Linux, Unix, and so on). Understanding in terms of Windows OS, shell serves a similar purpose DOS does. Different shells are available for Unix, Unix-like, or Linux OS. Some of the popular shells are Bourne shell (sh), C shell (csh), Korn shell (ksh), Bourne Again shell (bash), and Z shell (zsh).

In this book, we will be using Linux OS and Bourne Again shell, popularly known by its acronym bash. Linux-based systems generally have bash already installed. In case bash is not installed, try installing the bash package from your distribution's package manager. In order to know which shell currently your Linux console is using, run the following command in terminal:

$ ps -p $$

The output is as follows:

  PID TTY          TIME CMD
12578 pts/4    00:00:00 bash

In the preceding ouput, we see that the CMD column has value bash. This means, we are currently using bash shell in our current console.

If your console is not using the bash shell, then you can run the following command:

$ bash

Also, your shell will be bash now. To make bash as a default login shell, run the following command:

$ chsh -s /bin/bash

The output obtained is as follows:

Changing shell for user.
Password:******
Shell changed.

We are now set with bash shell and ready to learn shell scripting in detail. Shell scripts are nothing but plain text files with a series of commands that are run by bash in a specified order. Writing shell scripts is very useful when you have to perform a series of tasks by running various commands, as bash will read each line from a script file and run it without any need of user intervention. The general file extension used for shell scripts are .sh, .bash, .zsh, .ksh, and so on. Rather than using a file extension for shell scripts, it's preferred to keep a filename without extension and let an interpreter identify the type by looking into shebang (#!). Shebang is used in scripts to indicate an interpreter for execution. It is written in the first line of a script file, for example:

#! /bin/bash

It means use the bash shell to execute a given script. To run a shell script, make sure it has execute permission. To provide execute permission to an owner of a file, run the following command:

$ chmod u+x foo

Here, foo is the shell script file. After running this command, foo will have execute permission for the owner of the file.

Now, we are ready to proceed further on learning shell scripting concepts in detail. Each topic and subtopic covered in the chapters with examples will lead us progressively towards a good shell script programmer.

In this chapter, we will talk broadly about the following topics:

Hello World in shell

Whenever we learn a new programming language, we first learn how to write the Hello World program in it. It is the best way to know and interact with a new language. This also helps in confirming that the basic environment for a program in a given language has been set up and you are good to dive deep inside this language.

Interacting with shell

We can print the output of commands in console in an interactive way. Console is also known as a standard input and output stream. To print anything in a bash console, use the echo command followed by what is to be printed:

$ echo Hello World
Hello World

Alternatively, put the text to be printed in double quotes:

$  echo "Hello World"
Hello World

You can also put the text to be printed in single quotes:

$ echo 'Hello World'
Hello World

We can also use the printf command in shell programming for printing. The printf command also supports formatted printing, similar to what we have in C programming language— the printf( ) function:

$ printf "Hello World"
Hello World$

Here, after the output, we see the command prompt ($) because printf doesn't add a default newline after execution while echo does. So, we have to explicitly add the newline ( ) in the printf statement to add a newline:

$ printf "Hello World
"
Hello World

Similar to the C printf( ), we can specify formatted printing in bash. The syntax of bash printf is as follows:

printf FORMAT [ARGUMENTS]

FORMAT is a string that describes the format specifications and is specified within double quotes. ARGUMENTS can be the value or a variable corresponding to format specification. Format specification consists of the percentage (%) sign followed by format specifier. Format specifiers are explained in the following table:

Format specification	Description
%u	This prints an unsigned integer value
%i or %d	This prints an associated argument as a signed number
%f	This prints an associated argument as a floating point number
%o	This prints an unsigned octal value
%s	This prints a string value
%X	This prints an unsigned hexadecimal value (0 to 9 and A to F)
%x	This prints an unsigned hexadecimal value (0 to 9 and a to f)

The following examples demonstrate how to use format specification for printing different data type format in shell:

$ printf "%d mul %f = %f
" 6 6.0 36.0
6 mul 6.000000 = 36.000000
$ printf "%s Scripting
" Shell
Shell Scripting

We can also optionally specify a modifier in format specification to align an output to provide better formatting to the output. Format modifiers are placed between % and the format specifier character. The following table explains format modifiers:

Format Modifiers	Description
N	This is any number that specifies a minimum field width.
.	This is used together with field width. The field doesn't expand when the text is longer.
-	This is the left-bound text printing in the field.
0	This is used to fill padding with zeros (0) instead of whitespaces. By default, padding is done with whitespaces.

The following example demonstrates how to use format modifiers to improve printing formatting:

$ printf "%d mul %.2f = %.2f
" 6 6.0 36.0
6 mul 6.00 = 36.00

Let's make it scripted

Interactive printing is good if we have to print one or two lines, but for a lot of printing, it's good and preferred to write a script file. A script file will contain all the instructions and we can run a script file to perform the needed task.

Now, we are going to create a bash script file that makes use of the echo and printf commands and print messages:

#!/bin/bash
#Filename: print.sh
#Description: print and echo

echo "Basic mathematics"
printf "%-7d %-7s %-7.2f =	%-7.2f
" 23 plus 5.5 28.5
printf "%-7.2f %-7s %-7d =	%-7.2f
" 50.50 minus 20 30.50 
printf "%-7d %-7s %-7d =	%-7d
" 10 mul 5 50
printf "%-7d %-7s %-7d =	%-7.2f
" 27 div 4 6.75

The first line in bash script represents the path of the interpreter used. The second line is a comment line telling the filename of a script file. In shell script, we use # to add a comment. Furthermore, the echo command will print strings written within double quotes. For the rest, we have used printf to print formatted output.

To run this script, we will first provide execute permission to a user/owner of this script:

$ chmod u+x print.sh

Then, run the script file in console as follows:

$ ./print.sh

The result after running this script will look as follows: