Linux Shell Scripting part 1

Shell is a command line interpreter that executes commands read from the keyboard or from a file. It is not part of system kernel, but uses the system kernel to execute programs.

To find all available shells in your system type following command:

$ cat /etc/shells

To find your current shell type following command

$ echo $SHELL

Shell Scripting is series of command written in a plain text file and executing this text file without entering the commands one by one. Any text editor can be used for shell scripting.

Writing a simple shell script “first”

#
# My first shell script
#
clear
echo "My first shell script"

Explanation

1. # is used to represent a comment, which can be used to further explain what is happening in the script.

2. clear is clear command which clears the screen

3. echo is used to print on console

Setting execute permission for the script “first”

Syntax:
chmod permission script-name

Example:
$ chmod 755 first

Running the script “first”

$ ./first

Vriables in shell

1. System variables

Created and maintained by linux itself. Defined in capital letters.

Example: $HOME, $SHELL, $USERNAME

Using system variables

echo $HOME

2. User defined variables

Created and maintained by the user. Defined in simple letters.

Syntax: variable name=value

Example: name=abc

using user defined variables

name=abc

echo $name – this will print abc

echo name – this will print name

To access the user defined variables it is essential to write them followed by the $ sign.

Rules for Naming variable name

1. Variable name must begin with a letter or a number or an underscore, followed by one or more letters or numbers. Never use ?, * etc, for naming variable names.

2. When assigning value to variable never put spaces on either side of the equal sign
age=10 correct
age =10 incorrect
age= 10 incorrect
age = 10 incorrect

3. Variables are case-sensitive

Shell Arithmetic

Use to perform arithmetic operations.

Syntax:
expr operand1 operator operand2

Always put a space between the operand and the operator.

Examples:
expr 1 + 3
expr 2 - 1
expr 10 / 2
expr 20 % 3
expr 10 \* 3 multiplication use \* and not * since its wild card.
echo `expr 6 + 3` should be within back quotes(find back quote under ~) or it won’t print 9. If one uses single or double quotes it will print a + b instead of 9.