In this lesson, we will learn about Bash.
Bash, which stands for Borne Again Shell,
is a Unix shell and command language
used to write scripts.
Bash scripting concepts include both data structures
and control structures.
In Bash, data structures,
such as variables, arrays, and strings,
are essential for organizing
and storing data within the scripts.
By using these data structures,
Bash scripts can handle complex tasks
by processing and organizing multiple elements,
such as a series of commands,
configurations, or input data,
in a structured way.
Next, control structures in Bash,
such as loops like for and while
and conditional statements like if and else,
dictate the flow of script execution.
In this manner,
control structures enable those scripts
to perform repeated actions
and make decisions based on specific conditions.
Let's learn more about Bash data structures
and control structures.
First, we have Bash data structures.
In Bash, variables are a fundamental data structure
used to store single values.
In variable declaration,
Bash does not require the use of a keyword,
like var or let.
Some other scripting languages
do require a keyword like that
when declaring a variable.
In Bash, you simply assign a value
using the = operator.
However, in Bash, no spaces should surround the equal sign.
For example, in the code that appears on the screen,
we can see that the username is the variable,
and it holds the value admin.
Bash variables, like the username variable in this case,
can hold strings or numbers,
but everything in Bash is treated
as a string unless otherwise specified.
A string is a sequence of characters,
such as letters, numbers, and symbols,
used to represent text.
Accessing the value of a variable in Bash
requires a $ before the variable name
as seen in the echo command,
which is used to print out information on the screen.
Next, let's look at a script snippet that uses an array
and prints out an array element.
Arrays in Bash allow you to store multiple values
in a single variable,
and in an array, Bash uses zero-based indexing
to access array elements.
This means that the first element in an array
is in the 0 position,
and the second element is in the 1 position.
Furthermore, arrays are created
by assigning multiple values
within quotations separated by spaces,
as you can see in the script example.
In this example,
the servers array contains three values.
In the second part of the script,
echo is used to print out the First server
and then a colon to the screen.
Next, the ${servers[0]} retrieve the first value
or element of the array,
which is in the 0 position.
In this case, that is server1.
Overall, arrays are useful
when you need to store a list of items,
such as a list of server names,
and loop through them to perform operations.
Finally, Bash can be used to store and access strings.
Strings are simple data types in Bash,
but they are powerful
when combined with other data structures.
For examples, strings can be concatenated using variables
or direct text,
and special operators,
like the hash sign or the percent sign,
are used for string manipulation.
For example, the following Bash script snippet
allows you to find the length of a string.
This script snippet assigns the value Hello, world!
to the variable greeting.
Then it uses the echo command
to print the length of the string greeting.
In Bash, the ${#}
is used to identify the string
whose length should be counted.
So this command would print out
The length of the greeting is 13.
Second, we have Bash control structures.
Control structures dictate the flow of execution
by enabling decision making and repeated actions.
Conditional statements, such as if, elif, and else,
are used to evaluate conditions
and execute different blocks of code,
depending upon whether the conditions are met.
In this code snippet,
Bash uses the if keyword to open a conditional block
and the fi or fi to close the block.
As you can see in this example,
the if conditional statement checks
whether the variable user is equal to admin.
The evaluated condition is enclosed in square brackets.
If the condition is true,
it executes the code after then
to output to the screen "Welcome, admin!"
Otherwise, it runs the else block,
printing out to the screen "Access denied."
The if statement ends with a fi,
signaling the end of the if block.
Next, loops are another important control structure in Bash,
allowing you to execute commands repeatedly.
The for loop is often used to iterate
over lists or arrays,
making it a powerful tool
for automating repetitive tasks.
Take a look at the code example on the screen.
In this example,
the for loop iterates over the servers array,
and for each element in the array,
denoted by the square brackets with the @ sign within them,
it executes the echo command,
printing out to the screen "Connecting to server1",
"Connecting to server2", et cetera.
In this snippet,
the do and done keywords mark the beginning
and end of the loop block.
Next, in Bash,
a while loop continues to execute a block of code
as long as a specified condition remains true.
Similar to the if statement,
the condition is enclosed in square brackets,
and the block is marked with do and done.
Take a look at the code snippet on the screen.
In this code snippet,
the while loop runs
as long as the variable count is less than or equal to 5.
Each iteration that runs prints out the value of count
and increments the count variable
by one using the ((count++)) syntax.
Finally, case statements in Bash offer an alternative
to if statements
when there are multiple conditions to evaluate.
This is particularly useful
when you need to match a variable
against several possible values.
Let's review the following script snippet
that appears on the screen.
In this example,
the script evaluates the value of the variable Option
and executes a corresponding block of code.
The double semicolon at the end of each case block
is used to separate each case,
and the esac, which is case written backward,
marks the end of the entire case block.
So by combining control structures,
like if, for, while, and case,
Bash scripts can handle complex decision-making
and task automation,
making them highly flexible
for system management and security tasks.
So remember, Bash is a Unix shell
and command language designed
to automate system tasks,
including system management and security operations.
It uses data structures,
like variables, arrays and strings,
to store and organize data efficiently within scripts,
allowing for complex operations
by managing and processing multiple elements.
Control structures, such as loops
and conditional statements,
guide the flow of execution,
enabling scripts to perform repeated actions
or make decisions based on specific conditions.
These structures are key to automating repetitive tasks,
improving efficiency in system operations.
Overall, Bash scripting provides flexibility
for managing Unix-based systems through automation.