Shell Scripting Basics: Linux Command Examples

Nicholas Idoko 24 min read

Last Updated on March 9, 2024

Introduction to Shell Scripting

Let’s explore shell scripting Linux command examples.

Shell scripting is a powerful tool used in Linux for automating tasks and managing systems efficiently.

Shell scripting allows users to write commands that can be executed by the shell.

It is important because it saves time and effort by automating repetitive tasks.

The Linux command line interface provides a text-based environment for interacting with the operating system.

With the command line, users can execute commands and navigate the file system.

Shell scripting utilizes this interface to create scripts that can perform complex operations.

Scripts automate tasks like system administration, file management, and software installations.

Simple commands, when combined, can accomplish complex tasks easily.

Shell scripting also enables users to customize their environment and create shortcuts for frequently used commands.

Whether you are a system administrator or a regular user, learning shell scripting can greatly enhance your productivity.

It allows you to take full advantage of the Linux command line interface and automate repetitive tasks.

In the next sections, we will explore different aspects of shell scripting and provide examples to get you started.

By the end, you will have a solid foundation in shell scripting and be able to create your own scripts.

Stay tuned for the next section, where we will dive into the basics of writing shell scripts.

Basic Linux Commands

In the world of Linux, mastering shell scripting is essential for both beginners and experienced users.

With the power of the command line, you can automate tasks and perform complex operations quickly and efficiently.

This section focuses on the fundamental Linux commands that are crucial for shell scripting.

1. ls: Listing files and directories

The “ls” command allows you to view the contents of a directory, including files and subdirectories.

2. pwd: Displaying the current directory

“Pwd” stands for “print working directory” and helps you determine your current location within the file system.

3. cd: Changing directories

“cd” is used to navigate between different directories. It allows you to move to a specific directory or go back to the previous one.

4. mkdir: Creating directories

The “mkdir” command enables you to create new directories or folders. You can specify the name and location of the directory to be created.

5. touch: Creating files

“Touch” is used to create new files with a specified name and extension. It can also update the access and modification timestamps of existing files.

6. rm: Removing files and directories

With “rm,” you can delete files and directories permanently. It requires caution, as deleted files cannot be easily recovered.

7. cp: Copying files and directories

The “cp” command allows you to make a copy of files or directories. It requires specifying the source and destination locations.

8. mv: Moving or renaming files and directories

“Mv” is used to either move files and directories to a different location or rename them within the same directory.

Learning these basic Linux commands lays the foundation for efficient shell scripting.

Let’s explore them in more detail:

ls: The “ls” command provides various options like listing files in long format, sorting by size or time, and showing hidden files.
pwd: You can also use “pwd -P” to display the physical path without symbolic links.
cd: Moving to a specific directory requires using the absolute path or relative path.
mkdir: The “mkdir” command supports creating multiple directories simultaneously.
touch: Additionally, “touch” is often used to create empty files in batch operations.
rm: By including the “-r” flag, “rm” can remove directories and their contents recursively.
cp: When copying directories, the “-r” flag preserves their structure and files.
mv: “Mv” is commonly used for renaming files, but it can also relocate them between directories.

These commands serve as the building blocks for more complex shell scripts.

They allow for automating file and folder management tasks, facilitating efficient workflows.

By combining these commands with conditional statements, loops, and variables, you can create powerful scripts to accomplish various tasks with minimal effort.

Understanding and mastering the basics of Linux command line operations is crucial for effective shell scripting.

The commands mentioned above – ls, pwd, cd, mkdir, touch, rm, cp, and mv – form the backbone of Linux scripting.

Harnessing their power opens up a world of possibilities for automating tasks and increasing productivity on Linux systems.

Read: Creating a Simple ‘Hello World’ App with Java

Variables and Data Types in Shell Scripting

Declare variables in shell scripting with an equal sign following the variable name.

Assign text to string variables using single or double quotes.

Store numbers in numeric variables without specifying a data type.

Access system-related information through predefined environment variables in all shell scripts.

Variables hold values for use across the script, automating repetitive tasks and boosting efficiency.

To declare a variable, assign a value using an equal sign (=). For example, use name="John".

You can store text in string variables using single or double quotes, like name="John" or name='John'. Choose quotes based on the need to include special characters or variables.

Numeric variables store numbers. Shell scripts don’t need explicit data type declaration. Assign numeric values without quotes, for example, age=25.

Environment variables

Environment variables hold system and current shell session information. Shell scripts can access and modify them. Examples include PATH, HOME, and USER.

To access a variable’s value, use $ before the variable name. For instance, echo $name prints the value of name.

You can combine variables with command substitution to capture command outputs. For example, file_count=$(ls | wc -l) captures the current directory’s file count into file_count.

Shell scripting doesn’t enforce strong typing, so variables don’t need predefined data types. This flexibility supports dynamic data handling.

In conclusion, variables in shell scripting are essential for storing and manipulating values. They can be of string or numeric type, and environment variables hold system-related information. Understanding how to use variables is crucial for writing effective and efficient shell scripts.

Read: Writing Your First ‘Hello World’ Program in Python

Basic Shell Scripting Syntax

In this section, we will explore the basics of shell scripting in Linux with practical command examples.

Shell scripting is a powerful tool for automating tasks in Linux. Understanding the fundamental syntax is crucial for writing effective scripts.

Shell scripts are written in plain text using specific syntax rules. Let’s cover the essential components:

Shebang line

The shebang line is the first line of a script and starts with “#!”. It specifies the interpreter to run the script.

Comments

Comments provide explanatory notes to make scripts more readable. They start with “#” and are ignored during script execution.

Executing scripts

Before executing scripts, make sure you have execution permissions. Use the “./” prefix to run scripts located in the current directory.

Variables in scripts

Variables allow storing and manipulating data within scripts. They are declared using the “=” operator without spaces around it.

For example, let’s create a simple script that greets the user:

#!/bin/bash
# This script greets the user
name="John"
echo "Hello, $name!"

In this example, we set the variable “name” to “John” and use it in the echo command. When the script is executed, it displays “Hello, John!”

Understanding the basic syntax of shell scripting is a foundation to create more complex and functional scripts.

Stay tuned for more examples and advanced techniques!

Shell scripting is a powerful tool for automating tasks in Linux. Understanding the fundamental syntax is crucial for writing effective scripts.

Read: Automate Your Workflow with Python Scripts

Conditional Statements

Conditional statements are essential in shell scripting for executing commands based on certain conditions.

The if statement is used to perform a specific action if a condition is true.

For example, if a file exists, print “File exists” using the if statement.

The if-else statement allows us to execute different commands based on the condition’s result.

We can print “File exists” if the file exists, else print “File does not exist.”

Nested if statements are used when we have multiple conditions and need to execute commands accordingly.

For example, if a file exists and is writable, print “File exists and is writable.”

The case statement is used when we want to select a command based on a specific value.

We can use a case statement to perform different actions based on the value of a variable.

In shell scripting, these conditional statements help in decision-making and enable us to write dynamic scripts.

The if statement allows us to execute commands selectively, based on true or false conditions.

With the if-else statement, we can execute different commands based on whether a condition is true or false.

nested if statements

Nested if statements provide more complex decision-making capabilities by evaluating multiple conditions.

By nesting multiple if statements, we can handle various scenarios and execute commands accordingly.

case statement

The case statement simplifies the process of selecting commands based on specific values.

It eliminates the need for multiple if-else statements and provides a more efficient approach.

Using conditional statements in shell scripting enhances the flexibility and versatility of scripts.

They allow us to create dynamic scripts that can handle different situations and adapt to changing conditions.

By utilizing conditional statements effectively, we can automate tasks and improve overall script functionality.

These statements provide control flow in scripts, enabling us to make decisions based on specific conditions.

Moreover, they help us handle errors, validate user inputs, and optimize the execution of commands.

Conditional statements play a crucial role in shell scripting.

They allow us to create powerful and flexible scripts that can perform actions based on various conditions.

Whether it’s using the if statement, if-else statement, nested if statements, or case statement, understanding these concepts is vital.

Mastering conditional statements in shell scripting opens up endless possibilities for automating tasks and enhancing productivity.

Loops in Shell Scripting

Loops are an essential part of shell scripting as they allow us to repeatedly execute a certain section of code.

Different types of loops can be used in shell scripting, such as the for loop, while loop, until loop, and nested loops.

The for loop is used when we have a specific range or list of items that we want to iterate over.

It follows the structure of “for variable in list do … done”. The variable takes the value of each item in the list and the code inside the loop is executed.

For example, let’s say we want to print the numbers from 1 to 5 using a for loop:

for num in 1 2 3 4 5
do
echo $num
done

The while loop is used when we want to execute a block of code as long as a certain condition is true.

It follows the structure of “while condition do … done”. The code inside the loop is executed as long as the condition remains true.

For instance, let’s use a while loop to print the numbers from 1 to 5:

num=1
while [ $num -le 5 ]
do
echo $num
num=$((num+1))
done

The until loop is used when we want to execute a block of code until a certain condition becomes true.

It follows the structure of “until condition do … done”. The code inside the loop is executed until the condition becomes true.

Let’s print the numbers from 1 to 5 using an until loop:

num=1
until [ $num -gt 5 ]
do
echo $num
num=$((num+1))
done

Nested loops are loops within loops. They are used when we need to perform more complex iterations. We can have any type of loop inside another loop.

For example, let’s use a nested for loop to print the multiplication table from 1 to 5:

for i in 1 2 3 4 5
do
for j in 1 2 3 4 5
do
res=$((i*j))
echo "$i x $j = $res"
done
done

Loops are powerful tools in shell scripting that help us automate repetitive tasks.

The for loop, while loop, until loop, and nested loops provide different ways to iterate through lists or execute code based on conditions.

By utilizing these loops, we can make our scripts more efficient and effective.

Read: How Much RAM Do You Need for Coding? A Detailed Guide

Shell Scripting Basics: Linux Command Examples

Command Line Arguments

Command line arguments in shell scripts allow us to pass inputs to scripts while executing them.

This feature is essential for building interactive and flexible scripts in the Linux command line.

To access the command line arguments, we can use the special variables provided by the shell.

These variables are accessible within the script and hold the values provided during execution.

One such variable is “$0,” which represents the name of the script itself. It can be useful when we need to refer to the script within its own code.

The “$1” variable holds the value of the first command line argument passed to the script.

If additional arguments are provided, they can be accessed using “$2”, “$3”, and so on.

We can use these variables to create scripts that perform different actions based on the input provided.

For example, let’s say we have a script called “greet.sh” that greets a user by their name.

The script can be executed as follows:

$ ./greet.sh John

Inside the “greet.sh” script, we can access the command line argument using “$1”.

We can then use this value to personalize the greeting message.

#!/bin/bash

name="$1"
echo "Hello, $name! How are you today?"

When we run the script with the argument “John,” it will output:

Hello, John! How are you today?

This simple example demonstrates how command line arguments can enhance the functionality of shell scripts.

In addition to accessing specific arguments, we can also use special variables like “$@” to represent all the command line arguments as a list.

By using “$@”, we can iterate over each argument and perform operations accordingly. Let’s modify our “greet.sh” script to handle multiple names.

#!/bin/bash

for name in "$@"; do
echo "Hello, $name! How are you today?"
done

Now, executing the script with multiple arguments will greet each person individually:

$ ./greet.sh Alice Bob Charlie

Output:

Hello, Alice! How are you today?
Hello, Bob! How are you today?
Hello, Charlie! How are you today?

Using command line arguments allows us to create dynamic and powerful shell scripts.

We can prompt users for inputs, perform different actions based on the arguments, and automate repetitive tasks.

When working with command line arguments, it’s important to validate and sanitize the inputs, especially if they are used for critical operations or system commands.

Understanding how to access and utilize command line arguments in shell scripts is an essential skill for any Linux user or system administrator.

They provide versatility and interactivity, making scripts more user-friendly and powerful.

Read: Why Dual Monitors Can Boost Your Coding Efficiency

File Operations in Shell Scripting

In shell scripting, file operations are essential for manipulating data on Linux systems. Here are the basic operations:

Reading data from files: We can use the “cat” command to display the contents of a file on the terminal.

Writing data to files: The “echo” command allows us to write data into a file. For example, “echo ‘Hello World’ > file.txt” writes “Hello World” into file.txt.

Appending data to files: In shell scripting, we can use the “>>” operator to append data to an existing file. For instance, “echo ‘This is a new line’ >> file.txt” appends the text at the end of file.txt.

Deleting files: The “rm” command is used to remove files. Be careful while using this command, as it permanently deletes files.

To delete a single file, we can run “rm file.txt” where file.txt is the file we want to remove.

We can use wildcards like “*” to delete multiple files to represent multiple characters. For example, “rm *.txt” deletes all files with the .txt extension in the current directory.

In shell scripting, these file operations are frequently used for tasks such as data processing, log analysis, and automation.

Lists of Shell Scripting File Operations:

1. Reading data from files

Use the “cat” command to display the contents of a file.
Example: “cat file.txt” will show the contents of file.txt on the terminal.

2. Writing data to files

Use the “echo” command to write data into a file.
Example: “echo ‘Hello World’ > file.txt” writes “Hello World” into file.txt.

3. Appending data to files

Use the “>>” operator to append data to an existing file.
Example: “echo ‘This is a new line’ >> file.txt” appends the text at the end of file.txt.

4. Deleting files

Use the “rm” command to delete files.
To delete a single file, run “rm file.txt” where file.txt is the file to remove.
To delete multiple files, use wildcards like “*” to represent multiple characters.
Example: “rm *.txt” deletes all files with the .txt extension in the current directory.

These file operations provide flexibility and efficiency in managing data and files within a shell script.

By utilizing these commands, shell scripters can automate tasks, extract information, and manipulate data effectively.

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Advanced Shell Scripting Concepts

Shell scripting is a powerful tool that allows users to automate tasks in Linux using simple commands.

In this section, we will explore advanced concepts in shell scripting, including functions, input and output redirection, process control, and error handling.

Functions

Functions in shell scripting are similar to functions in programming languages.

They allow you to group a set of commands together and execute them as a single unit. This makes your scripts more modular and easier to maintain.

You can define custom functions using the syntax:

function_name() {
# commands
}

To call a function, you simply use its name followed by parentheses.

Input and Output Redirection

Input and output redirection in shell scripting allows you to control where the input for a command comes from and where the output goes.

The `>` operator is used to redirect the output of a command to a file. For example, `ls > files.txt` will write the output of the `ls` command to a file called `files.txt`.

The `<` operator is used to redirect input from a file to a command. For example, `sort < numbers.txt` will read the input for the `sort` command from a file called `numbers.txt`.

Process Control

Process control in shell scripting allows you to manage and control the execution of multiple processes.

This can be useful when you want to run multiple commands concurrently or sequentially.

The `&` operator allows you to run a command in the background, freeing up your terminal. For example, `command &` will run the command in the background.

The `wait` command is used to wait for the completion of all background jobs. This ensures that your script doesn’t continue until all background jobs are finished.

Error Handling

Error handling in shell scripting allows you to handle errors and exceptions that may occur during the execution of your script.

The `exit` command is used to terminate your script and return an exit status to the calling environment. You can specify an exit status between 0 and 255.

The `if` statement allows you to perform conditional execution based on the success or failure of a command. For example:

if [ $? -eq 0 ]; then
# commands to execute if the previous command was successful
else
# commands to execute if the previous command failed
fi

In this section, we have explored advanced shell scripting concepts such as functions, input and output redirection, process control, and error handling.

These concepts will help you to further enhance your shell scripts and make them more efficient and robust.