Getting Started with Linux

Getting Started with Linux#

Information

Details

Lead Author

Hantao Cui

Learning Objectives

• Navigate the Linux file system
• Perform basic file operations
• Use essential Linux commands

Prerequisites

WSL installation

Estimated Time

90 minutes

Topics

Linux basics, file navigation, directories, command line

Landing in the Linux Shell#

When you open a new Linux terminal, you will see a prompt like

User@Machine:~$

In this prompt, User is the username, Machine is the machine name, and ~ stands for the home directory, which is the current working directory. The colon : is a separator between the machine name and the working directory.

The prompt character is $ for a regular user and # for a superuser.

Working with Files and Folders#

To navigate the file system in the shell, the three most essential commands to use are:

  • cd: Change directory

  • ls: List files and directories

  • pwd: Print working directory

Change Directory#

The cd command is used to change the current working directory. When a single argument is passed, namely, the path to the directory, the current working directory is changed to the specified directory.

User@Machine:~$ cd /tmp
User@Machine:/tmp$ cd ..
User@Machine:/$ cd
User@Machine:~$

The first command cd /tmp specifies an absolute path /tmp, so the working directory is changed accordingly, as you see in the second command.

The second command cd .. changes the directory to the parent directory. .. is a relative path to the parent directory. As a result, the directory changed from /tmp to /. In Linux, / is known as the root directory. Every directory has an absolute path that starts from the root directory.

The third command cd without any argument changes the directory to the home directory. The home directory is the directory where the user is located. The directory change can be confirmed by the pwd command.

Case Sensitivity

Linux is case-sensitive, following the convention from UNIX. It is case sensitive, meaning that all the command and file names are case sensitive.

In the beginning, this might feel a bit confusing when you are used to Windows.

Understanding Paths#

There are two ways to tell the computer where to find files:

  1. Absolute paths - Start from the root /

    cd /home/username/Documents

  2. Relative paths - Start from current location

    cd Documents          # Go to Documents from current folder
cd ../Downloads      # Go up one, then to Downloads

Special path shortcuts:

  • . - Current directory

  • .. - Parent directory

  • ~ - Home directory

List Files and Directories#

The ls command is used to list the files and directories in the current directory.

ls can accept a path to a directory as an argument. It will then list the items in the specified directory.

ls can take arguments. A few commonly used arguments are

  • -a: Show all files and directories, including hidden ones

  • -l: Show detailed information about the files and directories

  • -t: Sort the files and directories by time

  • -r: Reverse the order of the files and directories

These options can be combined. For example, ls -lrt is equivalent to ls -l -r -t, which will list the files and directories in the current directory in reverse order of time. The combination is agnostic to the option order.

To list files that match a pattern, one can use the wildcard *. For example,

User@Machine:~$ ls f*.txt
file.txt

This command lists all files that start with f and end with .txt.

Hidden files and directories

Hidden files and directories are named to start with a dot .. They are not displayed by ls but are shown by ls -a.

Exercise

Use the ls command to list files and directories in the /var directory. Include hidden files in your listing. Then, sort the files by modification time.

Basic File Operations#

Make a Folder#

The mkdir command is used to make a folder. It can take a path to the folder as an argument.

Remove#

Two commands are used to remove files and directories. rm is used to remove files or directories.

rmdir is used to remove empty directories.

Create a directory and a file in it.

User@Machine:~$ mkdir test
User@Machine:~$ ls
test
User@Machine:~$ touch test/file.txt
User@Machine:~$ ls test
file.txt

The touch command creates a new & empty file.

Since the directory test is not empty, rmdir will not work.

User@Machine:~$ rmdir test
rmdir: test: Directory not empty

One can remove the directory test with rm -r test. The option -r stands for recursive, meaning that the folder and everything (files and subfolders) inside will be removed.

User@Machine:~$ rm -r test
User@Machine:~$ ls

No Recycle Bin on Linux

Linux bash does not have a built-in recycle bin. Files deleted with rm are gone forever.

If unsure, use mv to rename the file or move it to a temporary location.

Exercise

In your home directory, create a new directory named power_logs. Inside power_logs, create an empty file named session1.log.

Verify that the directory and file were created correctly.

Copy and Move#

The commands for copy and move are cp and mv.

The format for calling both commands is command SOURCE DESTINATION.

The example below makes a copy of file.txt to file2.txt.

User@Machine:~$ cp file.txt file2.txt
User@Machine:~$ ls
file.txt  file2.txt

The example below moves file2.txt to the directory test.

User@Machine:~$ mv file2.txt test
User@Machine:~$ ls test
file2.txt
User@Machine:~$ ls
file.txt

Exercise

Back to the power_logs folder, copy session1.log to a new file named session1_backup.log in the same directory.

Move session1_backup.log to a new directory under power_logs named backup.

File Types and Extensions#

Unlike Windows, Linux doesn’t strictly require file extensions to determine file types. However, extensions are commonly used for clarity:

  • .txt for text files

  • .sh for shell scripts

  • .py for Python scripts

  • .log for log files

  • .tar.gz or .zip for compressed archives

You can use the file command to determine a file’s type regardless of its extension:

file document.txt

Hidden Files#

In Linux, files and directories that start with a dot (.) are hidden by default:

  • .bashrc - Shell configuration

  • .ssh/ - SSH configuration directory

  • .gitconfig - Git configuration

To view hidden files:

ls -a  # Show all files including hidden ones
ls -la # Show all files in long format

Linux File System#

The Linux file system has a hierarchical structure. Unlike Windows, which uses letters (C:, D:, etc.) to represent different drives, Linux uses a single directory / as the root of the entire file system. All files and directories are logically stored in the / directory.

Structure and Common Directories#

The File System Hierarchy Standard (FHS) defines the directory structure and directory contents in Unix-like operating systems. The FHS is maintained by the Linux Foundation.

The following are some common directories in the Linux file system:

  • /home: This directory contains the home directories of all users. That includes your home directory.

  • /root: This is the home directory of the root user. It is only accessible by the superuser.

  • /bin: This directory contains essential system binaries. Common commands like ls, cp, and mv are stored here.

  • /sbin: This directory contains essential system binaries that are used by the superuser.

  • /usr: This directory contains user binaries, libraries, and documentation. This is not to be confused with the /home directory.

  • /etc: This directory contains system-wide configuration files.

  • /mnt: Mount point for mounting a filesystem temporarily.

Understanding Permissions#

In Linux, every file and directory has an owner and associated permissions that control who can read, write, or execute it. When you list files with the ls -l command, you can see the permissions:

User@Machine:~$ ls -l file.txt
-rw-r--r-- 1 User staff 13 Jun 10 12:34 file.txt

The permission string -rw-r--r-- has 10 characters:

  • First character: file type (- for regular file, d for directory)

  • Next 3 characters: owner permissions (rw- means read and write)

  • Next 3 characters: group permissions (r-- means read only)

  • Last 3 characters: others permissions (r-- means read only)

The three basic permission types are:

  • r: Read permission

  • w: Write permission

  • e: Execute permission (for files) or Access permission (for directories)

Changing Permissions with chmod#

The chmod command changes the permissions of a file or directory. There are two ways to specify permissions:

  1. Symbolic mode (using letters):

User@Machine:~$ chmod u+x file.sh  # Add execute permission for the user
User@Machine:~$ chmod g+w file.txt  # Add write permission for the group
User@Machine:~$ chmod o-r file.txt  # Remove read permission for others

Symbols used:

  • u: User (owner)

  • g: Group

  • o: Others

  • a: All (equivalent to ugo)

  • +: Add permission

  • -: Remove permission

  • =: Set permission exactly

  1. Numeric mode (using octal numbers):

# Set rwx for owner, rx for group and others
User@Machine:~$ chmod 755 script.sh

# Set rw for owner, r for group and others
User@Machine:~$ chmod 644 file.txt

Common numeric permissions:

  • 755 (rwxr-xr-x): Standard for executable scripts

  • 644 (rw-r–r–): Standard for regular files

  • 700 (rwx——): Private file, accessible only by owner

Directory Permissions

For directories, the permissions have slightly different meanings:

  • r: List directory contents

  • w: Create or delete files within the directory

  • x: Access the directory (cd into it)

A directory needs execute (x) permission to be accessed!

Changing Ownership#

There may be times you run into permission issues. This often happens when you use the root user (superuser) to run a command. For example, you probably have used sudo to install a package.

If you accidentally run a command as the superuser, all the files and directories created by the command will be owned by the superuser. A regular user will then not be able to modify or remove the files.

The chown command is used to change the ownership of a file or directory. The format is chown OWNER:GROUP FILES. For example,

User@Machine:~$ chown User:staff file.txt

changes the ownership of file.txt to the User user and the staff group.

If you need to change the ownership from root to yourself, you will need to use sudo, because only the superuser can relinquish its ownership.

User@Machine:~$ sudo chown User:staff file.txt

There are options like -R to apply the change recursively.

Caution

Caution with sudo

Always be careful when using sudo. It can easily mess up the permissions.

Exercise

  1. Create a shell script called hello.sh in your home directory with the command:

    echo '#!/bin/bash\necho "Hello, World!"' > hello.sh

  2. Try to execute it with ./hello.sh. What happens?

  3. Use chmod to make it executable and try again.

Quizzes#

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