Chapter 3.md

Introduction to Command Line

The Shell

When we speak of the command line, we are really referring to the shell. The shell is a program that takes commands and passes them to the operating system to carry out. Almost all Linux distributions supply a shell program from the GNU project called bash.

Understanding Commands

Commands are programs that we run from the shell instead of a GUI application.

Command Breakdown

PROGRAM [OPTIONS] [ARGUMENT]

Commands usually breakdown to three parts:

• PROGRAM: is the program name or path in the system, examples: firefox, java, etc...
• [OPTIONS]: are special argument that alter the default behaviour of the program, and most of the time they are optional.
• [ARGUMENT]: are the arguments that the program operate on, and also most of the time they are optional.

Note: There are two different formats for [OPTIONS]:

• Short format: usually starts with a dash '-' followed by a single letter, example: -a.
• Long format: usually starts with a double dash '--' followed by a word, like --all.

Take for example the following commands:

echo [OPTION] [STRING] : Displays string of text.

Option	Usage
-n	Do not output the trailing newline.
-e	Enable interpretation of backslash escapes.
-E	Disable interpretation of backslash escapes. (default)

osc@osc:~$ echo Hello World
Hello World
osc@osc:~$ echo -n "Hello, "
echo "World!"
Hello, World!
osc@osc:~$ echo -e "Hello\nWorld"
Hello
World
osc@osc:~$ echo -E "Hello\nWorld"
Hello\nWorld

clear : Clears the screen.

osc@osc:~$ echo Hello World
Hello World
osc@osc:~$ clear

Output:

osc@osc:~$

Linux File System Hierarchy

If you’re new to the Linux system, you may be confused by how it references files and directories. Before exploring the Linux system, it helps to have an understanding of how it’s laid out.

Linux stores files within a single directory structure, called a virtual directory. The virtual directory contains file paths from all the storage devices installed on the computer, merged into a single directory structure. The Linux virtual directory structure contains a single base directory, called the root. Directories and files beneath the root directory are listed based on the directory path used to get to them.

The tricky part about the Linux virtual directory is how it incorporates each storage device. The first hard drive installed in a Linux system is called the root drive (/). The root drive contains the virtual directory core. Everything else builds from there. On the root drive, Linux can use special directories as mount points. Mount points are directories in the virtual directory where you can assign additional storage devices. Linux causes files and directories to appear within these mount point directories, even though they are physically stored on a different drive.

Often system files are physically stored on the root drive. User files are typically stored on a separate drive or drives, as shown below.

'/' Directory

The '/' directory or the "root" directory is where everything begins on Linux.

No matter what you want to access, where it is, it will somehow connect to the root directory.

The '/' character is also used as a directory separator in file names. For example, if etc is a subdirectory of the / directory, you could refer to that directory as /etc. Likewise, if the /etc directory contained a file named issue, you could refer to that file as /etc/issue.

Top Level Directories

Directory	Content / Description
/	The root of the virtual directory. It is the starting point for the file system hierarchy
/boot	Boot directory, where boot files are stored (e.g, Linux kernel and other static files of the boot loader).
/media	Media directory, a common place for mount points used for removable media.
/mnt	Mount directory, another common place for mount points used for removable media.
/etc	System configuration files directory.
/bin	Binary directory, where many essential user command binaries are stored.
/sbin	System binary directory, where many system administration binaries are stored.
/usr	User binary directory, where the applications and files used by users are stored (/usr/ is the second major section of the filesystem (secondary hierarchy).
/tmp	Temporary directory, where temporary work files can be created and destroyed (these temporary files are generally deleted when the system is restarted).
/var	Variable directory, for files that change frequently which handled by services, such as logs, queues, caches, and spools.
/home	Home directory, where Linux creates normal user directories (non-root users).
/root	The home directory for the root user (administrative superuser).

Navigating the System

pwd : Prints the absolute path of the current working directory.

osc@osc:~/Downloads$ pwd
/home/osc/Downloads

cd [DIRECTORY] : Change directory.

To change our working directory, we use the cd command.

osc@osc:~$ cd /home/osc/Downloads
osc@osc:~/Downloads$

Now , our working directory here is Downloads.

Absolute Path

An absolute path begins with the root directory '/' and follows the tree branch by branch until the path to the desired directory or file is completed.

For example, there is a directory on our system in which most of our system's programs are installed. The directory’s path is "/usr/bin". This means from the root directory (represented by the leading slash '/' in the path), there is a directory called "usr", which contains a directory called "bin".

osc@osc:~$ cd /usr/bin
osc@osc:/usr/bin$ pwd
/usr/bin
osc@osc:/usr/bin$

Relative Path

Where an absolute path starts from the root directory '/' and leads to its destination, a relative path starts from the working directory.

The '.' notation refers to the working directory, and the '..' notation refers to the working directory's parent directory.

The working directory here is usr.

osc@osc:/usr/$ cd ./bin
osc@osc:/usr/bin$ pwd
/usr/bin

In almost all cases, we can omit the ./.

osc@osc:/usr/$ cd bin
osc@osc:/usr/bin$ pwd
/usr/bin

Command	Usage
cd ..	Change the current working directory to the parent directory of the current directory.
cd	Change the working directory to your home directory.
cd -	Changes the working directory to the previous working directory.
cd ~user_name	Changes the working directory to the home directory of user_name.

ls [OPTION] [FILE] : Lists directory contents.

osc@osc:~$ ls
Desktop  Documents  Downloads  Music  Pictures  Public  Templates  Videos

Besides the current working directory, we can specify the directory to list:

osc@osc:~$ ls /home
osc

Option	Usage
-l	Displays detailed information about files.
-a	List all files, including the hidden files.
-t	Sort files by modification time, with the newest files appearing first.
-r	Reverse the order of the sort to display files in reverse order.
-S	Sort files by size, with the largest files appearing first.

Exploring the System

cat [OPTION] [FILE] : Display contents of a file, concatenate files and print on the standard output.

osc@osc:~/Documents$ cat myFile
hello world

Option	Usage
-n	Number the lines of the output.
-s	Suppress repeated empty output lines.
-E	Display a dollar sign ($) at the end of each line.

file : Determine file type.

osc@osc:~/Documents$ file myFile
myFile: ASCII text

type : It is used to find out whether it is a built-in or external binary file.

osc@osc:~$ type cd
cd is a shell builtin