Linux Command Line Basics

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LINUX COMMAND LINE BASICS:
www.ihaveapc.com admin team

Contents:
1. PART 1: Navigation Page 03
2. PART 2: Linux Directory Structure and Permissions Page 13
3. PART 3: Managing Users And Groups Page 18
4. PART 4: Users, Groups And Permissions Page 32
5. PART 5: System Information Commands Page 47
6. PART 6: More System Information Commands Page 52
7. PART 7: vi Cheat Sheet Page 61

PART 1: Navigation
Linux has evolved as a great choice for desktop OS and most of the tasks can be
accomplished from within the GUI. But still, we may need to use the command line
interface in case of an emergency where GUI is not available or for remote
administration (Telnet, SSH etc.) from a place with a very low bandwidth.
Now we’ll learn how to navigate around using command line. The Linux version used
here is Linux Mint 9.0, which is a variant of Debian Linux. To practice, start the
‘Terminal’ from the ‘Accessories’ section of the main menu. Please note that Linux
commands are case sensitive.
1. pwd - Print Working Directory. This command outputs the current directory the
user is in. For a Linux desktop user, it is typically ‘/home/<user-name>’. Everything
in Linux is represented as a folder or a file. ‘/’ is the root directory and all other
directories fall under this directory (more about the Linux directory structure in the
next part). So, if your user name is ‘sam’, your home directory will be ‘/home/sam’.

2. ls - List. This command lists the contents of a directory.
To view the directory contents in long format, issue the command:
ls -l

In the above screenshot, you can see the output of list command in long format.
The first column represents user permissions, which will learn about in the upcoming
posts. Second column represents number of files/folders in that directory. Third
column represents the owner. Fourth column represents the owner’s group. Fifth
column represents the size in bytes. Sixth and seventh columns represent the date
and time modified respectively. Last column represents the file/folder name.
In Linux, a hidden file/directory name starts with a dot i.e. ‘.’. If you want to view
the hidden files/directories then issue the following command:
ls -la
Also note that in the above screenshot, the colored entries are directories and black
entries are files. Notice the second entry, which has ‘->’ in it’s name. It is a link,
which is similar to shortcut file in windows. In above case, ‘.mozilla-thunderbird’ is a
link to actual directory ‘/home/ihaveapc/.thunderbird’.
Also, note that the output has scrolled too fast for the user to read. To make the
output readable, we will use two things- a pipe (|) and ‘more’. Pipe will cascade
output of one command as a input of the second command. Command ‘more’ will
instruct the system to display output screen full at a time until space key is pressed.
Hence, if we issue command:
ls -la | more

then, we will get the same output as above but screen full at time until user presses
the space key to advance to the next screen.

3. cd – Change Directory. This command allows you to change the current working
directory. Syntax is simple: ‘cd <directory path>’. To go to the root directory, issue
the following command:
cd /
After changing the directory, you can use ‘pwd’ command to check your current
directory. You can issue command:
cd ~
from any directory to go back to your home directory.
You can issue command:
cd -
to switch back to previous directory in which you were working before issuing last
‘cd’ command.
Also, notice that when we changed to directory ‘/etc’ by issuing command ‘cd /etc‘,
the directory path is displayed on the left side of the ‘$’ sign. When we changed to
home directory by issuing command ‘cd /home/ihaveapc‘, ‘~’ is displayed on the left
side of the ‘$’ sign. Recall that ‘~’ represents home directory.

4. cp – Copy. The syntax is simple ‘cp <path to the file/folder to be copied> <path
where the file/folder is to be copied>’
In below screenshot, we have changed to directory ‘/home/ihaveapc/Pictures’ and
we want to copy the file ’10.jpg’ to ‘/home/ihaveapc/Desktop’. Hence, we issue the
following command:
cp 10.jpg /home/ihaveapc/Desktop
However, notice that we did not get any message on screen as the file was copied.
Hence, we can issue the ‘-avr’ option. ‘a’ means archive(preserve link and
permissions), ‘v’ means verbose(show what is being done), ‘r’ means recursive(copy
recursively). Hence, we issue the following command:
cp -avr 10.jpg /home/ihaveapc/Desktop
Note that the previously copied ’10.jpg’ in ‘Desktop’ directory was overwritten
without confirmation. To avoid this we will use the option ‘-i’. ‘i’ means
interactive(confirm before overwriting). Hence, the above command will be modified
to:
cp -iavr 10.jpg /home/ihaveapc/Desktop

5. mkdir – Make Directory. Syntax is simple ‘mkdir <directory name to be created>’
Now, we make a directory ‘pics’ in the ‘Desktop’ folder by issuing the following
command:
mkdir pics
6. mv – Move. The syntax is simple ‘mv <path to the file/folder to be moved> <path
where the file/folder is to be moved>’
We need to move the file ’10.jpg’ from directory ‘Desktop’ to directory ‘pics’. For this
we will issue the following command:
mv 10.jpg /home/ihaveapc/Desktop/pics

7. rm – Remove. This command deletes a file/directory. Syntax is ‘rm <file/directory
name to be deleted>’
We need to delete file ’10.jpg’ in directory ‘pics’. Hence, we issue the following
command:
rm 10.jpg

However, if we try to delete a directory which has files/folders in it, system will not
delete it and will tell you that the directory is not empty. Hence, we can use ‘-rf’
option in such cases. ‘r’ means recursive(delete recursively), ‘f’ means force(do not
prompt about non-existent files). Hence, we issue the following command to delete
‘pics’ directory:
rm -rf pics

[Be very careful while using the above command. Never run the above command on
root directory as a root user.]

PART 2: Linux Directory Structure and Permissions
We will now continue with the Linux command line basics. We will go through the
standard Linux directory structure before you explaining some concepts about Linux
file permissions so that you will be in a better position to understand user/group
management commands and the commands used to set file permissions in Linux.
Linux directory structure:
The Linux directory structure is like a tree. Standard Linux directory structure
generally contains the following directories-
1. / – This is the highest level directory in the tree. It is called ‘root’ and all other
folders and devices are its subdirectories. In Linux, everything is represented as a
file or a folder (even hardware devices!). Since, you can set permissions on files and
folders; you can secure access to these files and folders. This is the reason why
Linux is secure by design.
2. /bin – This directory contains essential binary applications required by the
system to operate.
3. /usr/bin – This directory contains the binary applications for the system’s users.
4. /sbin – This directory contains the binary applications required for system
administration by a super user. This folder contains critical binary applications that
system must use even before other directories are mounted.
5. /usr/sbin – This directory contains the binary applications required for system
administration by a super user but the applications are available only after booting is
completed.
6. /boot – This is the directory where Linux kernel and boot loader files are kept.
7. /dev – This directory contains all the hardware devices available to the system
represented as files. The system can read from and write to these files or both
depending on the type of device.
8. /etc – This directory contains the configuration files for the OS, applications and
the start up scripts.
9. /home – This is the directory which contains the user’s home directories. In
general, a normal user cannot write or make changes to the directories outside his
home directory. This reduces the chances of user accidently messing up the system.
Only root user (administrator) or super users can write or make changes anywhere
in the Linux directory structure.
10. /lib – This directory contains the shared libraries required for proper functioning
of the system.

11. /lost+found – This directory contains the lost and found files from the root
directory.
12. /mnt – This directory is the mount point for the mounted file systems in your
computer.
13. /media – This directory is the mount point for the file systems of the removable
devices.
14. /opt – This directory is a place for the optional applications that can be
installed.
15. /proc – This is a virtual directory that contains entries corresponding to the
processes and threads running in the system.
16. /srv – This directory acts as a temporary location for the data to be used by the
servers.
17. /sys – This directory contains system-specific information to be used as a
reference for other applications.
18. /tmp – This directory is used for temporary storage of files.
19. /usr – This directory contains files/folders that support applications that are
available to all the users.
20. /var – This directory contains files that change while the system is running.

File / Folder permissions in Linux:
In Linux, there are 3 types of file permissions-
r (read) – Allows user to view the file – numerical value = 4
w (write) – Allows user to edit the file – numerical value = 2
x (execute) – Allows user to run the file as an executable – numerical value =1
While listing a directory in long format via ls -l command, you will notice the that the
first column has the following format-
drwxrwxrwx
or
-rwxrwxrwx
This column represents the file / folder permissions. Let us see how-
- r w x r w x r w x
The first character from left represents the whether the listed entry is a file
(represented by ‘-’), directory (represented by ‘d’) or a link (represented by ‘l’).

- r w x r w x r w x
The red characters represent the permissions provided to the owner of the file.
Presence of a character(r, w or x) represents that the permission for that action
(read, write or execute) is granted. Presence of a ‘-’ instead of the character
represents that the permission for that action (read, write or execute) is denied.
In above case the owner has permission to read, write and execute the file. Note
that, r+w+x = 4+2+1 = 7.
- r w x r w x r w x
The green characters represent the permissions provided to the members of the
owner’s group for the file. In above case the group members have permission to
read, write and execute the file. Note that, r+w+x = 4+2+1 = 7.
- r w x r w x r w x
The blue characters represent the permissions provided to all the other users for the
file. In above case the other users have permission to read, write and execute the
file. Note that, r+w+x = 4+2+1 = 7.
The numerical value of the above permission is represented as 777.
Here are few examples of the file permissions-
rwx------ : (4+2+1, 0+0+0, 0+0+0 = 700) Owner can read, write and execute
the file. Group users and other users do not have any permission for the file.
rw-rw-rw- : (4+2+0, 4+2+0, 4+2+0 = 666) All users can read and write to
the file.
rw-r--r-- : (4+2+0, 4+0+0, 4+0+0 = 644) Owner can read and write to the
file. Group users and others can only read the file.
The directory permissions are calculated in similar manner with just the following
differences-
r (read) – Allows user to list the files in the directory – numerical value = 4
w (write) – Allows user to create new files and delete the files in the directory –
numerical value = 2
x (execute) – Allows user to change to the directory via cd command – numerical
value =1
Here are few examples of directory permissions-

rwxrwxrwx : (4+2+1, 4+2+1, 4+2+1 = 777) Allows owner, group members
and others to list files in directory, create files in directory, delete files from the
directory and to change to the directory.
rwxr-xr-x : (4+2+1, 4+0+1, 4+0+1 = 755) Allows owner to list files in
directory, create files in directory, delete files from the directory and to change to
the directory. Group members and others can change to the directory and list the
files only.
rwx------ : (4+2+1, 0+0+0, 0+0+0 = 700) Allows owner to list files in
directory, create files in directory, delete files from the directory and to change to
the directory. Group members and others do not have any permission on the
directory. This makes the directory private to the owner.

PART 3: Managing Users And Groups
We will learn how to manage users and groups.
1. adduser – Add user. Syntax is ‘sudo adduser <user name>’. We need to use
‘sudo’ as root privileges are required to run this command. A root user can run the
command without ‘sudo’. To add a new user named ‘mint1′, issue the following
command in the terminal:
sudo adduser mint1
Enter the user password when prompted. A new user called ‘mint1′ will be created.
A home directory will be created for the new user in ‘/home’ location. The name of
new user’s home directory will be same as his user name. [Note that the default
structure of the home directory is defined in '/etc/skel' (skel=skeleton). Every time a
home directory is to be created for a new user, files are copied from '/etc/skel']. The
system will now ask you to create a password for this new user. Type the password
and re-type it when prompted. Ask the user to change this password to a desired
one by using ‘passwd’ command when the user logs in for the first time. You can
also enter the full name and contact details of the new user when prompted. Type
‘y’ to confirm the information and hit enter.

In similar manner, to add a new user named ‘mint2′, issue the following command
in the terminal:
sudo adduser mint2
Note that two new home directories have been created for users ‘mint1′ and ‘mint2′
in ‘/home’ location.

2. deluser – Delete user. Syntax is ‘sudo deluser <user name>’. We need to use
‘sudo’ as root privileges are required to run this command. A root user can run the
command without ‘sudo’. To delete the user ‘mint1′ issue the following command at
the terminal:
sudo deluser mint1

Note that the above command does not delete user’s home directory. It is still
present in ‘/home’ location.

If you want to delete a user as well as his home directory, use the option ‘-remove-
home’. In order to delete user ‘mint2′ as well as his home directory issue the
following command at the terminal:
sudo deluser mint2 -remove-home

Note that the user’s directory has been deleted from the ‘/home’ location.

3. addgroup – Add group. Syntax is ‘sudo addgroup <group name>‘. We need to
use ‘sudo’ as root privileges are required to run this command. A root user can run
the command without ‘sudo’. To add a group called ‘team1′, issue the following
command at the terminal:
sudo addgroup team1
To add a group called ‘team2′, issue the following command at the terminal:
sudo addgroup team2

Now, click on ‘Menu>Administration>Users and Groups’.
Click on ‘Manage Groups’.

Notice that groups ‘team1′ and ‘team2′ have been added. Select ‘team1′ and click
‘Properties’.

You can view the list of users who are members of this group. Click ‘OK’ to close the
window.
You can also select a user and click ‘Advanced Settings’.

Enter user password when prompted and click ‘Authenticate’.
In the ‘Change Advanced User Settings’ window, you can modify user’s main group,
if required.

4. delgroup – Delete group. Syntax is ‘sudo delgroup <group name>’. We need to
use ‘sudo’ as root privileges are required to run this command. A root user can run
the command without ‘sudo’. In order to delete groups ‘team1′ and ‘team2′, issue
the following commands at the terminal:
sudo delgroup team1
sudo delgroup team2
5. passwd – Change user password. Syntax is ‘passwd <user name>’. You can
change the password for the user whose name has been entered.
Simply issuing command ‘passwd’ enable you to change your own password. Issuing
the command ‘sudo passwd’ will enable you to change root user’s password.
To change your password, issue the following command at the terminal:
passwd

You need to enter your current password, new password and then re-type the new
password.
6. who – Shows list of logged on users. To view the users which are logged on,
issue the following command at the terminal:
who

Note that the users ‘root’ and ‘ihaveapc’ are logged in the system (entries 1 and 2).
Entry 3 (pts/0) denotes the pseudo terminal slave session of user ‘ihaveapc’ (as we
have opened a terminal in the GUI after logging in to the Gnome desktop, that
session becomes the slave session of our current session in the GUI).

PART 4: Users, Groups And Permissions
We’ll now learn how to add users to groups via command line and how to modify
file/folder permissions.
Let’s create two groups in the system- ‘team1′ and ‘team2′
Issue the following commands at the terminal:
sudo addgroup team1
sudo addgroup team2
1. useradd – adds a new user to an existing group.
Syntax 1 – sudo useradd -g <group-name> <user-name>
The above command will add a new user to an existing group. The option ‘-g’ will
make the specified group new user’s primary group. In order to add new users
‘user1′ and ‘user2′ with primary group ‘team1′, issue the following commands at the
terminal:
sudo useradd -g team1 user1

sudo useradd -g team2 user2
Syntax 2 – sudo useradd -G <group-name> <user-name>
The above command will add a new user to an existing group. The option ‘-G’ will
make the specified group new user’s secondary group. In order to add new users
‘user3′ and ‘user4′ with secondary group ‘team2′, issue the following commands at
the terminal:
sudo useradd -G team2 user3
sudo useradd -G team2 user4

2. usermod – adds an existing user to an existing group.
Syntax 1 – sudo usermod -g <group-name> <user-name>
The above command will add an existing user to an existing group. The option ‘-g’
will make the specified group existing user’s primary group. In order to add existing
users ‘user3′ and ‘user4′ with primary group ‘team1′, issue the following commands
at the terminal:
sudo usermod -g team1 user3
sudo usermod -g team1 user4

Syntax 2 – sudo usermod -a -G <group-name> <user-name>
The above command will add an existing user to an existing group. The options ‘-a’
and ‘-G’ will make the specified group existing user’s secondary group. In order to
add existing users ‘user1′ and ‘user2′ with secondary group ‘team2′, issue the
following commands at the terminal:
sudo usermod -a -G team2 user1
sudo usermod -a -G team2 user2

3. chmod – modifies file/folder permissions.
Syntax – chmod <permission value> <file/folder name>
Issue the following command at the terminal:
chmod 777 firefox-3.6.8.tar.bz2

The above command will allow everybody to read, write and execute the file.

The above command will allow the file owner to read, write and execute the file.
Group users can read and execute the file. Others can only execute the file.

The above command will allow file owner to read, write and execute the file. Group
users and others have no permissions on the file.
Now, let us look how ‘chmod’ is used to modify folder permissions.
Issue the following command at the terminal (‘test’ is folder in our home directory):
chmod 777 test

The output of ‘ls -la’ command shows that the above command allows owner, group
members and others to list files in directory ‘test’, create files in directory ‘test’,
delete files from the directory ‘test’ and to change to the directory ‘test’.
chmod 755 test

The output of ‘ls -la’ command shows that the above command allows owner to list
files in directory ‘test’, create files in directory ‘test’, delete files from the directory
‘test’ and to change to the directory ‘test’. Group members and others can change to
the directory ‘test’ and list the files only.
chmod 700 test

The output of ‘ls -la’ command shows that the above command allows owner to list
files in directory ‘test’, create files in directory ‘test’, delete files from the directory
‘test’ and to change to the directory ‘test’. Group members and others do not have
any permission on the directory ‘test’.
4. chown – changes file ownership.
Syntax – sudo chown <user-name(of new owner)> <file-name>
sudo chown user1 firefox-3.6.8.tar.bz2

The output of command ‘ls -la firefox-3.6.8.tar.bz2’ shows that ‘user1′ is the new
owner of the file.
5. chgrp – changes file’s group ownership
Syntax – sudo chgrp <group-name(of new owner group)> <file-name>
sudo chgrp team1 firefox-3.6.8.tar.bz2

The output of the command ‘ls -la firefox-3.6.8.tar.bz2′ shows that the file’s group
ownership has changed to ‘team1′.

PART 5: System Information Commands
Now we’ll learn a few Linux system information commands:
1. df – disk free. This commands displays file system and disk space usage for all
partitions. If option ‘-h’ (human-readable) is used with the command, it will generate
the report using KB/MB/GB units instead of number of blocks which are displayed
when the command is used without any options.
2. free – This command displays information about the amount of used and free
system memory. If option ‘-m’ is used with the command, report is generated using
MB unit.

3. top – This command provides a real-time information about Linux system uptime,
number of users, system load, number of tasks, and utilization of system resources
such as cpu, memory and swap partition. You can press ‘Ctrl’ + ‘Z’ keys to quit the
program at any time.

4. uname – unix name. This command provides the name, kernel version and other
system architecture details about the current system and the Linux OS running on it.
If option ‘-a’ is used with the command, it will print machine type, network node
host name, processor type, OS release and OS version.
5. lsb_release – This command provides Linux Standard Base and the distribution
information. When ‘-a’ option is used with the command, it will print LSB version,
distributor ID, description of distribution, release number and codename of the
distribution.

PART 6: More System Information Commands
Let us learn a few more Linux system information commands.
1. ifconfig – interface configuration. This command displays information about the
network interfaces in a system.
2. iwconfig – This command is similar to ‘ifconfig’ command but it only displays
information about the wireless network interfaces. (You won’t see any wireless
network interface in the screenshot as it has been taken inside a virtual machine.)

3. ps – process status. This command shows you all the processes running in the
system.

4. lspci – This command lists all PCI buses and devices connected to them.

5. lsusb – This command lists all USB buses and devices connected to them.
6. lshal – This command lists all the devices that HAL(Hardware Abstraction Layer) is
aware about i.e. most of the hardware connected to your system.

7. lshw – This command lists hardware present in the system including information
about manufacturer, device type and where it is connected.

PART 7: vi Cheat Sheet

Linux Command Line Basics

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