Introductory Terms and Concepts#

• Kernel: The core of the operating system that manages the hardware and software resources.
• Shell: The interface that allows you to interact with the operating system.
• Terminal: The text-based interface that allows you to interact with the shell.
• Command: An instruction given to the shell to perform a specific task.
• Filesystem: The structure used to organize and store files on a storage device.
• Directory: A container for files and other directories.
• Path: The location of a file or directory in the filesystem.

The Linux Filesystem#

Basic Commands in Linux#

• pwd: Print the current working directory.
• whoami: Display the current user.
• cd: Change the current directory.
• ls: List the contents of a directory.
• man: Display the manual page for a command.

Finding Stuff#

• locate: Search for files and directories.

locate filename

• whereis: Locate the binary, source, and manual page files for a command.

whereis command

• which: returns the location of the binaries in the PATH variable

which command

• find: Search for files and directories based on various criteria.

find directory options expression
find / -type f -name filename

• grep: Search for patterns in files.

ps aux | grep apache2

Modifying Files and Directories#

• cat: The cat command followed by a filename will display the contents of that file, but to create a file, we follow the cat command with a redirect,

cat > filename

• touch: Create an empty file or update the access and modification times of a file.

touch filename

• mkdir: Create a new directory.

mkdir directoryname

• cp: This creates a duplicate of the file in the new location and leaves the old one in place.

cp source destination

• mv: can be used to move a file or directory to a new location or simply to give an existing file a new name.

mv source destination
mv filename newfilename

• rm: Remove files or directories.

rm filename

• rmdir: Remove empty directories.

rmdir directoryname

NOTE

It’s important to note that rmdir will not remove a directory that is not empty

Viewing Files#

• cat: Display the contents of a file.

cat filename

• head: Display the first few lines (by default 10 lines) of a file.

head filename
head -20 filename

• tail: Display the last few lines (by default 10 lines) of a file.

tail filename
tail -20 filename

• nl: Display the contents of a file with line numbers.

nl filename

Filtering Text with grep#

Using sed to Find and Replace#

• sed: Stream editor for filtering and transforming text.

sed 's/old/new/g' filename

NOTE

g stand for global, it will replace all the occurences of the old string with the new string

Viewing Files with more and less#

• more: Display the contents of a file one screen at a time.

more filename

• less: Display the contents of a file one screen at a time with backward and forward navigation.

less filename

Analyzing Networks with ifconfig#

• ifconfig: Display and configure network interfaces.

Checking Wireless Network Devices with iwconfig#

• iwconfig: you can use the iwconfig command to gather crucial information for wireless hacking such as the adapter’s IP address, its MAC address, what mode it’s in, and more.

Changing Your Network Information#

ifconfig eth0 192.168.191.212
ifconfig eth0 192.168.181.115 netmask 255.255.0.0 broadcast 192.168.1.255

ifconfig eth0 down
ifconfig eth0 hw ether 00:11:22:33:44:55
ifconfig eth0 up

dhclient eth0

Manipulating the Domain Name System#

DNS is a critical component of the internet, and although it’s designed to translate domain names to IP addresses, a hacker can use it to garner information on the target.

nano /etc/resolv.conf
echo "nameserver 8.8.8.8"> /etc/resolv.conf

nano /etc/hosts

Using apt to Handle Software#

• apt: The Advanced Package Tool (APT) is a package management system that allows you to install, update, and remove software on your system.

apt-cache search keyword
apt update
apt upgrade
apt install package
apt remove package # purge if you want to remove the configuration files as well
apt autoremove
apt search package

Adding Repositories to Your sources.list File#

The servers that hold the software for particular distributions of Linux are known as repositories.

nano /etc/apt/sources.list

Installing Software with git#

some software is not available in the official repositories, and you need to install it manually. One way to do this is by using git, a version control system that allows you to clone repositories from github.

git clone repositoryUrl

Different Types of Users#

• Root: The superuser who has complete control over the system.
• Regular User: A user who has limited control over the system.
• Group: A collection of users who share the same permissions.
• Other: Everyone else who has access to the system.

Granting Permissions#

The three levels of permission are as follows

• Read (r): This grants permission only to open and view a file.
• Write (w): This allows users to view and edit a file.
• Execute (x): This allows users to execute a file (but not necessarily view or edit it).

chown username filename

chgrp groupname filename

Checking Permissions#

• ls -l: –l (long) switch to display the contents of a directory in long format—this list will contain the permissions.

On each line, we get information about:

• The file type (d for directory, - for file)
• The permissions on the file for owner, groups, and users, respectively
• The number of links to the file
• The owner of the file
• The group that owns the file
• The size of the file
• The date and time the file was last modified
• The name of the file

Changing Permissions#

chmod permissions filename

chmod u=rwx,g=rx,o=r filename

Setting More Secure Default Permissions with Masks#

• umask: is a three-digit octal number corresponding to the three permissions digits, but the umask number is subtracted from the permissions number to give the new permissions status. This means that when a new file or directory is created, its permissions are set to the default value minus the value in umask

umask 022

Special Permissions#

• Set User ID (SUID): the SUID bit says that any user can execute the file with the permissions of the owner but those permissions don’t extend beyond the use of that file.
  • To set the SUID bit, enter a 4 before the regular permissions, so a file with a new resulting permission of 644 is represented as 4644 when the SUID bit is set.
• Set Group ID (SGID): SGID also grants temporary elevated permissions, but it grants the permissions of the file owner’s group, rather than of the file’s owner. This means that, with an SGID bit set, someone without execute permission can execute a file if the owner belongs to the group that has permission to execute that file.
  • The SGID bit works slightly differently when applied to a directory: when the bit is set on a directory, ownership of new files created in that directory goes to the directory creator’s group, rather than the file creator’s group. This is very useful when a directory is shared by multiple users. All users in that group can execute the file(s), not just a single user.
  • The SGID bit is represented as 2 before the regular permissions, so a new file with the resulting permissions 644 would be represented as 2644 when the SGID bit is set. Again, you would use the chmod command for this—for example, chmod 2644 filename.

Viewing Processes#

• ps: Display information about running processes.

ps columns

USER The user who invoked the process PID The process ID %CPU The percent of CPU this process is using %MEM The percent of memory this process is using COMMAND The name of the command that started the process

Managing Processes#

nice -n 10 command

nice -n -10 /bin/slowprocess
renice 19 PID

command &

fg PID

Scheduling Processes#

• at: is used to set up the daemon—a background process atd, which is useful for scheduling a job to run once at some point in the future