KL005 – Becoming Familiar with Kali Linux

Basic Linux Commands

Basic Command Structure

The following describes how GNU/Linux Bash is structured:

Note that GNU/Linux is case-sensitive. So, IT means ssh is not the same as Ssh

pompt$ command -option keywork/argument

• Option – Options (Also known as switches) usually come after hyphen -. They intend to modify the output of the command. A command might have none or some options. Each option might have no argument or one argument. For example, -c 4 in ping -c 3 8.8.8.8
• Keyword – This is specified parameter defined by OS. For example, start ssh in systemctl start ssh
• Argument – is not predefined and is assigned by the user. For example 8.8.8.8 in ping 8.8.8.8

As a side note, remember the Linux commands are case sensitive. Implies that Training is not the same as TRAINING.

Manual Pages

Command man is used to view the manual pages. For example you have so far used command systemctl quiet a few times lets use man command in conjunction with systemctl command to get the manual page for systemctl command.

┌──(kali㉿kali)-[~]
└─$ man systemctl

SYSTEMCTL(1)                                                                               systemctl                                                                               SYSTEMCTL(1)

NAME
       systemctl - Control the systemd system and service manager

SYNOPSIS
       systemctl [OPTIONS...] COMMAND [UNIT...]

DESCRIPTION
       systemctl may be used to introspect and control the state of the "systemd" system and service manager. Please refer to systemd(1) for an introduction into the basic concepts and
       functionality this tool manages.

COMMANDS
       The following commands are understood:

Manual page systemctl(1) line 1 (press h for help or q to quit)

apropos

This command is helpful for finding a particular command based on description. Let’s say we want to change our password and we don’t know what command to use.

┌──(kali㉿kali)-[~]
└─$ apropos "change user password"
chage (1)            - change user password expiry information
passwd (1)           - change user password

We can then check the manual pages for the aforementioned commands to understand which command caters to our need.

sudo

By default, Shell restricts access to certain parts of the system. SuperUser DO (sudo) is used to temporarily elevate your user privilege as sudoer privilege.

Manual page of sudo best describes what it does:

sudo allows a permitted user to execute a command as the superuser or another user.

You have previously used sudo command in order to be able to start or stop a service using systemctl.

Listing Files

The ls command lists the files on the current directory or on a specified path.

┌──(kali㉿kali)-[~]
└─$ ls
Desktop  Documents  Downloads  Music  Pictures  Public  Templates  Videos

┌──(kali㉿kali)-[~]
└─$ ls Desktop
file1  file2

Exercise:

Use man page to see the different switches/options for ls and try different switches to list the directories. For example, try these:

ls -a
ls -l
ls -la
ls -lha
ls -lhat
ls -lhatr

The Linux Filesystem

Filesystem Hierarchy Standard (FHS) describes the Linux/Unix file hierarchy. ls -l / shows you the directories in FHS. The directories you will find most useful are:

• / is the root
• /bin – common programs (ls, cd, cat, etc.)
• /sbin – system programs (fdisk, mkfs, sysctl, etc)
• /etc – system configuration files. similar to Control Panel in Windows
• /tmp – temporary files (typically deleted on boot)
• /usr/share – application support and data files
• /home – home directory of the user

You can use command man file-hierarchy for more information on your GNU/Linux distribution.

┌──(kali㉿kali)-[~]
└─$ man file-hierarchy
NAME
       file-hierarchy - File system hierarchy overview

DESCRIPTION
       Operating systems using the systemd(1) system and service manager are organized based on a file system hierarchy inspired by UNIX, more specifically the hierarchy described in the File
       System Hierarchy[1] specification and hier(7), with various extensions, partially documented in the XDG Base Directory Specification[2] and XDG User Directories[3]. This manual page
       describes a more generalized, though minimal and modernized subset of these specifications that defines more strictly the suggestions and restrictions systemd makes on the file system
       hierarchy.

       Many of the paths described here can be queried with the systemd-path(1) tool.

GENERAL STRUCTURE
       /
           The file system root. Usually writable, but this is not required. Possibly a temporary file system ("tmpfs"). Not shared with other hosts (unless read-only).

       /boot/
           The boot partition used for bringing up the system. On EFI systems, this is possibly the EFI System Partition (ESP), also see systemd-gpt-auto-generator(8). This directory is
           usually strictly local to the host, and should be considered read-only, except when a new kernel or boot loader is installed. This directory only exists on systems that run on
           physical or emulated hardware that requires boot loaders.

       /efi/
           If the boot partition /boot/ is maintained separately from the EFI System Partition (ESP), the latter is mounted here. Tools that need to operate on the EFI system partition should
           look for it at this mount point first, and fall back to /boot/ — if the former doesn't qualify (for example if it is not a mount point or does not have the correct file system type
           MSDOS_SUPER_MAGIC).

       /etc/
           System-specific configuration. This directory may or may not be read-only. Frequently, this directory is pre-populated with vendor-supplied configuration files, but applications
           should not make assumptions about this directory being fully populated or populated at all, and should fall back to defaults if configuration is missing.

       /home/
           The location for normal user's home directories. Possibly shared with other systems, and never read-only. This directory should only be used for normal users, never for system
           users. This directory and possibly the directories contained within it might only become available or writable in late boot or even only after user authentication. This directory
           might be placed on limited-functionality network file systems, hence applications should not assume the full set of file API is available on this directory. Applications should
           generally not reference this directory directly, but via the per-user $HOME environment variable, or via the home directory field of the user database.

       /root/
           The home directory of the root user. The root user's home directory is located outside of /home/ in order to make sure the root user may log in even without /home/ being available
           and mounted.

       /srv/
           The place to store general server payload, managed by the administrator. No restrictions are made how this directory is organized internally. Generally writable, and possibly
           shared among systems. This directory might become available or writable only very late during boot.

       /tmp/
           The place for small temporary files. This directory is usually mounted as a "tmpfs" instance, and should hence not be used for larger files. (Use /var/tmp/ for larger files.) This
           directory is usually flushed at boot-up. Also, files that are not accessed within a certain time may be automatically deleted.

           If applications find the environment variable $TMPDIR set, they should use the directory specified in it instead of /tmp/ (see environ(7) and IEEE Std 1003.1[4] for details).

           Since /tmp/ is accessible to other users of the system, it is essential that files and subdirectories under this directory are only created with mkstemp(3), mkdtemp(3), and similar
           calls. For more details, see Using /tmp/ and /var/tmp/ Safely[5].

RUNTIME DATA
       /run/
           A "tmpfs" file system for system packages to place runtime data, socket files, and similar. This directory is flushed on boot, and generally writable for privileged programs only.
           Always writable.

       /run/log/
           Runtime system logs. System components may place private logs in this directory. Always writable, even when /var/log/ might not be accessible yet.

 Manual page file-hierarchy(7) line 1 (press h for help or q to quit)

You might be wanting to look at the post here for more information about FHS.

Work On the Command Line

Absolute Path vs Relative Path

A combination of / and alphanumeric characters is called path and it points to a location to a file or a directory. For example: /home/kali/TRAINING/.

Absolute Path

When you represent the location of a file or a directory from root directory which is /; you are using absolute path. For example: /etc/passwd.

Relative Path

Relative path is relative your present working directory (pwd).

┌──(kali㉿kali)-[~]
└─$ ls TRAINING/
file1  file2

The above snippet is the same as using absolute path ls /home/kali/TRAINING

To reference a directory from the parent directory, we use ../. For example

┌──(kali㉿kali)-[~]
└─$ ls ../
kali

As another example when we employ ../../ it means we reference a directory from a grandparent directory:

┌──(kali㉿kali)-[~]
└─$ ls ../../
bin  boot  dev  etc  home  initrd.img  initrd.img.old  lib  lib32  lib64  libx32  lost+found  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var  vmlinuz  vmlinuz.old

We also have ./ which refers to current directory. Try, cd ./VIDEOS

Some Keyboard shortcuts

Ctrl+L clears the screen.

Tab Completion: In Bash shell you can hit tab key to complete filenames and directory paths.

Up arrow ↑ writes the last command you entered in Bash shell to the command prompt. Conversely, down arrow ↓ scrolls forward in history.

Ctrl+R invokes the reverse-i-search wherein you can type a set of letters to match for the most recent command in your history that contains those letters.

Ctrl+C stops the running command immediately. For example, when you ping 1.1.1.1 the program won’t stop unless you hit this key composition.

Exercise

Run few command such as the ones which shown below, then let’s try to invoke reverse-i-search

sudo systemctl start ssh
sudo systemctl enable ssh
sudo systemctl status ssh
sudo systemctl start apache2
sudo systemctl enable apache2
sudo systemctl status apache2
ping 8.8.8.8
^C
ping 1.1.1.1
^C

Only with Ctrl+R, 2 letters (do not use digits), and Return (Enter) key, try to invoke the following commands.

sudo systemctl status ssh
ping 8.8.8.8

Moving Around

To check the current working directory use command pwd.

┌──(kali㉿kali)-[~]
└─$ pwd
/home/kali

To traverse to different directories, use cd following by the directory name. using cd with no arguments, brings you back to the home directory.

┌──(kali㉿kali)-[~]
└─$ cd Desktop

┌──(kali㉿kali)-[~/Desktop]
└─$ pwd
/home/kali/Desktop

Creating and Removing the Directories

You can create folder(s) using mkdir command followed by the name(s) of the folder(s).

┌──(kali㉿kali)-[~]
└─$ mkdir Documents/TRAINING

┌──(kali㉿kali)-[~]
└─$ ls Documents/
TRAINING

┌──(kali㉿kali)-[~]
└─$ cd Documents/TRAINING

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ pwd

/home/kali/Documents/TRAINING

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ mkdir CCIE 2022

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ ls

2022  CCIE

In the example above we intended to create a directory called CCIE 2022, but instead we created two distinct directories called CCIE and 2022. We can use command rf -rm to remove the directories. Pay careful attention that once the directories/files are removed from terminal, you cannot undo the operation. Now, Let’s remove all the contents in TRAINING folder we have just created above. First, we make sure we are in the right directory.

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ pwd
/home/kali/Documents/TRAINING

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ rm -rf CCIE/ 2022/

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ ls

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$

Note that, if you have too many directories and files and you wanted to remove all those, instead of calling their names, you could use wildcard *. So, the command would be rm -rf *.

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ touch file1 file2

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ rm -rf *
zsh: sure you want to delete all 2 files in /home/kali/Documents/TRAINING [yn]? y

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$

touch

touch command with no option and argument is used to create an/multiple empty file(s). See the example above. We created two files in the last example. Can you see the filenames?

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ touch file1 file2

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ ls -l
total 0
-rw-r--r-- 1 kali kali 0 Apr  3 21:20 file1
-rw-r--r-- 1 kali kali 0 Apr  3 21:20 file2

If you touch an existing file, you will update the timestamp of the file to the current timestamp.

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ touch file1

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ ls -l
total 0
-rw-r--r-- 1 kali kali 0 Apr  3 21:22 file1
-rw-r--r-- 1 kali kali 0 Apr  3 21:20 file2

Copy, Move, and Rename

copy

Command cp copies files and directories

┌──(kali㉿kali)-[~/Documents]
└─$ cd TRAINING

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ ls
file1  file2

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ cp file1 file3

┌──(kali㉿kali)-[~/Documents/TRAINING]
└─$ ls
file1  file2  file3

┌──(kali㉿kali)-[~/Documents]
└─$ cp -r TRAINING ../

┌──(kali㉿kali)-[~/Documents]
└─$ cd ..

┌──(kali㉿kali)-[~]
└─$ ls

Desktop  Documents  Downloads  Music  Pictures  Public  TRAINING

Move

Command mv moves the files. Moving means that it copies the file(s) from original path to a new path. It will then removes the file from the old path. Analogous to cut and paste.

┌──(kali㉿kali)-[~/Documents]
└─$ mv TSHOOT../

┌──(kali㉿kali)-[~/Documents]
└─$ ls

┌──(kali㉿kali)-[~/Documents]
└─$ cd ..

┌──(kali㉿kali)-[~]
└─$ ls
Desktop  Documents  Downloads  Music  Pictures  Public  TRAINING  TSHOOT

Rename

Command mv also renames the files. You only need to specify the new name to the final destination

┌──(kali㉿kali)-[~/TRAINING]
└─$ ls
file1  file2  file3

┌──(kali㉿kali)-[~/TRAINING]
└─$ mv file3 file4

┌──(kali㉿kali)-[~/TRAINING]
└─$ ls
file1  file2  file4

echo

The echo command displays a line of text

┌──(kali㉿kali)-[~]
└─$ echo "Hello World"
Hello World

Environment Variable

Environment Variable is a variable which holds a value. Here are some environment variable when you open a new terminal:

• USER
• PWD
• HOME
• PATH

When we want to call a variable we prepend a $ sign to that variable.

┌──(kali㉿kali)-[~]
└─$ echo $USER
kali


┌──(kali㉿kali)-[~]
└─$ echo $PATH
/usr/local/sbin:/usr/sbin:/sbin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games:/home/kali/.dotnet/tools

The command env shows you the Environment Variable defined by Kali Linux.

┌──(kali㉿kali)-[~]
└─$ env

USER=kali
LOGNAME=kali
HOME=/home/kali
PATH=/usr/local/sbin:/usr/sbin:/sbin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games:/home/kali/.dotnet/tools
SHELL=/usr/bin/zsh
TERM=xterm
DISPLAY=localhost:10.0
XDG_SESSION_ID=764
XDG_RUNTIME_DIR=/run/user/1000
DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
XDG_SESSION_TYPE=tty
XDG_SESSION_CLASS=user
MOTD_SHOWN=pam
COMMAND_NOT_FOUND_INSTALL_PROMPT=1
POWERSHELL_UPDATECHECK=Off
POWERSHELL_TELEMETRY_OPTOUT=1
DOTNET_CLI_TELEMETRY_OPTOUT=1
LANG=en_US.UTF-8
SSH_CLIENT=10.0.0.1 28203 22
SSH_CONNECTION=10.0.0.1 28203 10.0.0.80 22
SSH_TTY=/dev/pts/1
SHLVL=1
PWD=/home/kali
OLDPWD=/home/kali/TRAINING
LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:mi=00:su=37;
LESS_TERMCAP_mb=
LESS_TERMCAP_md=
LESS_TERMCAP_me=
LESS_TERMCAP_so=
LESS_TERMCAP_se=
LESS_TERMCAP_us=
LESS_TERMCAP_ue=
_=/usr/bin/env

cat

The command cat prints the content of the file on the standard output (You will learn more about standard output later)

┌──(kali㉿kali)-[~]
└─$ cat /proc/sys/kernel/version
#1 SMP Debian 5.15.15-2kali1 (2022-01-31)

head, tail

The command head prints the first 10 lines of a files. You can modify the default numbers of the lines with option -n. For example head -n 5.

┌──(kali㉿kali)-[~]
└─$ head -n5  /etc/passwd
root:x:0:0:root:/root:/usr/bin/zsh
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync

Conversely, the command tail prints the last 10 lines of the file. You can modify the default numbers of the lines with option -n. For example tail -n 3.

┌──(kali㉿kali)-[~]
└─$ tail -n 3 /etc/passwd
geoclue:x:132:140::/var/lib/geoclue:/usr/sbin/nologin
king-phisher:x:133:141::/var/lib/king-phisher:/usr/sbin/nologin
kali:x:1000:1000:Kali,,,:/home/kali:/usr/bin/zsh