Cyber Security Multi-part Question

 

 

 

 

Student Name:

 

An Overview of Linux

A typical Linux operating system is made up of three parts; the kernel, the shell and the applications.

Kernel: The kernel of UNIX is the hub of the operating system: it allocates time and memory to programs and handles the file store and communications in response to system calls.

Shell: The shell acts as an interface between the user and the kernel. When a user logs in(into command line only environment), the login program checks the username and password, and then starts another program called the shell. The shell is a command line interpreter (CLI). It interprets the commands the user types in and arranges for them to be carried out. The commands are themselves programs: when they terminate, the shell gives the user another prompt . A graphical user interface for shell is called Terminal.

Desktop Environment: Linux operating systems also have a graphical user interface (GUI) through which users can access applications and system resources. Unlike other operating systems Linux OSs has many Desktop Environments available. Users are free to install the GUI of their choice. Some popular ones are GNOME, KDE, XFCE, Ubuntu Unity etc. When you install a Linux System it comes with a default GUI. You can install other GUIs any time you want but obviously you can only use one GUI environment at a time. The Kali Linux comes with GNOME.

Linux Directory Structure: In linux files and directories (folders) are laid out on disk in hierarchical fashion in a tree structure. Top directory is called root folder, it is denoted by  “/”  (slash).  The sum of all files and directories in a Linux machine is technically referred as “file-system”.

Path (location) of a file or directory on the file-system is written starting from root the directory (/) followed by all intermediate directory names separated by a slash with the actual file name (or directory name) in the end of the path. Ex: the file path of the file “report.doc” in above figure is written as /home/its/ug1/ee51vn/report.doc

Notes:

  1. In the path /home/its/ug1/ee51vn/report.doc, do not get confused between first slash / which denotes the root directory and subsequent slashes-/ which are used to separate directories at each level of path.
  2. There are no spaces in a file path. Ex: The file path “/home/its/ug1/ee51vn/report.doc” is one single string with no spaces between slashes and subsequent folder names

 

Question 1: Write path for directory staff  in above figure

There are different default directories under the / directory which are used for specific purpose.

/bin : all executable binary files (including commands)are present in this directory.

/dev : contains device files for hardware devices (usb, cdrom etc.)

/etc : Contains Application’s configuration files, startup, shutdown, start, stop script for every individual program.

/home : Home directory of the users. Every time a new user is created, a directory in the name of user is created within home the directory which contains other directories like Desktop, Downloads, Documents, etc.

/lib : The Lib directory contains kernel modules and shared library images required to boot the system and run commands in the root file system.

/lost+found : This Directory is installed during installation of Linux, useful for recovering files which may be broken due to unexpected shut-down.

/media : Temporary mount directory is created for removable devices viz., media/cdrom.

/mnt : Temporary mount directory for mounting file system.

/opt : Optional is abbreviated as opt. Contains third party application software. Viz., Java, etc.

/proc : A virtual and pseudo file-system which contains information about running process with a particular Process-id aka pid.

 

Linux-Intro

There is huge number of flavors of Linux Operating Systems i.e. Linux is not a single operating system. But all of them are created from same Linux Kernel. An estimate is there are around 3500 types of Linux OSs! such as Kali, Redhat, Ubuntu, Debian, CentOS etc.

 

You will use Ubuntu 20.04 for this lab

You can download the OS from OSboxes.org.

 

 

Use following credential for OS login

Username: osboxes

Password : osboxes.org

Sudo Password: osboxes.org

 

Alternately, you can download the zipped version of image from this link Download the VDI from this link : https://sourceforge.net/projects/osboxes/files/v/vb/55-U-u/20.04/Ubnt-20.04-VB-64bit.7z/download

 

 

You need to install virtualbox from this link:

https://www.virtualbox.org/wiki/Downloads

 

 

Download ubuntu 20.04 from osboxes.org and extract the file in a drive/folder. Now create a virtual machine in virtual box. Click on Machine-> New

 

 

 

On Create Virtual machine window give a suitable name select machine folder, select type “Linux” and Version “Ubuntu 64 bit” and click Next

 

Provide at least 2 gb ram and click Next

 

 

 

 

 

 

 

 

 

Select “Use an existing virtual hard disk file” and click on the icon as shown below

 

 

Press Add button and add the disk image previously downloaded/extracted from osboxes.and click on open as shown below

 

 

 

 

 

 

 

 

 

Select ubuntu 20.04 from the hard disk selector window and click on “choose”

 

 

Now, click on create

 

 

 

Now start the machine

 

Go to the Network section and change the Attached to NAT

 

 

Press Ok button and go again to the Machine option and press Reset.

 

 

This is how the default desktop environment on a Ubuntu 20.04 Desktop OS looks like.

 

  1. To Open Terminal: Click on activities and type terminal. Click in the “Terminal” icon.
  2. To Open Web Browser: Click on the FireFox Browser

Linux Terminal -Intro

 

  1. Open Terminal
  2. You will see a Terminal window like this one:

 

 

The text [email protected]:~$ is the shell prompt. It means user osboxes, is logged into machine osboxes. $ indicates normal user. If you find # instead of $ then it is a root user.

 

The ~ (tilde) symbol indicates that the user is currently in his home directory. For the root user the home directory is /root. For the user “osboxes” the home directory would be /home/osboxes. In later sections I’ll explain the difference between a super user root and a normal user.

 

 

Linux Command Format:

A typical linux command has 3 parts: command, options and arguments. Options starts with “-“ symbol (hyphen or minus symbol). There are always spaces in between command, option and arguments

Ex: “ls -l /home”. In this “ls” is command, “–l”  is option, “/home” is argument. This command lists all file and folders in the folder /home.  Note that there are always spaces in between command, arguments and options. You have to type this command in this way: ls<space>-l<space>/home.

 

Notes:

  1. To separate commands from normal text I used double quotes but when you are typing commands in shell do it without quotes. DO NOT USE QUOTES. In every tutorial (on line tutorials or Text Books) it is customary to use some kind of text scheme to separate command from explanation. By end of the lab you’ll be able to differential between a normal text and a Linux command.

 

  1. The italic part in command is a sample name – you have to use the actual name according to your environment. For example if I asked you to type “/home/username, if your username is “bob” you have to type /home/bob.

 

  1. All Commands in Linux are case sensitive. pwd and Pwd are not same.

User Account Management

  1. Creating a new user account for you:
  2. Type “sudo su”. The system will ask for the root password. Type “osboxes.org” as the password. The shell prompt will change to # and you are working now as a user root.
  3. Type “adduser username”. Give a desired user name in place of In the below screenshot, I have given my username as alice.
  4. Enter a password for your account. You have to do it twice.

Note: Passwords are not shown in terminal as you type them. Usually while typing password you’ll see *****(stars) or in ……… (dots). But here you don’t see anything on a Linux Terminal.

 

  1. Add further user details. These details are optional-you can just hit enter to skip for Full Name, Number, Phone etc.) –See the below screenshot.
  2. Type “y” at “Is this information correct?”

 

  1. To change any user password use passwd command

Ex: passwd alice

  1. Add new user to sudoers file to give the user super user permissions.
  2. Type “nano /etc/sudoers”. (nano is command line text editor, it is explained in detail in Text Editors section of this Lab document).
  3. In the file add following line:

Username ALL=(ALL:ALL) ALL. You add this line next to line the

root ALL=(ALL:ALL) ALL. My username is alice. You have to give your newly created username.

  1. Press Control+O, Press Enter, when asked for confirmation, Press Control+X to exit
  2. Close the terminal window.
  3. Go to the upper menu and press in the right corner icon and click on poweroff.

In the new window click Restart

 

 

  1. Login with your new user after the VM has restarted.

Question 2: Login to your new user account, open a terminal and type “sudo su”, enter sudo password for your account, and submit a screen shot. Screenshot should clearly show the prompt. It looks like [email protected]:/home/alice.

  1. Type exit from the current root user prompt
  2. Create another user from your account.
    1. Open Terminal after logging into your account
    2. Type “sudo adduser bob”. Enter sudo password for your account.

Note: the user password is asked when you use the “sudo” command for first time in a terminal. “sudo” is used to elevate a normal user to root user for execution certain high privilege commands.

  1. Add a (unix) password for new user bob twice and rest of the details as earlier.

 

  1. To change to root user from normal user
  2. Type “sudo su –“ .
  3. Type the root user password at the prompt. After successful login, you should observe a change in shell prompt from your user to root user.
  4. Type “exit” to exit from root user. Now you should be back to you user prompt.

 

Question 3: Type “tail /etc/passwd” (/etc/password is one string –no spaces in between.)

Submit a screenshot of the above step.

 

Root user is administrator of system and has full privileges by default, while normal user has limited privileges. He is referred to super user in Linux. Normal user need to use “sudo” command to gain certain high privilege commands, temporarily. Ex: to install software, to change to root user in terminal, to change system settings etc.

Usage of Some Basic Commands:

Files and Directories

  1. pwd – present working directory

Type “pwd” at command prompt (without quotes). This command displays the directory in which you are currently. When a user starts a shell the default directory is user’s home directory: /home/username. (Username is your login name with which you logs into the Kali VM).

 

  1. cd – change directory

This command is used to change to a desired directory from current directory.

  1. Type “cd /home” (without quotes). This will move the user to the /home directory.
  2. Type “pwd”. What is it showing
  3. Type “cd -”. This will take you back to previous directory. i.e your home directory
  4. You can use “cd ~” to go back to your home directory from. (~ Tilde symbol)
  5. . (single dot) refers to current directory, ..(double dots) refer to parent directory.

“cd .” keeps you in same directory, while “cd ..” moves to parent directory, a level above. “cd ../..” moves you two levels above the current directory.

  1. ls – list

ls command simply lists file and directories in present directories. Option “–l” which gives more details about files which include file size, time created, file permissions etc

  1. Type “ls –l”

To display details of a directory or file: ls –l directory or file name. Ex: ls –l Desktop

  1. Type “ls –la”

 

Question 4: what output “ls –la” gives (take a screenshot)? What difference did you observer between the results of ls –l and ls –la? What is option “–a” means (hint: refer to man page: man ls. See man help section below. Or you can just google)

 

  1. mkdir– to create a directories
  2. Type “mkdir firstdir”. It creates a new directory called firstdir. You can also create multiple directories and also nested directories
  3. Type “mkdir dir1 dir2 dir3.” It creates 3 directories in present directory.
  4. Type “mkdir –p dir11/dir22/{dir33, dir44}” . It creates 4directories: dir11, dir22 inside dir11, dir33 and dir44 inside dir22.
  5. Type “ ls”

 

  1. rm-delete a file or directory
  • type “rm –r dir3”. You can also use “rmdir” but it only deletes empty directories

 

  1. touch-to create empty files
  • type “touch file1 file2 file3”.
  1. mv-to move or rename files and directories
  2. type mv file1 dir1. this moves (cut and paste) the file file1 to directory dir1
  3. type mv file2 file22.
  4. Type “ls”
  5. cp- copy files and directories

Type the following commands in order

“touch file4” .Creates a file

“mkdir dir4” .creates a directory dir4

“cp file4 dir4” .copies file4 to dir4

“mkdir dir5” .creates another directory dir5

“cp –r dir4 dir5” .Copies dir4 to dir5

 

Question 5: Why do we have to use “-r” option to copy directories (Hint use man cp)?

 

  1. find– to find files and directories.
  • Type “find /root –name file1.”

 

Question 6: What is the result?

 

You are searching for file called file1 in directory /root. This command will search for the file in /root and in all its sub directories. You gave /root as search location as you think file1 must be somewhere in it. You are totally clueless where the might be on file system, you should start searching for it from top most directory in file-system, i.e /. the command would be find / -name file1

 

Absolute and relative path

  1. The absolute path of a file or directory is its path written from the root of the file system. For example, consider the directory dir44 you created earlier in Step 5. Its absolute path is /home/username/dir11/dir22/dir44.

 

  1. Relative path of file or directory is its path from the current directory. Now you are in your home directory which is /home/The relative path of dir44 from this directory is dir11/dir22/dir44. You start writing relative path from directory directly below i.e. dir11

 

  1. You can access a file or directory either using relative path or absolute path
  • Type “touch /home/username/dir11/dir22/dir44/file44”.(Replace username you actual username).
  • Type “dir11/dir22/dir44/file55”

Either way you are able can access dir44

 

Note: Relative path only makes sense if the object you are accessing below your current directory. Else you have to use the Absolute path. For example, you have to access /etc/wireshark/init.lua file, you don’t have any relative path to it from your home directory. But you can use a relative path you are in / or /etc.

 

File Compression and archiving

  1. To compress and archive files: In Linux file compression and archiving (grouping file together) and compression are two different activities. First, we archive multiple files with “tar” command. A file with “.tar” extension is created. Then we zip (compress the file) with gzip command. (gzip-stands for gunzip is compression tool in linux. Bunzip (bzip2) and zip are other commonly used compression tools)

 

  1. Type “touch a b c d”. Creates 4 files a, b, c and d
  2. tar –cf compressed.tar a b c d. We are creating a tar file tar from files a b c and d
  3. gzip –vf compressed.tar. This should create a file compressed.tar.gz

Note:  For “tar” command, you should supply output filename along with .tar extension. For gzip, it automatically creates a zipped file with .gz extension. You no longer have .tar file after you compress it with gzip.

 

  1. You can also do it in single step which is usually the preferred way:

type “tar –zcvf compressed2.tar.gz a b c d”.

 

  1. To extract and uncompress files
  2. Type “rm –rf a b c d”. Deleting the files
  3. Type “ls”
  4. Type “gzip –dvf compressed.tar.gz”-you’ll gettar file
  5. Type “tar –xvf compessed.tar”. You’ll get original files a, b, c and d
  6. type “ls”
  7. You can just uncompress and extract the compressed2.tar.gz in a sigle step:
  8. Type “tar –zxvf compressed2.tar.gz”

Getting help- the “man” command:

“man” command is used to get help for any command in Linux. man stands for manual. To pull up a man page of a command, type man command.  Ex: man ls. This command displays list of options available with the command. Man pages explain different options and syntax of a command and are usually difficult to understand for beginners. If you want to know usage and example for a command, google is the best place to look.

 

Type “q” to quit from man page.

File Permissions and Access Control

In Linux a file has read, write and execute permissions. And permissions are assigned to users of three categories: owner, group and others. The user who creates a file will be the owner, group is group of users who has access. Others are all the users other than owner and users in the group

  1. Understanding file/directory permissions:

Type ls –l filename to list file permissions.

Below illustration explains various fields of a file details printed

Each file (and directory) has associated access rights, which may be found by typing ls -l. Also, ls -lg gives additional information as to which group owns the file (beng95 in the following example):

-rwxrw-r– 1 ee51ab beng95 2450 Sept29 11:52 file1

In the left-hand column is a 10 symbol string consisting of the symbols d, r, w, x, -, and, occasionally, s or S. If d is present, it will be at the left hand end of the string, and indicates a directory: otherwise – will be the starting symbol of the string.

The 9 remaining symbols indicate the permissions, or access rights, and are taken as three groups of 3.

  • The left group of 3 gives the file permissions for the user that owns the file (or directory) (ee51ab in the above example);
  • the middle group gives the permissions for the group of people to whom the file (or directory) belongs (eebeng95 in the above example); For every user in Linux,a group with the same name is also created. This group will be default group on newly created files.
  • the rightmost group gives the permissions for all others.

The symbols r, w, etc., have slightly different meanings depending on whether they refer to a simple file or to a directory.

 

  1. Access rights on files.
  • r (or -), indicates read permission (or otherwise), that is, the presence or absence of permission to read and copy the file
  • w (or -), indicates write permission (or otherwise), that is, the permission (or otherwise) to change a file
  • x (or -), indicates execution permission (or otherwise), that is, the permission to execute a file, where appropriate

 

  1. Access rights on directories.
  • r allows users to list files in the directory;
  • w means that users may delete files from the directory or move files into it;
  • x means the right to access files in the directory. This implies that you may read files in the directory provided you have read permission on the individual files.
  • So, in order to read a file, you must have executed permission on the directory containing that file, and hence on any directory containing that directory as a subdirectory, and so on, up the tree.

Some examples

-rwxrwxrwx a file that everyone can read, write and execute (and delete).
-rw——- a file that only the owner can read and write – no-one else
can read or write and no-one has execution rights (e.g. your
mailbox file).

 

  1. Changing access rights(permisions) on a file
  2. chmod (changing a file mode):
  3. Only the owner of a file can use chmod to change the permissions of a file. The options of chmod are as follows
Symbol Meaning
u user
g group
o other
a all
r read
w write (and delete)
x execute (and access directory)
+ add permission
take away permission

 

  1. type “touch access” (to create a file called access)
  • type “ls –l access”
  1. To remove read write and execute permissions on the file access for the group and others

type  “sudo chmod go-rwx access”. This will leave the other permissions unaffected.

  1. To give read and write permissions on the file access to all,

Type “sudo chmod a+rw access”

 

  1. chgrp-this command is used to change the group of a file
  2. Type “sudo chgrp bob access”. This will change the default group on file “access” to bob from your username.
  3. Type “ls –l access”

 

  1. chown- this command is used change ownership of a file or directory. This can also be used to change the group like chgrp.
  2. Type “touch own_file”
  3. Type “ls –l own_file”
  • Type “sudo chown bob own_file”. This change owner of the file to bob from you.
  1. Type “mkdir own_dir”
  2. Type “ls –ld own_dir”. The –ld option displays properties of directory. If you only use –l it shows properties of contents of own_dir.
  3. Type “sudo chown bob:bob own_dir” .This change both owner and group to bob on folder own_dir.
  • Type “ls –ld own_dir”

 

                            Question 7: Submit a screen shot of terminal

 

Note that chown only change owner of directory but not its contents. To change ownership of files and directories within own_dir, along with is permissions use chown with “–R” option.

Ex: “sudo chown –R bob:bob own_dir”

 

Installing Software

  1. To install a software package apt-get command is used in Ubuntu Linux.
  • Type “sudo apt-get update”
  • Type “sudo apt-get install chromium” or  “sudo apt-get install chromium-browser”

It checks for file size to be downloaded and ask for confirmation. Type “y” and hit enter. This command installs Chromium web browser.

 

Question 8: Take a screenshot of the notification that installation is complete.

If you don’t know the exact name of the package you can search for it with “apt-cache” command

Ex: apt-cache search openoffice. Relevant packages will be shown.

 

  1. Installing software From GUI

You can also install software using Ubuntu Application icon on desktop

Click Applications

 

 

  1. To download a file from internet use wget command

Type

wget https://kekeseen.files.wordpress.com/2013/03/linux_commands_08.pdf

 

This will down the pdf file. Similarly, you can down load file from web page if its URL is known.

 

Text Editors-Nano & Vim

Nano:

  1. To open a file in nano:

Type “nano newfile”. This will open a blank file called newfile.

If there is already a file by name newfile nano will open that. Only if there exists no file by the name supplied it opens a new blank file with the supplied name.

  1. To edit and save

To write data to file, move cursor with arrow keys to location where you want to add content and start writing. Delete content using backspace.

Press Ctrl+O, it ask for confirmation: File name to write: newfile. Hit Enter.

  1. To exit nano

Press Ctrl+X

  1. To search for a string or keyword

Press Ctrl+W. A search: prompt appears at the bottom, type keyword you want to search and press enter.

Note: You have all the above explained controls at the bottom of the nano editor with brief description

 

 

  1. VI Editor:

 VI Editor has 2 different modes: Command Mode and Insert Mode.

  1. Command mode:

Type: “vi blankfile”. This opens a new file-blank file if a file doesn’t exist by the name. A file is opened into command mode by default. In this mode you can issue commands to edit text.

 

  1. Insert mode:

Hit letter “i”, which moves to insert mode from command mode. You can edit, add content to the file in this mode. Use arrow keys to navigate, backspace to delete.

To save changes to the file:

  1. Press Escape key-it takes file back to command mode.
  2. Press “Shift + :” A : prompt is created at bottom of file.
  • Type letter “w”-it saves your file

Note: If you don’t want save the file at this point but want to go back , hit escape-to go to command mode and then “i” to go to insert mode. You always have to be in command mode before you go to insert mode.

  1. Again Press “Shift + :”– and type “q” to exit vi editor. If file has unsaved modifications, vi asks for confirmation. Typing q! Forcefully quits without saving.

 

  1. More about Command mode: This mode takes commands from key board, to do operation like copy, paste, cut delete, etc. You can navigate using arrow keys.
  2. dd-delete current line (in which cursor is present). ndd –deletes n number of lines. Ex: 5dd
  3. yy-copy current line (in which cursor is present). nyy- copies n number of lines.
  4. p-paste copied line(s) below current line

 

  1. To print content of text file to terminal
  2. Type “cat newfile”. It will dump all the content in newfile to terminal.
  3. Type “less newfile”. This will open newfile on terminal but only partially display its content. Press enter to browse the file one line at a time. Press Q to exit less.

 

There are many other text editors like emacs, pico etc, but we limit our demonstration to nano and vi. There are many GUI test editors as well. A of couple them I mentioned in Kali Linux Intro Section.

 

List of Useful commands

This is not an exhaustive list of commands. But this list covers most of the commands which are regularly used.Note: Always use the command “man” to view a command’s manual pages. The command’s manual page gives a complete list of functional usage of a command. a  alias       Create an alias of a command   apropos  Search Help manual pages (man -k)  apt-get   Search for and install software packages (Debian/Ubuntu for Centos use yum)  aptitude Search for and install software packages (Debian/Ubuntu)  aspell     Spell Checker  awk        Find and Replace text, database sort/validate/indexb  basename Strip directory and suffix from filenames  bash    GNU Bourne-Again SHell   bc        Arbitrary precision calculator language   bg        Send to background  break   Exit from a loop •  builtin  Run a shell builtin  bzip2   Compress or decompress named file(s)c  cal      Display a calendar  case    Conditionally perform a command  cat      Concatenate and print (display) the content of files  cd       Change Directory  cfdisk  Partition table manipulator for Linux  chgrp   Change group ownership  chmod  Change access permissions  chown   Change file owner and group  chroot   Run a command with a different root directory  chkconfig System services (runlevel)  cksum   Print CRC checksum and byte counts  clear     Clear terminal screen  cmp      Compare two files  comm   Compare two sorted files line by line  command  Run a command – ignoring shell functions •  continue Resume the next iteration of a loop •  cp         Copy one or more files to another location  cron     Daemon to execute scheduled commands  crontab Schedule a command to run at a later time  csplit   Split a file into context-determined pieces  cut       Divide a file into several partsd  date     Display or change the date & time  dc        Desk Calculator  dd       Convert and copy a file, write disk headers, boot records  ddrescue Data recovery tool  declare  Declare variables and give them attributes •  df        Display free disk space  diff     Display the differences between two files  diff3    Show differences among three files  dig      DNS lookup  dir      Briefly list directory contents  dircolors Colour setup for `ls’  dirname  Convert a full pathname to just a path  dirs     Display list of remembered directories  dmesg Print kernel & driver messages   du       Estimate file space usagee  echo    Display message on screen •  egrep   Search file(s) for lines that match an extended expression  eject     Eject removable media  enable  Enable and disable builtin shell commands •  env      Environment variables  ethtool  Ethernet card settings  eval     Evaluate several commands/arguments  exec    Execute a command  exit     Exit the shell  expect Automate arbitrary applications accessed over a terminal  expand Convert tabs to spaces  export  Set an environment variable  expr     Evaluate expressionsf  false    Do nothing, unsuccessfully  fdformat Low-level format a floppy disk  fdisk   Partition table manipulator for Linux  fg        Send job to foreground   fgrep   Search file(s) for lines that match a fixed string  file      Determine file type  find     Search for files that meet a desired criteria  fmt      Reformat paragraph text  fold     Wrap text to fit a specified width.  for       Expand words, and execute commands  format Format disks or tapes  free     Display memory usage  fsck     File system consistency check and repair  ftp       File Transfer Protocol  function Define Function Macros  fuser    Identify/kill the process that is accessing a fileg  gawk     Find and Replace text within file(s)  getopts  Parse positional parameters  grep      Search file(s) for lines that match a given pattern  groupadd Add a user security group  groupdel  Delete a group  groupmod Modify a group  groups   Print group names a user is in  gzip       Compress or decompress named file(s)h  hash     Remember the full pathname of a name argument  head     Output the first part of file(s)  help      Display help for a built-in command •  history  Command History  hostname Print or set system namei  iconv Convert the character set of a file  id       Print user and group id’s  if       Conditionally perform a command  ifconfig Configure a network interface  ifdown Stop a network interface   ifup      Start a network interface up  import  Capture an X server screen and save the image to file  install  Copy files and set attributesj  jobs     List active jobs •  join     Join lines on a common fieldk  kill     Stop a process from running  killall Kill processes by namel  less    Display output one screen at a time  let      Perform arithmetic on shell variables •  link    Create a link to a file   ln       Create a symbolic link to a file  local  Create variables •  locate Find files  logname  Print current login name  logout  Exit a login shell •  look     Display lines beginning with a given string  lpc       Line printer control program  lpr       Off line print  lprint   Print a file  lprintd Abort a print job  lprintq List the print queue  lprm    Remove jobs from the print queue  ls         List information about file(s)  lsof     List open filesm  make    Recompile a group of programs  man      Help manual  mkdir   Create new folder(s)  mkfifo  Make FIFOs (named pipes)  mkisofs  Create an hybrid ISO9660/JOLIET/HFS filesystem  mknod  Make block or character special files  more     Display output one screen at a time  mount   Mount a file system  mtools  Manipulate MS-DOS files  mtr      Network diagnostics (traceroute/ping)  mv      Move or rename files or directories  mmv   Mass Move and rename (files)n  netstat  Networking information  nice      Set the priority of a command or job  nl          Number lines and write files  nohup   Run a command immune to hangups  notify-send  Send desktop notifications  nslookup Query Internet name servers interactivelyo  open   Open a file in its default application  op       Operator access p  passwd   Modify a user password  paste    Merge lines of files  pathchk  Check file name portability  ping     Test a network connection  pkill     Stop processes from running  popd    Restore the previous value of the current directory  pr        Prepare files for printing  printcap Printer capability database  printenv Print environment variables  printf   Format and print data •  ps        Process status  pushd  Save and then change the current directory  pv        Monitor the progress of data through a pipe   pwd     Print Working Directoryq  quota    Display disk usage and limits  quotacheck Scan a file system for disk usage  quotactl Set disk quotasr  ram      ram disk device  rcp       Copy files between two machines  read     Read a line from standard input •  readarray Read from stdin into an array variable •  readonly Mark variables/functions as readonly  reboot   Reboot the system  rename Rename files  renice   Alter priority of running processes   remsync Synchronize remote files via email  return  Exit a shell function  rev       Reverse lines of a file  rm       Remove files  rmdir    Remove folder(s)  rsync    Remote file copy (Synchronize file trees)s  screen   Multiplex terminal, run remote shells via ssh  scp     Secure copy (remote file copy)  sdiff   Merge two files interactively  sed     Stream Editor  select Accept keyboard input  seq     Print numeric sequences  set      Manipulate shell variables and functions  sftp    Secure File Transfer Program  shift   Shift positional parameters  shopt Shell Options  shutdown Shutdown or restart linux  sleep    Delay for a specified time  slocate Find files  sort      Sort text files  source  Run commands from a file ‘.’  split     Split a file into fixed-size pieces  ssh       Secure Shell client (remote login program)  strace   Trace system calls and signals  su         Substitute user identity  sudo     Execute a command as another user  sum      Print a checksum for a file  suspend  Suspend execution of this shell •  sync     Synchronize data on disk with memoryt  tail     Output the last part of file  tar      Store, list or extract files in an archive  tee      Redirect output to multiple files  test     Evaluate a conditional expression  time   Measure Program running time  timeout  Run a command with a time limit  times    User and system times  touch    Change file timestamps  top        List processes running on the system  traceroute Trace Route to Host  trap      Run a command when a signal is set(bourne)  tr          Translate, squeeze, and/or delete characters  true      Do nothing, successfully  tsort     Topological sort  tty        Print filename of terminal on stdin  type     Describe a command •u  ulimit     Limit user resources •  umask    Users file creation mask  umount  Unmount a device  unalias   Remove an alias •  uname    Print system information  unexpand Convert spaces to tabs  uniq      Uniquify files  units     Convert units from one scale to another  unset    Remove variable or function names  unshar   Unpack shell archive scripts  until      Execute commands (until error)  uptime  Show uptime  useradd  Create new user account  userdel   Delete a user account  usermod Modify user account  users       List users currently logged in  uuencode Encode a binary file   uudecode Decode a file created by uuencodev  v         Verbosely list directory contents (`ls -l -b’)  vdir    Verbosely list directory contents (`ls -l -b’)  vi        Text Editor  vmstat Report virtual memory statisticsw  wait     Wait for a process to complete •  watch  Execute/display a program periodically  wc       Print byte, word, and line counts  whereis  Search the user’s $path, man pages and source files for a program  which    Search the user’s $path for a program file  while    Execute commands  who      Print all usernames currently logged in  whoami   Print the current user id and name (`id -un’)  wget     Retrieve web pages or files via HTTP, HTTPS or FTP  write    Send a message to another user x  xargs    Execute utility, passing constructed argument list(s)  xdg-open Open a file or URL in the user’s preferred application.  yes        Print a string until interrupted  zip        Package and compress (archive) files.  .            Run a command script in the current shell  !!          Run the last command again  #           Comment / Remark  //           Comment / Remark

 

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