Linux Basics

Commands and programs that all Linux users need to know (but many don't!)

TODO: Add screenshots/code examples for each command; put commands in tables; clean and organize all (issue #7)

Command-line Basics

Command	Description
man $command	Get help with a command
history	View history of commands that have been typed into the terminal
!<number>	Repeat a specific command from command history
Ctrl + r	Search through command history: then cycle with Up or Down arrows. (Do not need to type history command first)
alt + .	Cycle through previously used command arguments
ctrl + [arrow_keys]	Move between "words" on a command line
clear	Clear all text off the terminal window
echo $text	Print string to terminal. Most useful when piped into other commands. Can be used to display environment variables such as $USER, $HOME, $PATH
Ctrl + Shift + c	Copy selected text
Ctrl + Shift + v	Paste clipboard contents
lp $filename	Print from file to printer
cd $directory	Change directories
cd ..	Move up one directory
cd ~	Change directory to current user's home directory
cd -	Return to previous directory
exit	Exit terminal session

Special Symbols

Symbol	Purpose
|	Send the output of one command to another.
>	Redirect output to a file.
<	Redirect input from a file.
>>	Append output to an existing file.
/	Separator used in path names.
\	Used to escape characters and to send multi-line commands.
.	Current directory.
..	Parent directory.
$$	displays the process ID of the current shell instance.
&	Process command in the background (and give control of the terminal back).
&&	Run the next command only if the previous completed successfully.
*	Match any number of characters in file name.
?	Match any single character in file name.
[ ]	Match any one of the enclosed characters in file name.
;	Run commands in sequence, regardless if the previous succeeded.
( )	Group commands.
{ }	Used to feed multiple parameters to a single command. Separate parameters by ,
!	Followed by a digit will repeat the command from the history file that corresponds.
!!	Repeat the previous command.
0	Shortcut that stands for Standard Input (STDIN)
1	Shortcut that stands for Standard Output (STDOUT)
2	Shortcut that stands for Standard Error (STDERR)

Filesystem Basics

Standard Linux Directories and Files

Everything in Linux is a file, even directories and devices. Directories have some special restrictions, but for the most part can be treated like files.

Listing and viewing Directories and Files

Command	Description
ls -a	List files in a folder, to include hidden files: Hidden files in Linux begin with a . these files can still be accessed normally, but the . must be added to the name.
ls -la	List files with attributes (filesize, permissions, etc.)
ls -lS	List files, sorted by Size
ls -R	List files in current folder and all subfolders (Recursive)
find -L / -samefile $file	Locate all files that symlink to a file
which $file	Searches for files in a $PATH directory only.
locate $file	Uses a database to search for files. Update the database with sudo updatedb
df	List the size, used space, and available space on the mounted filesystems of your computer
cat $file	Print the contents of a file to the command line
cat $file1 $file2 > $newfile	Combine the contents of two text files
diff $file1 $file2	Compare two files and show differences (Only for text-based files)
grep $string $file	Search for string inside a file
head $file	Displays the first 10 lines of a file. Specify the number of lines with -#
tail $file	Displays the last 10 lines of a file. Specify the number of lines with -# -f - Update the output continuously.
file $file	Displays the filetype of a file, determined by the hexadecimal " magic bytes".

File and directory creation and deletion

Command	Description
touch $fileName	Create a new blank file with this name
cp $file [/path/to/]$newFile	Copy file from one location to another. If no location is specified, creates the copy in the same directory. [Path optional]
mv $file [/path/to/]$newFile	Move file from one location to another. If no location is specified, renames the file in same directory (removes the old file).
rm $file	Removes (deletes) a file.
rm *	Removes (deletes) all files in the directory.
rm -rf *	Recursively deletes all files in the directory and all subdirectories and files. Will not prompt for approval with -f.
mkdir [/path/to/]$dir	Makes a new empty directory
mkdir -p test/{test1,test2}	The -p flag creates multiple directories at once. In this example we use brace expansion to create test/ and two subdirectories under it simultaneously.
rmdir $dir	Deletes an (empty) directory
sudo rm --force $(which $file)	Removes all instances of a specified filename. Only searches PATH directories. You could also use find or locate instead of which to find more files. With --force will not prompt for approval!

File & text manipulation

Command	Description
cat $file1 $file2	Concatenates the contents of two files
wc	Counts the lines, words, and bytes in a file. -l will count only lines, -m will count only characters, -c will count only bytes, -w will count only words
awk	A programming language for text processing. Can do many many things.
sed	Performs text editing on a stream of text. Useful for replacing text in a file and much more. Example: Replace all occurrences of 1001 with 0 in /etc/passwd. sed -i -e 's/1001/0/g' /etc/passwd
cut	Extract a section of text. -f selects the field, -d sets the delimiter.
sort
uniq
comm $file1 $file2	Compare two files and show differences. Output is in three columns: Lines that are unique to the first file Lines that are unique to the second file Lines that are shared by both files.
diff $file1 $file2	Compare two files and show differences. Has two modes: -c Context format -u Unified Format
vimdiff $file1 $file2	Opens two files in Vim side-by-side and highlight the differences. Some shortcuts: [ctrl] w - Switch to the other split window do - Gets changes from the other window into the current one dp - Puts the changes from the current window into the other one ]c - Jump to the next change [c - Jump to the previous change

cat > $fileName
 [Type your file contents]
 [Press `Ctrl+d` to return to your terminal]

File Permissions

The permissions for a file (for example, viewed with the ls -l command) are typically written as:

-rwxrwxrwx owner group [metadata] $filename

r = read w = write x = execute

Breaking down this format gives us four parts:

1. The first character tells if it is a file or a directory. if it is a - (hyphen) then it is a file. However if the first character is a d, then the file is a directory. (Remember, technically everything in Linux is a file, even directories).

2. The next three characters specify the permissions of the owner of the file.

3. The following three characters specify the permissions of the group that owns the file.

4. The final three characters specify the permissions of all other users.

The permissions -rwxrwxrwx mean that the anyone can read, write and execute the file.

In the above example, the owner, group, and everyone permissions are all rwx; hence anyone can read, write, and execute this file.

The chmod command

The chmod command is used to set the permissions on a file. This is usually expressed in one of two different formats, ugoa+rwx and octal notation. The command is used as follows:

chmod [permissions] $file

Octal notation

In octal notation, the permissions are assigned using triple octal (base8) digits. The first digit is the cumulative permissions for the owner, the second for the group, and the third for everyone else.

Permissions	Binary notation	Octal notation	Description
---	000	0	No permissions
--x	001	1	Execute permission only
-w-	010	2	Write permission only
-wx	011	3	Write and execute
r--	100	4	Read permission only
r-x	101	5	Read and execute permission
rw-	110	6	Read and write permission
rwx	111	7	Read, write and execute

From the above table we can easily derive :

Read = 4    Write = 2     Execute = 1

Therefore, if you want to give only the owner read and write permissions, they would be assigned 600 (4+2=6).

Taking the same example from above, to assign the permissions -rwxrwxrwx the command would be:

chmod 777 $file

That is: read (4), write (2), and execute (1) permissions for the owner, group, and all others.

ugoa+rwx notation

In this notation format, there are three main components:

1. Who. The users to modify permissions for: u = user (owner), g = group, o = others, and finally a = u+g+o (all).

2. What. The modifier: = to set permissions, + for adding permissions, - for removing permissions.

3. Which. The permissions to set, add, or remove: one or more of rwx as above.

As you can see, this notations allows for easier and more explicit control over exactly which permissions are given to whom.

Examples:

To give all users the write permission:

chmod a+w $file

To remove write and execute permissions from the 'other' group:

chmod o-wx $file

These permission changes can also be chained by adding a comma between the permission changes.

To add read/write permissions for the file owner and group, while making it read only for everyone else:

chmod ug+rw,o=r $file

Advanced permissions (TODO: Finish cleaning this up. Add descriptions of SUID/GUID)

Other than just read and write, you can also set some other permissions like SUID and GUID.

chmod 4000 file

chmod +s file

Both the above examples would add the setuid bit to the file.

chmod 2000 file

chmod +g file

Both the above examples would add the getuid bit to the file.

The sticky bit

https://en.wikipedia.org/wiki/Sticky_bit <- pull more information from here and add

The "sticky bit" is added to folders in order to prevent anyone else from deleting the folder or any of its contents. It is represented by a t at the end of the permissions d--r--r--rt. When a sticky bit is set, nobody other than the owner or the root can delete the folder or the file.

chmod 1000 folder

chmod +t folder

Both the above examples set the sticky bit to the folders

Examples: chmod 1744 file

This would set the sticky bit, give all permissions to the owner and only read permission to the group and others

chmod 0600 file

This would only give the owner read and write permission, but not execute permission.

The chown command

The chown command can be used to change the owner of a file or a directory.

chown $user $group $file

The above command would change the owner of the file to $user and also the group to $group.

File Attributes

Read attributes of files on Linux with lsattr

lsattr lists the file attributes on a second extended file system. See chattr below for a description of each attribute.

Useful options:

Argument	Description
-R	Recursively list attributes of directories and their contents.
-a	List all files in directories, including files that start with . (hidden files).
-d	List directories like other files, rather than listing their contents.
-l	Print the options using long names instead of single character abbreviations.

You can chain together these options to recursively list the attributes of all files and folders in a directory with long names:

lsattr -Ral /home/

Change attributes of files on Linux with chattr

chattr changes the file attributes on a Linux file system.

The format of a symbolic mode is +-=[aAcCdDeFijmPsStTux].

Symbol	Meaning
+	Add the following attributes the to specified file
-	Remove the following attributes from the specified file
=	Set the attributes of the specified file to be the following

The letters aAcCdDeFijmPsStTux select the new attributes for the specified files:

Attribute	Description
a	append only
A	no atime updates
c	compressed
C	no copy on write
d	no dump
D	synchronous directory updates
e	extent format
F	case-insensitive directory lookups
i	immutable
j	data journaling
m	don't compress
P	project hierarchy
s	secure deletion
S	synchronous updates
t	tail-merging
T	top of directory hierarchy
u	undeletable
x	direct access for files

The following attributes are read-only and may be listed by lsattr but not modified by chattr:

Attribute	Description
E	encrypted
I	indexed directory
N	inline data
V	verity

See the chattr manpage for more detailed descriptions of each attribute.

File compression and encryption (TODO:)

Command	Description
unzip
gunzip
tar

System Information

Command	Description
uname -a	List OS, hostname, kernel build number, CPU architecture
ps	List running processes (current user)
ps aux	List running processes for all users (if permitted)
top	Similar to Windows Task Manager, lists running processes with details of hardware usage
systemctl list-unit-files	Show list of all services installed with status

Networking

Command	Description
ifconfig	Get networking information (IP, Subnet mask, MAC, etc.); On some systems may require sudo rights
ip a	Get networking information (IP, Subnet mask, MAC, etc.); No sudo required. Newer
ifconfig $interface $ip/$CIDR	Set IP address for an interface
ifconfig $interface mtu $size	Change MTU size for an interface
ifconfig $interface hw ether $new_MAC	Change MAC address (or use macchanger)

Managing connections

TODO: add more information about Managing connections in Linux (Issue #9)

• Add commands such as telnet, SSH, nc, curl, wget

• Add commands for listing information about open network connections: lsof -i, ss, netstat

• include description and examples

Command	Description
telnet
ssh
nc
curl	Transfer data to or from a server using a variety of protocols including IMAP/S, POP3/S, SCP, SFTP, SMB/S, SMTP/S, TELNET, TFTP, and others.
wget	Downloads files using the HTTP,HTTPS, or FTP protocols.
axel	Download files using concurrent connections -a - Show progress indicator -n # - # number of connections to use -o - Specify the output file's name

nc listener: nc -lvnp <port>

list open network connections

Command	Description
lsof -i
ss	Shows State, data sent/recieved, local process:port, remote address:port
ss -anlp	Get all connections that are listening, do not resolve names, show process information
netstat

Shared folders

Command	Description
showmount -e $ip	Show available shares to mount
smb://$ip/$share_name	Connect to Windows SMB share folder

TODO: add more information on mounting and using network shares (issue #10)

• Add information on creating, mounting, and connecting to network shares (Samba, SMB, etc.)

• pull more information from HTB Writeups (Resolute, Remote, and Fuse for example, as well as outside sources)

• Add commands such as smbclient, smbpass, showmount, mount, Downloading Files, Detecting ADS, smbmap,

DNS

Command	Description
dig @$server $domain_or_ip $record_type	Look up DNS information for a site
dig -x $ip	Reverse look up a domain from an IP
host $hostname	Look up the IP address for a host- or domain-name.

Installing and Managing Programs

Command	Description
sudo apt update	Update repository database
sudo apt upgrade	Update installed programs and packages (must update repository database first). Adding -y will accept all prompts and install automatically. Specifying a package name after "upgrade" will upgrade only that package.
sudo apt dist-upgrade
sudo apt full-upgrade
apt search $keyword	Search for packages (unknown name) to install from repositories
apt-cache search $keyword	Search for package in repositories
apt show $package	Show details about the specified package
sudo apt install $package	Installs the specified package (and any dependencies).
sudo apt remove --purge $package	Uninstalls the specified package
dpkg -i $deb_file	Installs the specified .deb package file (Does not install dependencies).
alien $file.rpm	Convert rpm to Debian packages

Users and Groups

TODO: Add information about Linux Users and Groups (issue #11)

• Add information about creating, modifying, and deleting users and passwords

• Add information about use, creation, modification, and deletion of groups

• Add commands such as adduser, groups, passwd, etc.

• Add commands related to listing users, seeing who is logged in, etc. (id, w, last -a)

• add descriptions and examples

Command	Description
env	Displays all environment variables for the current context.

groups

Add a new user: adduser

addgroup

id

w

last -a

User Privileges

Command	Description
sudo $command	Execute commands with elevated privileges
sudo -u $username $command	Execute sudo command using another user's privileges
sudo -l	List sudo privileges for current user with
sudo -k	Stop remembering credentials and re-prompt for password
/etc/sudoers	Configuration file for sudo

Environment Variables

Command	Action
env	List all current environment variables and their values.
export [variable_name]	Define the value of an environment variable. Can be a new or existing variable. Exported variables only work in the context of the current shell.
echo $PATH	List the values in the PATH environment variable.
echo $USER	Show the current username.
echo $PWD	Show the current working directory.
echo $HOME	Show the current user's home directory
echo "$$"	Show the process ID of the current shell.
stty size	Show number of rows and columns in the current shell.

$PATH

• To make $PWD part of path so you don't need ./ when running commands/scripts: (NOT RECOMMENDED for home/production use!) export PATH='pwd':$PATH

• Add new $PATHs to the .profile file rather than .bashrc. Then, use the command source ~/.profile to use the newly added PATHs.

• The best way to add a path (e.g., ~/opt/bin) to the PATH environment variable is:

export PATH="${PATH:+${PATH}:}~/opt/bin"
#for appending (instead of PATH="$PATH:~/opt/bin")

export PATH="~/opt/bin${PATH:+:${PATH}}"
#for prepending (instead of PATH="~/opt/bin:$PATH")

(from https://unix.stackexchange.com/questions/26047/how-to-correctly-add-a-path-to-path)

$HISTCONTROL

The HISTCONTROL environment variable can be used to whether the bash history removes duplicate commands, commands that start with a space, or both. The default behavior is to remove both.

export HISTCONTROL=ignoredups

ignoredups - Ignore Duplicates

$HISTIGNORE

The HISTIGNORE environment variable can be used to filter commands so they do not appear in the history.

export HISTIGNORE="ls:[bf]g:exit:history"

This example causes the history command to not log common commands such as ls,bg,fg,exit,and history. Uses standard bash text shortcuts such as [ ] to indicate options.

$HISTTIMEFORMAT

The HISTTIMEFORMAT environment variable controls date/time stamps in the output of the history command.

export HISTTIMEFORMAT='%F %T '
#show date and time before each command in history

Startup Scripts

Add script to run at startup: update-rc.d </path/to/the/script> defaults (needs 755 permissions)

Delete script from default autorun: update-rc.d -f </path/to/the/script> remove

Make a Linux live boot USB

On Windows (easiest way!):

1. Download and run Rufus.

2. Select the USB device and ISO you want to use, giving the volume a name if you wish.

3. If you want to use persistence,

   1. Click "Show advanced drive options".

   2. Select the amount of storage to use for persistence.

4. Click "Start" and wait for it to finish.

For Kali live persistent boot USBs you will need the additional step of adding a persistence.conf by following the instructions below.

1. Verify your USB devices persistent storage partition with the command fdisk -l.

2. After locating your partition (in this example it is /dev/sdb3), label it persistence.

   e2label /dev/sdb3 persistence

3. Create a mount point, mount the new partition there, and then create the configuration file to enable persistence. Finally, unmount the partition.

   mkdir -p /mnt/my_usb
   mount /dev/sdb3 /mnt/my_usb
   echo "/ union" > /mnt/my_usb/persistence.conf
   umount /dev/sdb3

Recover an unresponsive terminal

1. Press the RETURN/ENTER key.

   You may have typed a command but forgotten to press RETURN to tell the shell that you’re done typing and it should now interpret the command.

2. If you can type commands, but nothing happens when you press RETURN, try typing CTRL-J. If this works, your terminal needs to be reset to fix the RETURN key. Some systems have a reset command that you can run by typing CTRL-J reset CTRL-J. If this doesn’t work, you may need to log out and log back in or turn your terminal off and on again.

3. If your shell has job control type CTRL-Z.

   This suspends a program that may be running and gives you another shell prompt. Now you can enter the jobs command to find the program’s name, then restart the program with fg or terminate it with kill.

4. Use your interrupt key (typically DELETE or CTRL-C).

   This interrupts a program that may be running. (Unless a program is run in the background as the shell will wait for it to finish before giving a new prompt. A long-running program may thus appear to hang the terminal.) If this doesn’t work the first time, try it once more, though doing it more than twice usually won’t help.

5. Type CTRL-Q.

   If output has been stopped with CTRL-S, this will restart it. (Note that some systems will automatically issue CTRL-S if they need to pause output; this character may not have been typed by the user from the keyboard.)

6. Check that the SCROLL LOCK key is not toggled on.

   This key stops the screen display from scrolling upward. If pressing it once does not work, make sure you’ve pressed it an even number of times as this leaves the key in the same state it was when you started.

7. Type CTRL-D at the beginning of a new line.

   Some programs (like mail) expect text from the user. A program may be waiting for an end-of-input character from you to tell it that you’ve finished entering text. However, typing CTRL-D may cause you to log out, so you should only try this as a last resort.

8. If you’re using a windowing system, close (or terminate) the terminal window and open a new one.

Fixing command-not-found errors

https://stackoverflow.com/questions/19873430/command-not-found-message-when-i-try-to-add-command-in-bashrc/26976325

If you encounter errors on your system when you mistype a command or try to run a program that is not installed try these steps to fix the command-not-found command.

TODO: screenshot or type out example so people know what I am referring to...

sudo apt purge command-not-found #uninstall
sudo apt install command-not-found #reinstall
sudo update-command-not-found #rebuild the database
sudo chmod ugo+r /var/lib/command-not-found/commands.db* #fix database permissions

https://bugs.launchpad.net/command-not-found/+bug/1824000

Fork Bomb

A fork bomb is a type of denial-of-service attack against Unix-based systems, which makes use of the fork operation (or equivalent functionality) whereby a running process spawns another running process indefinitely. This attack works by creating a large number of processes very quickly in order to saturate the available resources of the operating system.

Once this code is executed, within seconds the target system will freeze and will have to hard rebooted.

A common succinct bash fork bomb looks like:

:(){:|:&};:

Which can be explained as:

Function	Description
:()	define a function named : . Whenever we call :, execute the commands inside the { }
`:	:`	load a copy of the : function into memory and pipe its output to another copy of the : function, which has to also be loaded into memory separately.
&	Disowns the other functions. If the first : is killed, all of the functions that it started should NOT also be killed.
;	Ends the function definition and tells the interpreter to run what is next as a command
:	Call function : initiating a chain-reaction: each call of : will start two more

It can also be written as:

forkbomb() { forkbomb | forkbomb & } ; forkbomb

References

• https://www.kali.org/docs/usb/kali-linux-live-usb-persistence/

• https://linuxconfig.org/linux-tutorials

• https://explainshell.com/

• https://unix.stackexchange.com/questions/26047/how-to-correctly-add-a-path-to-path

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