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
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
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
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
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
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:
-cContext format-uUnified 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 windowdo- Gets changes from the other window into the current onedp- Puts the changes from the current window into the other one]c- Jump to the next change[c- Jump to the previous change
File Permissions
The permissions for a file (for example, viewed with the ls -l command) are typically written as:
r = read w = write x = execute
Breaking down this format gives us four parts:
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 ad, then the file is a directory. (Remember, technically everything in Linux is a file, even directories).The next three characters specify the permissions of the owner of the file.
The following three characters specify the permissions of the group that owns the file.
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:
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.
---
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 :
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:
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:
Who. The users to modify permissions for:
u= user (owner),g= group,o= others, and finallya= u+g+o (all).What. The modifier:
=to set permissions,+for adding permissions,-for removing permissions.Which. The permissions to set, add, or remove: one or more of
rwxas 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:
To remove write and execute permissions from the 'other' group:
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:
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.
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:
-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:
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].
+
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:
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:
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:)
unzip
gunzip
tar
System Information
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
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
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
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
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.)
Add commands such as
smbclient,smbpass,showmount,mount, Downloading Files, Detecting ADS,smbmap,
DNS
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
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
env
Displays all environment variables for the current context.
groups
Add a new user: adduser
addgroup
id
w
last -a
User Privileges
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
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
$PWDpart of path so you don't need./when running commands/scripts: (NOT RECOMMENDED for home/production use!)export PATH='pwd':$PATHAdd new $PATHs to the
.profilefile rather than.bashrc.Then, use the commandsource ~/.profileto use the newly added PATHs.The best way to add a path (e.g., ~/opt/bin) to the PATH environment variable is:
(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.
ignoredups - Ignore Duplicates
$HISTIGNORE
The HISTIGNORE environment variable can be used to filter commands so they do not appear in the 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.
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!):
Download and run Rufus.
Select the USB device and ISO you want to use, giving the volume a name if you wish.
If you want to use persistence,
Click "Show advanced drive options".
Select the amount of storage to use for persistence.
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.
Verify your USB devices persistent storage partition with the command
fdisk -l.After locating your partition (in this example it is
/dev/sdb3), label itpersistence.Create a mount point, mount the new partition there, and then create the configuration file to enable persistence. Finally, unmount the partition.
Recover an unresponsive terminal
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.
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.
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.
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.
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.)
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.
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.
If you’re using a windowing system, close (or terminate) the terminal window and open a new one.
Fixing command-not-found errors
command-not-found errorsIf 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...
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:
:()
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:
References
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