Basics
https://linuxjourney.com/
1. The Shell
Navigating
pwd → Print working directory
cd → Change directory (Ex: cd /home/isildur/Documents change to Documents directory)
cd .. → Change directory to parent directory
cd ~ → Change directory to your home directory
cd - → Go back to previous directory
Looking at files
ls → List files in directory
ls -la → Shows a detailed list of files in long format (-l) and lists all hidden files (-a)
file → Shows info about a file (what type of file it is) and a description of the file's contents
cat → Output the content of a file
less → Output the content of a file in a page manner, so you can navigate through a text file page by page
more → Output the file but hust a little bit at a time, similar to less
Working with files
touch → Create a new file
cp → Copy
mv → Move files and also renames them
mkdir → Make a directory
mkdir -p new/thisonetoo/and/this/one → Make entire directory structure
rm → Remove file
rm -rf ./directory → Remove recursively and its content. Very dangerous!
rmdir → Remove empty directory
General
history → Show commands history
sudo → Used before commands to execute this commands as root
sudo -l → List what rights the sudo user has
cat /etc/sudoers → Find which users have sudo rights
Finding Files
find → Slower command to locate files but a lot more thorough. You can search for files recursively and with regex and lof of more features
find /home -name picture.jpg → Find if there is a file named picture.jpg in the home directory
find /home -type d -name MyFolder → Find a directory (-type d) named MyFolder in the home directory
find / -name file 2>/dev/null → Send all permisions denied outputs to dev/null
locate → Locates files or directories really fast on an internal database
sudo updatedb → Updates database in order to run the locate command
which → Outputs the path of the binary that you are looking for, searching through the directories that are defined in you $PATH variable
2. String manipulation
stdout (Standard Out)
echo Hello World > new.txt → Adds "Hello World" to the new.txt file, overwritting it
echo Hello World >> new.txt → Adds "Hello World" to the new.txt file without overwritting it (appends)
stdin (Standard In)
cat < new.txt > other.txt → Redirects new.txt content as stdin to the other.txt file (< used for stdin redirection)
stderr (Standard Error)
ls /fake/directory 2> new.txt → Redirect stderr to the new.txt file
ls /fake/directory > new.txt 2>&1 → Redirect stderr and stdout to the new.txt file
Pipe and tee
ls -la /etc | less → Pipe less with the ls -la command
ls | tee new.txt → Displays the content of ls in the screen and inside the new.txt file
env (Environment)
echo $HOME → Path to your home directory
echo $USER → Returns username
env → Returns information about the environment variables
echo $PATH → Returns a list of paths separated by a colon that the system searches when it runs a command
Editing Text
sed "1d" → Removes first line of file/stream
cut -c 5 new.txt → Outputs the 5th character in each line of the new.txt file
cut -f 2 new.txt → Extract contents by a field, by default everything separated by a TAB is a field.
cut -f 1 -d ";" new.txt → Extract first field separated by ;
head → Outputs the first 10 lines of a file
head -n 15 → Outputs the first 15 lines of a file
tail → Outputs the last 10 lines of a file
tail -n 15 → Outputs the last 15 lines of a file
join file1.txt file2.tx → Joing file1.txt and file2.txt
sort new.txt → Sort new.txt in alphabetic order
sort -r new.txt → Sort new.txt in reverse alphabetic order
sort -n new.txt → Sort new.txt by numerical value
tr a-z A-Z → Translate all lower case character to uppercase characters
cat new.txt | tr -d "." → Remove all dots
cat new.txt | tr "." "_" → Remove all dots and replace them with underscores
uniq new.txt → Remove all adjacent duplicates in new.txt
uniq -c new.txt → Get the count of how many occurences are on a line
uniq -u new.txt → Gets only unique values
uniq -d new.txt → Gets only duplicate values
sort new.txt | uniq → Remove all duplicates in new.txt even if they are no adjacent
wc /etc/passwd → Shows the toal count of words in a file
nl new.txt → Check the count of lines on a file
grep pattern new.txt → Finds "pattern" inside the new.txt file
grep -i pattern new.txt → Finds case insensitive "pattern" in the new.txt file
ls /somedir | grep '.txt$' → Returns all files ending with .txt in somedir
awk '{print}' filename → Prints every line of a file
awk '/192.16.40.10/' error.log → Filters out specific IP address
awk '/172.16.40.10.81/ {print $4}' error.log → Print out the forth column of a file
awk '{print $2,$5;}' error.txt → Prints out columns 2 and 5
awk -F ':' '{print $1}' test.txt → Prints first column of test.txt with : as a delimiter
3. User management
su → Opens a root shell
adduser NameOfUser → Add a new user
useradd NameOfUser → Another way to add a new user
adduser NameOfUser sudo → Add user to sudo-group
echo "username ALL=(ALL) ALL" >> /etc/sudoers → On some machines we might not be able to adit the sudoers file because we don't have an interactive shell, in this case we can just redirect the text into the file
cat /etc/group | grep sudo → Check which users are in the sudo group
su NameOfUser → Switch user in terminal
sudo userdel NameOfUser → Remove/delete user
4. Permissions
ls -la → Show all the files and directories and their permission settings
chmod +x → Give executable permissions to all users
chmod -x → Remove executable permissions to all users
chmod 777 file → Give read, write and execute permissions to all users on that file
sudo chown myfile → Give ownership of myfile to all users
passwd → Change password of current user
sudo chmod u+s myfile → Modify SUID permissions to grant read, write and execyte to all users
5. Processes
ps → list all running processes
ps aux → Displays a detailed list of all processes running that don't have TTY associated with.
top → monitor processes at real time
kill PID-number → Kill process running on the PID number
kill -9 PID-number → Another way to kill the process
ls /proc → List process information stored in the filesystem
6. Packages
Install package
sudo apt install → Install something with apt
sudo dpkg -i → Install .deb file
Remove packages
dpkg --list → Find package
dpkg -r something.deb → Remove debian package
sudo apt --purge remove nameOfProgram → Remove program
sido apt autoremove → Remove dependecies left behind
Organizing $PATH variable
source /etc/environment && export PATH → Define the path
If using zhs:
Adding a path
export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin → Non-presistent way to add binaries to the path
tar and gzip
gzip myfile → Compress a file with gzip
gunzip myfile.gz → Decompress a file ith gzip
tar cvd myfile.tar myfile1 myfile2 → Create and add files to a tar archive
tar xvf myfile.tar → Unpacking archives with tar
tar vzf myfile.tar.gz → Create a compressed tar file with tar and gzip
tar xzf myfile.tar → Uncompress and unpack tar file with tar and gzip
7. Cronjobs
There are two ways to configure cronjobs. The first one is by putting scripts in the following folders.
The second way is to write the command in the crontab
8. Devices
fdisk -l → List all devices
9. The Filesystem
Difference between sbin and bin
sbin → System Binaries
bin → Normal Binaries
Onry root and users with sudo privileges have access to sbin Binaries.
Mount
Everything on the linux-filesystem belongs to some part of the filesystem-tree. So if we plug in some device we need to mount it to the filesystem. That pretty much means that we need to connect it to the filesystem. Mount is like another word for connect.
mount /dev/usb /media/usb → Mount usb device to be able to browse its content
umount /media/usb → Umount usb device
10. Controlling Services
Systemctl
Systemctl can be used to enable and disable various services on your linux machine.
netstat -apnt → Verify services listening for connection
systemctl start → Start service
systemctl status → See service's status
systemctl stop → Stop service
systemctl enable → Enable service start upon boot
Init.d
nit.d is just a wrapper around Systemctl.
11. Kernel
The Kernel is responsible for talking between the hardware and the software, and to manage the systems resources.
The Linux Kernel differs from Windows in that it contains drivers by default. So you don't have to go around looking for drivers like you do on windows when you want to install a printer, or something like that.
sudo apt update → Update system
sudo apt upgrade → Upgrade system
sudo apt dist-upgrade → Upgrade system to the lates Linux kernel
14. Logging
Logs can be viewed on /var/log/ in debian distributions
16. Network basics
Network interfaces
ifconfig → Display network interfaces
ifconfig eth0 192.168.2.1 netmask 255.255.255.0 up → Create an interface and bring it up
ifup eth0 → Bring interface up
ifdown eth0 → Bring interface down
ip link show → Show interface information for all interfaces
ip -s link show eth0 → Show the statistics of an interface
ip address show → Show ip addresses allocated to interfaces
ip link set eth0 up → Bring interface up
ip link set eth0 down → Bring interface down
ip address add 192.168.1.1/24 dev eth0 → Add an ip address to an interface
Route
sudo route add -net 192.168.2.1/23 gw 10.11.12.3 → Add a new route
sudo route del -net 192.168.2.1/23 → Delete a route
ip route add 192.168.2.1/23 via 10.11.12.3 → Add route with ip command
ip route delete 192.168.2.1/23 → Delete route with ip command
dhclient
sudo dhclient → Obtain a fresh ip
arp
arp → View arp cache
ip neighbour show → View arp cache with ip command
Troubleshooting
ping -c 3 [www.google.com](<http://www.google.com>) → Test whether or not a packet can reach a host and stop sending echo request packets after the count (3) has been reached
traceroute [google.com](<http://google.com>) → See how packets are getting routed
sudo tcpdump -i wlan0 → Capture packet data on an interface
netstat -anpt → Find out what services are listening for connection on the machine
sudo service network-manager restart → Restart the network manager
rfkill list → List wifi to see if its blocked or not
rfkill block 0 → Block wifi interface
rfkill unblock 0 → Unblock wifi interface
DNS
nslookup [www.google.com](<http://www.google.com>) → Query name servers to find information about resource records
dig [www.google.com](<http://www.google.com>) → Getting information about DNS name servers
16.1 Iptables
Iptables is a firewall tool in linux. A firewall is basically a tool that scans incoming and/or outgoing traffic. You can add rules to the iptables to filter for certain traffic.
Types of chains
So you can filter traffic in three different ways input, forward, and output. These are called three different chains.
INPUT → This is for incoming connections. If someone wants to ssh into your machine. Or a web-server responds to your request.
FORWARD → This chain is used for traffic that is not aimed at your machine. A router for example usually just passes information on. Most connections are just passing through.
OUTPUT →This chain is used for outgoing traffic.
Active rules
iptables -L → View active rules
Return iptables to default settings:
iptables -A INPUT -s 192.168.1.30 -j DROP → Block an ip address (-A for append and -s for source)
iptables -A INPUT -s 192.168.1.0/24 -j DROP → Block an entire range
iptables -L -v --line-numbers → Output the rules with line-numbers
iptables -D INPUT 2 → Remove on specific rule
iptables -F → Remove all rules
sudo /sbin/iptables-save → Save changes made to iptables
Measuring bandwidth usage
iptables -L -v → List the rules with some verbosity
iptables -Z → Restart iptables count
iptables -F → Remove all the rules and FLUSH them
iptables -I INPUT 1 -p tcp -j ACCEPT → Add another rule
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