Linux

In this page you'll find Linux commands and other useful information.

1. System Related Commands

Command	What does it do?
cat /etc/*release*	Check system version
ls -ltrh /dev/video*	List all video devices picked up by the kernel
lsusb	List all devices attached with USB
lspci	List all devices attached with PCI
top	Displays real time view of performance related data of all running processes in a system
htop	Graphical top. Shows usage of CPU and Ram
uptime	Shows the uptime and the load-average of the system
sudo pkill <service>	Kill the service with the name or PID
clear	Clear commandline
which python3	Which will tell where the file/program is in the system. If it Exist
history	Shows the history of the commands in the command line
!40	Rerun the command with ID 40 without the need to rewrite the command
!-2	Rerun the 2-nd previous command
!p	Rerun the command that starts with the letter (python)
!!	Rerun the previous command
sudo !!	Rerun the previous command with sudo
^helllo^hello	Rerun the command with fixed grammar
systemctl status apache2	Checking the services that are running on the machine
sudo systemctl disable apache2	Removing the "Loaded" so the process does not start with the machine
sudo systemctl enable apache2	Adding "Loaded" so the process does starts with the machine
sudo systemctl stop apache2	This command will stop the process
sudo systemctl start apache2	This command will run the process
sudo systemctl restart apache2	This command will restart the process
tail -n 4 /var/log/syslog	Viewing only the last 4 rows of logs
head -n 4 /var/log/syslog	Viewing only the first 4 rows of logs
jouurnalctl -u apache2	View the logs only of the apache2
jouurnalctl -fu apache2	View the logs in real time only of the apache2
<command> > file.txt	Overwrites the output to the file
<command> >> file.txt	Appends the output to the file
ls -l > file.txt	Saving the ls -l command output to the file.txt
`	`
`ls -l	grep file`
`cat file.txt	sort
grep	grep limits the search criteria
grep -v file.txt	Don't grep rows that contain the word file.txt ( -v is the same as not in python)

status can be:

• Loaded: Enabled #Start when the machine is started.
• Active (running) #Telling us that the process is running.
• Active Time #We can check when did the process started

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2. Resource Management Commands

Command	What does it do?
free	Tells us the amount of Memory used by the system
free -m	The same but in MB
df	Means disc free. Tells us how much diskspace is being used
df -h	Makes it better and easier to read. "h" is human-readible.

"free" memory is the memory that is not used at all by the system. "available" memory is the memory that exist but reserved for caches/system logs and stuff.

It's weird but Linux has a finite number of files. So we can have free disk space but if the nr.files is full we can't add more. These are called i-nodes.

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3. File System Commands

Command	What does it do?
pwd	Print Working Directory. Tell where you are
.	Current Directory
ls	List Files and Folders in the working Dir
ls -l	Give Long Listing
ls -a	List Everything. Even Hidden Files ("." before the name)
cd <dir_name>	Change Directory to the specified one
cd ..	Go Above 1 Dir
cd -	Go to the Previous Dir
cd /	Go to Root Folder
cd /home/<user_name>	Go to the Home Directory. Where Everything is. Like C-Drive
cd ~	Go to the Home Directory. Same as the above command
mkdir <folder_name>	Make a new Directory
touch <file_name>.<extention>	Create a file If it doesn't Exist
cat > <file_name>.<extention>	Create a file from cat and write to it. CTR + C for closing it
echo "" > <file_name>.<extention>	Create a file
cat <file_name>.<extention>	Open file to check it
cp <file_name>.<extention> <file_name_2>.<extention>	cp stands for copy
rm test.txt	rm stands for remove. If you use rm it will remove the file from the system. You can't get it back
mv test.txt ini	mv stands for move. File to the Directory
mv *txt ini	Move all the files ending with txt to the Directory
mv *txt ..	Move all the txt files to the Dir above
mv ../test.txt .	2 dots = parent Directory. 1 dot = current Directory. Move the File from the parent to the current Dir
mv test1.txt test2.tx	mv is still used to rename files. Be careful with this. May overwrite without permission
scp * <user>@<ip_address>:~/	Secure copy all files from a Dir to another device at it's Home Dir
scp -P <port_nr> <user>@ip_address:~/	Secure copy files from a device to another at remote specified port
mv <file_name>.<extention> <dir_path>	Move files from a directory to another
mv <file_name>.<extention> <file_name_2>.<extention>	Rename files
nano test.txt	Open nano = text editor to edit the test.txt file. Press "control + o" to save the edit
cat test.txt	Will show the content of the file
`cat test.txt	head - 2`
echo $?	Special variable. Returns 0 if the last command was succesful or not 0 if the last command was with error
find / -name *.log	Find everyfile in the root-folder. Search by name that ends with .log
find tes*	Find files that start with "tes"
find *tes*	Find files that has "tes" somewhere in the name

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4. Network Commands

Command	What does it do?
ssh <user>@<ip_address>	Connect with SSH client with specified user
`sudo lsof -i -P -n	grep LISTEN`
`sudo netstat -ap	grep <port_nr>`
ping -I wlan0 time.google.com	Ping with defined network interface. Here wlan0

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5. Shortcut Commands

Command	What does it do?
CTRL + Z	Shunts the process into the background, suspended
CTRL + C	Interrupt (kill) the current foreground process running in in the terminal
CTRL + D	Close the bash shell. Similar to exit
CTRL + L	Clear the screen. This is similar to running the “clear” command
CTRL + A or Home	Go to the beginning of the line
CTRL + E or End	Go to the end of the line
CTRL + ALT + (UP or Down)	Go to the other Workspace in Ubuntu

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6. User management

Root is the user with the most proviledge in the System. Permission string that we get when we press ls -l

Beginning	Groups	Meaning
-rwxrwxr-x	4 Sections(groups of 3 letters)	d = directory or - =file
		rwx = user. Which user has the file
		rwx = group
		r-x = the others that are not in the group
		r = read access
		w = write access
		x = execute access
		- = No access/permission

This command shows us all the users:

~$ cat /etc/passwd

:shows ini : x : 1000: 1000 : /home/ini: /bin/bash   #X is for the password.
                                                     #The numbers are the group ID and user ID

Command	What does it do?
sudo cat /etc/shadow	Show the password-Hash saved in the system
sudo !!	!! are used to rewrite the code that was used before
groups	shows all the groups
groups ini	shows the group ini is part of
adduser batman	Adding the user with the name batman
sudo - batman	Logging in into the new user
logout	Logging out from the user
sudo su - batman	Logging in into the new user without the need for the password
passwd	Changing the password of the user
sudo su -	Change to root. Do what ever you want
sudo userdel -r batman	Removing the user with the name batman
sudo groupadd heroes	Creating a group in the system
sudo usermod -aG heroes ini	Adding the used ini to the group heroes. (logout and login to work)
sudo gpasswd - d ini heroes	Removing a user from a group
sudo groupdel heroes	Removing a group

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7. MISC

spd-say command can be a nifty tool.

Use this tool, for the system to say something or to inform you that a task finished:

~$ sudo apt-get update; spd-say "Task Finished"

install gnome-tweak tool [sudo apt install gnome-tweak-tool] you can actually choose between coherent or independent behavior of your multiple monitors across workspaces. Once installed, launch tweak and then click on workspaces. then choose " Workspaces span displays" (instead of the default choice "Workspaces on primary display only" !)

\time #time option doesn't work #time is not a builtin; I mean it's not treated as a command; it's a keyword. #That's why tricks like \time

Streams in Linux: Input, Output and Error Streams

stdin (standart input) Input: stdout (standart output) Output: Every thing that returns info from the terminal is a standart Output Stream stderr (standart error) Error : >> ls ini3 #cannot access. #This is Error stream

Bash Alias

To create Bash aliases we do the following

~$ alias <alias_name>="<command>"

#Examples

~$ alias ll="ls -lh"                              #Better List 
~$ alias lt="ls -h --size -1 -S --classify"       #List files by size
~$ alias myip="curl ipinfo.io/ip"                 #Get Public IP

To make this alias permanent we need to modify the ./bashrc file.

~$ nano ~/.bashrc

After adding the alias in the file and saving the changes we close the file and run:

~$ source ~/.bashrc

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Extra Tools and Programs:

vim

vim is a text editor like nano. vim works with "modes". Normally is the command mode.

Command	What does it do?
:q	Exit vim
:q!	Exit vim without saving changes
:x	Exit vim saving changes
:wq	Exit vim saving changes, same as x
i	Enter the Insert Mode
:w	Save the file
:w test.txt	Save a created file
SHIFT + a	Enter Insert Mode and jump to the end of the Line
o	Create a new line
0	Go to the beginning of the line
$	Go to the end of the line

When we are in Command Mode

Command	What does it do?
x	Delete a Characters one at a time
dd	Delete a Line
3dd	Delete 3 Lines
.	Redo the same command u used before
/text	Find the first use of text string in the file
n	Check the next time it was used
Shift + n	Check the previous used case of the string
:%s/string/string2	Replace the string with the other string
u	Undo the last change
u then CTRL + r	Undo the previous changes
30 then SHIFT + G	Go to line number 30

Screen

If you run a long running process or task on a machine that you are connected with SSH, and the connection drops, we lose the console we were connected to and the work with it. To be able to leave the task running, even if we lose connection, we can use the tool screen. Screen uses "virtual separate terminals". Even if we lose connection we can still attach to the session terminal.

To install screen we run:

~$ sudo apt-get install screen

To use screen, the most useful commands are:

~$ screen -S <name_screen_session>   # Create a new screen terminal with a given name
~$ screen -ls                        # List all the screen terminals currently running
< CTRL + a + d on_keyboard >         # Detach from screen terminal. Leave it running on background
~$ screen -r <id_screen>             # Reattach to the screen terminal specified
~$ screen -d <id_screen>             # Detach to the screen terminal specified

watch

watch is a very useful command in Linux to check the result of a command for regular interval times. We use it when we want to check the command output and how it changes with time. In order to not write the command ourselfs many time, we use this tool.

~$ watch -n 1 <linux_command>

The -n tag is used to set the time interval that the command will be run. Default is 2 seconds. In order to use watch with commands that have a pipe | we need to put all the command in brackets

~$ watch -n 1 "<linux_command> | <linux_command>"

wondershaper

wondershaper is a tool that is used to limit the bandwidth on a specific network card. Install wondershaper from Github. Do not install it with sudo apt-get because it will install an older not maintained version.

Command	What does it do?
sudo wondershaper -c -a eth0	Clean the limits from adaptor. Needs to be done everytime
sudo wondershaper -a eth0 -u 4096 -d 50000	Limit Downlink and Uplink Bandwidth of Client

iperf3

iperf3 is a tool used to measure the maximum bandwidth of a link. See Iperf Documentation for more information. This tool is a CPU intensive one and the results we get are always lower than theoretical maximum speed of a network link.

Command	What does it do?
iperf3 -s	Iperf Server
iperf3 -s -D	Iperf Server in background as a Daemon
iperf3 -c 192.168.1.8	Iperf client
iperf3 -c 192.168.1.8 -t 10	Time in seconds to transmit
iperf3 -c -u 192.168.1.8	Use UDP instead of TCP. (Check Documentation)
iperf3 -c 192.168.1.8 -R	-R. Client in reverse test mode – Server sends, client receives

crontab

crontab is a tool used to schedule jobs in fixed time intervals. It is used to automate commands either time based or on system reboot See Crontab Documentation for more information.

To undertand more use this cool online tool Crontab Guru

Commands that need admin rights should be added to the root crontab file, that can be accessed with sudo:

 ~$ crontab -e
 ~$ sudo crontab -e

# .---------------- minute       (0 - 59)
# |  .------------- hour         (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month        (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week  (0 -  6) (Sunday= 0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name  command to be executed

Command	What does it do?
* * * * *	Every Minute
1 * * * *	At Minute 1 each Hour Day etc
5 0 * * *	At 00:05 each Day etc
* * 1 * *	At each Minute each Day of month 1 (January)
15 14 1 * *	At 14:15 each day of January
0 22 * * 1-5	At 22:00 for Monday till Friday
23 0-23/2 * * *	At 00:23 01:23 02:23 each Day
5 4 * * sun	At 04:05 each Sunday
0,20,40 * * * *	Every 20 minute
*/20 * * * *	Every 20 minute. Same as Above

To add a new route in the route table on reboot we write the following command in the crontab file:

@reboot sleep 60 && route add -net <ip_address>/<CIDR> gw <gateway_ip> dev <device_name>

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References

crontab

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