Linux Basic Commands

Linux is a family of open-source free operating systems built using the Linux kernel. Linux is an incredibly powerful operating system when compared to other types of operating system for certain tasks, mostly command line based tasks. This means, it has a steep learning curve and you will be learning new things everyday. In this post I will be listing some of the basic commands that you might find useful to get started and to perform day to day activities in Linux.

Network

Identifying the IP address

ip addr show

ifconfig

Installing OpenVPN client and import *.ovpn (Windows) file to configure

sudo apt-get install openvpn

sudo apt-get install network-manager-openvpn-gnome

Now goto the network settings on the system tray -> “vpn connections” -> “configure vpn” -> “Add” -> select “Import saved VPN configuration” from the drop down list - > “create” -> select the *.ovpn file -> finally, type the username and password and click “ok”. Now go to the network settings and connect to the configured VPN server.

User management

Changing the default password

passwd

Find UID of a user

id -u user_name

Find GID of a user

id -g user_name

Find all the groups a user belongs to

id -G user_name

groups username

Find all the groups associated with UID

id user_name

Disable sudo password for a particular user

create a file in /etc/sudoers.d for example

sudo vim /etc/sudoers.d/dont_prompt_user

add the following to the above file

user_name ALL=(ALL) NOPASSWD:ALL

Bash

Enable debug

set -x

Disable debug

set +x

Deleting an entry from history

history -d line_number

Storage

Display the amount of disk space available

df -h # -h: human readable output format

Check the capacity of a directory

du -sh /path/directory

h: human readable output; s: total disk space used by the directory

scp copy

scp -r /path/to/file username@a:/path/to/destination

scp -r username@b:/path/to/file /path/to/destination
### Executing a command at a fixed interval using watch command
watch -n x command
# "x" is the repeat time in seconds;"command" is the command that you want to execute

System

Users connected to the system

who

Running a command at a fixed interval

while true; do
cmd >> output.txt
sleep 2
done

while true; do sleep 2; cmd >>output.txt; done &

Running a command in background

nohup command &

Show the list of installed packages in Ubuntu or Debian

dpkg --get-selections

Installing ffmpeg from ppa depository (add ppa to apt apt-get and install) - (source)

sudo add-apt-repository ppa:mc3man/trusty-media
sudo apt-get update
sudo apt-get dist-upgrade
sudo apt-get install ffmpeg

Get the version of the libav-tools (source)

apt-cache search libav | grep libav

Execute modified command when called by adding alias to the bashrc file (source)

echo "alias ls='ls -alh'" | tee -a ~/.bashrc; . ~/.bashrc
# alternatively, you can edit this file by opening this file from your home directory
vim .bashrc

Counting the number of files in a directory and sub-directories (source)

find . -type f | wc -l
# . - this directory and all subdirectories
# -type f - find all files
# | - piped into wc - word count and the -l - tells to count the lines from wc

# or

find . | wc -l
# this counts the files and directories within the current directory.

Extract a process detail from top command based on process name

top -c -p $(pgrep -d',' -f ffmpeg)

Spitting a MP3 files into multiple files (source)

ffmpeg -i 01.mp3 -f segment -segment_time 100 -c copy 01_%03d.mp3

Joining MP3 files to a single track:[pre class=”brush:bash”]

cat *.mp3 > out.mp3

rsync

Copy/Sync a Directory on Local Computer

rsync -avzh /root/rpmpkgs/folder/ /tmp/backups/folder/ --delete

Syncing files using rsync across network

rsync -av -e "ssh -T -o Compression=no -x" user@10.1.1.1:/source/path/ /destination/path/

rsync -avzhe "ssh -i ~/.ssh/id_rsa" user@10.1.1.1:/source/path/ /destination/path/

chmod

chmod 700 myDir #7-user; 0:Group; 0: Others - read(4), write(2), execute(1)
# Permission rwx Binary
#7 read, write and execute rwx 111
#6 read and write rw- 110
#5 read and execute r-x 101
#4 read only r-- 100
#3 write and execute -wx 011
#2 write only -w- 010
#1 execute only --x 001
#0 none --- 000

| No | Permission | rwx | Binary | |——|——|——|——| | 7 | read, write and execute | rwx | 111 | | 6 | read and write | rw- | 110 | | 5 | read and execute | r-x | 101 | | 4 | read only | r– | 100 | | 3 | write and execute | -wx | 011 | | 2 | write only | -w- | 010 | | 1 | execute only | –x | 001 | | 0 | none | — | 000 |

List all the directories with sizes

du -sh *
du: disk usage
# -s: Display an entry for each specified file
# -h: human readable format
du -sh * | sort -n # sort folders by size
du -sh * | tail -r # sort by largest first

List all the processes associated with a process-name

ps -fC process-name

List all the installed packages

apt list --installed

Changing password of another user

sudo passwd user_name

Terminal execute commands as another user

sudo su user_name

Renaming all files in a folder (Amend for eg: unix_)

rename 's/^/Unix_/' *

List the number of threads for a process ID

ps -o nlwp processID
cat /proc/processID/status | grep Threads
ls /proc/processID/task/ | wc -l

Setting time zone from terminal

sudo dpkg-reconfigure tzdata

Importing ssh key from github

ssh-import-id-gh user_id

Changing default ssh port

vi /etc/ssh/sshd_config
#locate the Port 22 line and change it
service sshd restart

Compressing PDF files using ghostscript

sudo apt install ghostscript
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/prepress -dNOPAUSE -dQUIET -dBATCH -sOutputFile=compressed_PDF_file.pdf input_PDF_file.pdf
# some example dPDSETTINS: /prepress /printer /ebook /screen

Ubuntu .deb package installation/uninstallation

installing

sudo dpkg -i package_file.deb

list the packages installed with name urserver

sudo dpkg -l '*urserver*'

remove the package itself (without the configuration files

sudo dpkg -r urserver

delete (purge) the package completely (with configuration files)

sudo dpkg -P urserver

check if the package has been removed successfully

sudo dpkg -l urserver

Check CPU microarchitecture

cat /sys/devices/cpu/caps/pmu_name

OS Details

OS release - works with atest ubuntu

cat /etc/os-release

another way to check os details

lsb_release -a

Find command

sudo find / -name "name_of_the_file"

add a user to sudo

usermod -aG sudo username