# List files and directories
ls
ls /home
ls –al
ls –al /home/student
# Change current directory
cd /home
# Print current directory
pwd
# Copy files
cp a.txt /home
cp a.txt b.txt
# Move or rename file
mv a.txt /home
mv a.txt b.txt
# Remove empty directory
rmdir test
# Remove file or Non empty directory
rm a.txt
rm –r test
# Create directory
mkdir test
# Print file content
cat a.txt
# Search for text in file
grep "word" a.txt
# Display the first 10 lines of a file
head a.txt
head –n 5 a.txt
# Display the last 10 lines of a file
tail a.txt
tail –n 5 a.txt
tail –f a.txt
# Display text from file in one screen
less a.txt
# Display list of running processes
ps aux
# Display list of open files
lsof –i
# Display network connections
netstat –antp
# Display network information
ifconfig
# Sort content of a file
sort a.txt
# Remove duplicate lines (sort first)
uniq a.txt
# Display information about a file
stat a.txt
# Test network connectivity
ping google.com
# Display current user
whoami
# Change user passwd
passwd student
# Terminate process
kill 1845
# Search on files
find / -name a.txt
find / -name "*.txt"
# Text editor (Save: Ctrl+X)
nano filename
Create link file
# Soft link
ln –s file link
# Hard link
ln file link
Special Characters
# Directory separator ( / )
cd /home/student
# Escape character ( \ )
mkdir test\ dir
# Current directory ( . )
ls .
cat ./a.txt
# Parent directory ( .. )
ls ..
cat ../a.txt
# User home directory ( ~ )
cd ~
# Run in background ( & )
gedit &
# Represent one or more characters ( * )
ls *.txt
# Represent single character ( ? )
ls a?.txt
# Represent range of values ( [ ] )
ls a[0-9].txt
# Command separator run both commands anyway ( ; )
pwd ; whoami
ay7aga ; whoami
# Command separator run second command if the first succeed ( && )
pwd && whoami
ay7aga && whoami
# Command separator run second command if the first failed ( || )
pwd || whoami
ay7aga || whoami
Redirection
# stdout (1)
whoami > a.txt
whoami 1> a.txt
# stderr (2)
ay7aga 2> err.txt
# stdin (0)
wc < a.txt
wc 0< a.txt
# append value to the end of file
pwd >> a.txt
# print the output to out.txt, and print the error to err.txt
ls > out.txt 2> err.txt
# print the output and error to all.txt
ls > all.txt 2>&1
Piping
# Pass the output of the first command to the input of the second command.
ls –l | grep "Desktop"
cat /etc/passwd | grep ":0:"
ping google.com | grep "64 bytes"
cat a.txt | sort | uniq
# Create user
useradd testuser
# Set or change user password
passwd testuser
# Create group
groupadd testgroup
# Add user to group
gpasswd –a testuser testgroup
# Delete user
userdel testuser
# Delete group
groupdel testgroup
Important Files
/etc/passwd
# Example
# root:x:0:0:root:/root:/bin/bash
1- Username
2- Encrypted password
3- User ID
4- Group ID
5- Comment (Full username)
6- Home directory
7- Shell type
/etc/shadow
# Example
# root:$6$KILMHVxNbzVXTmbwlh6GiH6k3u4zrMsvlmTgWRF9m7SW:18184:0:99999:7:::
1- Username
2- Encrypted password
Look at the first 3 characters
$1$ = MD5 encryption
$5$ = SHA-256 encryption
$6$ = SHA-512 encryption
3- Last password change date
4- Minimum date
5- Maximum date
6- Warn date
7- Inactive date
8- Expire date
/etc/group
# Example
# root:x:0:hassan,ahmed
1- Group name
2- Group password if exist
3- Group ID
4- Members of this group
SU
# SU switch to another user
su
su testuser
# SU also can switch to another user with similar environment as the user loggedin
su -
su - testuser
# SU can also run command directly without full shell
su username -c command
Sudo
# Sudo can run single command as root
sudo cat /etc/shadow
# And you can also have full root shell
sudo -i
Linux Boot Process
BIOS: Performs some system integrity checks then executes MBR.
MBR: It contains information about boot loader then loads and executes the boot loader (GRUB).
GRUB: displays a splash screen then executes kernel.
Kernel: mount partitions then executes init script.
Init: determine the Linux run level then executes runlevel programs.
Runlevel: responsible for which services started at which runlevel.
Linux Run Levels
# Get current runlevel
runlevel
# Get default runlevel
systemctl get-default
# Set default runlevel
systemctl set-default runlevelX.target
# List services at specific runlevel (S means start, K means kill/stop)
ls /etc/rcX.d
Services
# Start service
systemctl start apache2
# Stop service
systemctl stop apache2
# Restart service
systemctl restart apache2
# Print status of service
systemctl status apache2
# Enable or disable service at boot time
systemctl enable/disable apache2
File System
/: Root directory, every thing starts from there.
/root: Root home directory, contains Desktop, Downloads, Documents and so on.
/bin: Contains users binaries (ls, cp, cat).
/sbin: Contains system binaries (reboot, ifconfig, fdisk).
/etc: Contains system configuration files.
/home: Home directory for all users (/home/student, /home/testuser).
/boot: Contains boot load files and kernel files.
/lib: Contains system libraries.
/var: Contains variable data which is continuously change in size (/var/log).
/usr: Contains user programs and it contains another bin (/usr/bin) & sbin (/usr/sbin)
which contains second level user and system binaries.
/mnt: Mount directory where system admin can mount any partitions here.
# Update repository (always must run first)
apt-get update
# Upgrade all softwares
apt-get upgrade
# Upgrade all the system include kernel
apt-get dist-upgrade
# In most cases you will need to run this
apt-get update && apt-get dist-upgrade
# Install software
apt-get install software-name
# Remove software
apt-get remove software-name
# Remove software with its configuration files
apt-get purge software-name
# Search on software
apt-cache search software-name
If the software don’t exist in the repository then you can go to the main website of the software and download the deb package.
