# Basics ## Core Commands ```shell # 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 ![](/files/qnqUGPQXkHvamIwBvyvu) ```shell # Soft link ln –s file link # Hard link ln file link ``` ## Special Characters ```shell # 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 ![](/files/BltAR0cPX3ojAbwuqdTf) ```shell # 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 ```shell # 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 ``` ## Environment Variables ```shell # Run "env" command HOSTNAME=Debian USER=student PWD=/home/student PATH=/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games SHELL=/bin/bash # Print environment variables echo $HOSTNAME echo $PATH echo $SHELL # Set variables PATH=$PATH:/home/user SHELL=/bin/sh ``` ## User Management ```shell # 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 ```shell # 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 ```shell # 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 ```shell # Example # root:x:0:hassan,ahmed 1- Group name 2- Group password if exist 3- Group ID 4- Members of this group ``` ## SU ```shell # 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 ```shell # Sudo can run single command as root sudo cat /etc/shadow # And you can also have full root shell sudo -i ``` ## Linux Boot Process ![](/files/IZAnh4EEvBJZayoJTyAP) * **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 ![](/files/BdhujlP6iA92twF0ulAs) ```shell # 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 ```shell # 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 ![](/files/L2lj16QJ6egNh8l19vf9) * **/**: 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. * **/tmp**: Temporary files (delete at reboot). ## File Permissions ![](/files/1TetJFCGjGQhqGjclxok) ```shell # Permission groups 1- owner (u) 2- group (g) 3- others (o) # Permission types 1- read (r=4) 2- write (w=2) 3- execute (x=1) # Example 1 chmod u+rwx filename chmod g+rw filename chmod o+r filename chmod go+rw filename chmod a-x filename # Example 2 chmod 777 filename chmod 700 filename chmod 764 filename chmod 755 filename chmod 644 filename ``` ## Installing Software ```shell # 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. ```shell # Install .deb package dpkg –i nmap.deb # List all packages dpkg –l dpkg –l | grep nmap # Remove package dpkg –r nmap ``` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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