Linux Command Reference

📁 Directory Commands

• cd – Change directory
• mkdir – Create a new directory
• ls – List directory contents
• touch – Create an empty file
• pwd – Print current working directory
• rm – Remove files or directories
• cp – Copy files or directories
• mv – Move or rename files or directories

🧠 Process Management

• ps -ef – List all running processes
• top – Display real-time CPU and memory usage
• htop – Interactive and visual process viewer
• free – Show memory usage statistics
• nproc – Show number of available CPU cores
• kill – Terminate a process by PID
• killall – Terminate all processes by name

📝 Text Processing

• grep – Search for lines matching a pattern
• awk – Pattern scanning and text processing tool
• sed – Stream editor for filtering and transforming text
• | – Pipe: send output of one command as input to another
• cat – Display file contents
• echo – Print text to the terminal
• vi – Open the Vi text editor
• nano – Simple and beginner-friendly text editor

🔐 Permissions and Users

• chmod – Change file permissions (read/write/execute)
• chown – Change file owner or group
• usermod – Modify user account (e.g., add to a group)
• passwd – Change user password

🌐 Networking

• ping – Check connectivity to a host
• curl – Transfer data from or to a server
• wget – Download files from the web
• ifconfig – Show network interfaces (deprecated, but still used)
• ip a – Show IP addresses (modern alternative to ifconfig)
• netstat – Display network connections (can be replaced by ss)
• ss – Display detailed socket statistics

📦 Package Management (Debian-based)

• apt update – Update package lists
• apt upgrade – Upgrade installed packages
• apt install <package> – Install a package
• apt remove <package> – Remove a package

🔧 System and File Utilities

• df -h – Show disk space usage
• du -sh <folder> – Show disk usage of a folder
• uname -a – Show system information
• history – Show command history
• clear – Clear the terminal screen
• man <command> – Show manual page for a command \

🐚 Shell Scripting

• set -x – Enable debug mode: print each command before execution
• set -e – Exit immediately if any command fails
• set -o pipefail – Return the exit code of the leftmost failing command in a pipeline

Others

• hash -r - Reset the shell Cache

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Linux User & Group Management

📁 Important Files

File Path	Description
/etc/passwd	Stores user account information
/etc/shadow	Stores user passwords and password aging data
/etc/group	Stores group information
/etc/sudoers	Defines users/groups allowed to run sudo

👤 User Management

• View user info:

    grep -i ^bob /etc/passwd

  Example Output:

  bob:x:1002:1002::/home/bob:/bin/sh
  

  USERNAME:PASSWORD:UID:GID:GECOS:HOMEDIR:SHELL
  

• Change password:

  passwd             # For current user
  passwd <username> # For specific user

• Delete a user:

  userdel <username>

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🔐 Password Details

• View password entry:

  grep -i ^bob /etc/shadow

  Example Output:

  bob:$6$...:18188:0:99999:7:::
  

  USERNAME:PASSWORD:LASTCHANGE:MINAGE:MAXAGE:WARN:INACTIVE:EXPDATE
  

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👥 Group Management

• Add a group:

  groupadd <groupname>

• Delete a group:

  groupdel <groupname>

• View user's group memberships:

  grep -i ^bob /etc/group

  GROUPNAME:PASSWORD:GID:MEMBERS
  

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