The Linux file system follows the Filesystem Hierarchy Standard (FHS). Key directories include:

• / - Root directory, the top-level directory
• /bin - Essential user binaries (commands)
• /boot - Boot loader files and kernel
• /dev - Device files
• /etc - System configuration files
• /home - User home directories
• /lib - Essential shared libraries
• /media - Mount points for removable media
• /mnt - Temporary mount points
• /opt - Optional software packages
• /proc - Virtual filesystem with process information
• /root - Root user's home directory
• /run - Runtime data
• /sbin - System administration binaries
• /tmp - Temporary files
• /usr - User programs and data
• /var - Variable data (logs, databases, etc.)

Linux uses a three-tier permission system:

• Owner (u): The file owner
• Group (g): The file's group
• Others (o): All other users
  Each tier has three permissions:
• Read (r): Permission to read the file (4)
• Write (w): Permission to modify the file (2)
• Execute (x): Permission to execute the file (1)
  Example: rwxr-xr-- means owner has read/write/execute, group has read/execute, others have read only.

# Change permissions using chmod
chmod 755 file.txt        # rwxr-xr-x
chmod u+x script.sh       # Add execute for owner
chmod g-w file.txt        # Remove write for group
chmod o=r file.txt        # Set others to read only
# Change ownership
chown user:group file.txt
chgrp newgroup file.txt

Multiple commands are available for finding files:
find command (most powerful):

# Find by name
find /path -name "filename"
find . -name "*.txt"
# Find by type
find /home -type f        # Files only
find /home -type d        # Directories only
# Find by size
find /var -size +100M     # Files larger than 100MB
find . -size -1k          # Files smaller than 1KB
# Find by permissions
find . -perm 755
find . -perm -u+x         # Files executable by owner
# Find and execute action
find /tmp -name "*.tmp" -delete
find . -name "*.log" -exec rm {} \;

locate command (faster, uses database):

locate filename
updatedb              # Update locate database

which/whereis commands:

which python           # Find executable in PATH
whereis python         # Find binary, source, manual

Several commands help manage processes:

# View processes
ps aux                  # All processes with details
ps -ef                  # Full format listing
ps -u username          # Processes for specific user
pstree                  # Process tree view
# Real-time process monitoring
top                     # Interactive process viewer
htop                    # Enhanced version of top
atop                    # Advanced system monitor
# Process information
jobs                    # Current shell jobs
pgrep process_name      # Find process IDs by name
pidof process_name      # Get PID of running process

Process states:

• R: Running
• S: Sleeping (waiting)
• D: Uninterruptible sleep
• Z: Zombie
• T: Stopped

# Kill by PID
kill PID                # Send TERM signal (15)
kill -9 PID             # Send KILL signal (forceful)
kill -15 PID            # Send TERM signal (graceful)
# Kill by name
killall process_name    # Kill all processes with name
pkill pattern           # Kill processes matching pattern
# Common signals
kill -l                 # List all signals
kill -HUP PID          # Reload configuration (1)
kill -INT PID          # Interrupt (2)
kill -QUIT PID         # Quit (3)
kill -KILL PID         # Force kill (9)
kill -TERM PID         # Terminate (15)
kill -STOP PID         # Stop process (19)
kill -CONT PID         # Continue process (18)

Foreground processes: Run in the terminal and accept input
Background processes: Run independently without blocking terminal

# Run command in background
command &               # Start in background
nohup command &         # Continue after logout
# Job control
jobs                    # List current jobs
fg %1                   # Bring job 1 to foreground
bg %1                   # Send job 1 to background
Ctrl+Z                  # Suspend current process
Ctrl+C                  # Terminate current process
# Detach from terminal
nohup long_command > output.log 2>&1 &
disown %1               # Remove job from shell

# User management
useradd username         # Add new user
useradd -m -s /bin/bash username  # Create with home directory
userdel username         # Delete user
userdel -r username      # Delete user and home directory
usermod -aG groupname username    # Add user to group
passwd username          # Change user password
chage -l username        # View password aging info
# Group management
groupadd groupname       # Create new group
groupdel groupname       # Delete group
gpasswd -a username groupname     # Add user to group
gpasswd -d username groupname     # Remove user from group
# View user/group information
id username              # Show user ID and groups
groups username          # Show user's groups
getent passwd username   # Get user info from database
getent group groupname   # Get group info