Deep Thought

Linux Exercises 1-8 Cheatsheet

11 min read

# ===============================================
# Linux Cheat Sheet
# ===============================================
 
# -------------------------------
# USER MANAGEMENT
# -------------------------------
 
sudo useradd -m username           # Add a new user and create a home directory
sudo passwd username               # Set a password for the user
sudo userdel -r username           # Delete a user and remove their home directory
sudo usermod -l newname oldname    # Change a user's login name
sudo usermod -d /new/home username # Change a user's home directory
 
# -------------------------------
# GROUP MANAGEMENT
# -------------------------------
 
sudo groupadd groupname             # Create a new group
sudo groupdel groupname             # Delete an existing group
sudo usermod -aG groupname username # Add user to supplementary group(s)
sudo usermod -g groupname username  # Change user's primary group
groups username                     # Show groups that a user belongs to
id username                         # Show detailed user and group IDs
 
# -------------------------------
# USEFUL FILES
# -------------------------------
# /etc/passwd  -> User account information
# /etc/shadow  -> Hashed passwords (root-only)
# /etc/group   -> Group definitions and memberships
 
# -------------------------------
# FILE AND DIRECTORY PERMISSIONS
# -------------------------------
 
ls -l # List files with permissions, owners, and groups
# Permissions breakdown: rwx for user, group, others
# Example: -rw-r--r-- means owner can read/write, others read-only
 
ls -a # List files including hidden files
 
# -------------------------------
# MODIFYING PERMISSIONS
# -------------------------------
 
chmod u+x file.txt                # Add execute permission for the user
chmod g+w file.txt                # Add write permission for the group
chmod o-r file.txt                # Remove read permission for others
chmod a=r file.txt                # Set read-only for everyone
 
chmod 755 script.sh               # Set rwxr-xr-x (owner full, others read/execute)
chmod 644 file.txt                # Set rw-r--r-- (owner read/write, others read-only)
chmod 700 secret.txt              # Set rwx------ (owner only)
 
chmod -R 755 myfolder             # Apply permissions recursively to folder and contents
 
# -------------------------------
# OWNERSHIP CHANGES
# -------------------------------
 
chgrp teachers notes.txt          # Change group ownership of a file
chgrp -R teachers projects/       # Change group recursively
 
chown alice notes.txt             # Change file owner
chown alice:teachers notes.txt    # Change both owner and group
chown -R alice:teachers projects/ # Change owner/group recursively
 
# -------------------------------
# DEFAULT PERMISSIONS WITH umask
# -------------------------------
 
umask                             # Show current umask value
umask 0007                        # Temporarily change umask (new files 660, dirs 770)
 
# -------------------------------
# SPECIAL PERMISSION BITS
# -------------------------------
# setuid (4xxx)  -> Run file with owner’s privileges
# setgid (2xxx)  -> Run file with group’s privileges
# sticky (1xxx)  -> Restricts file deletion in shared dirs
 
# Example:
# chmod 4755 script.sh   -> Set setuid bit on a script
# chmod 2755 dir/        -> Set setgid bit on a directory
# chmod 1777 /tmp        -> Set sticky bit (only owners can delete own files)
 
# ===============================================
# Investigating File Contents in Linux
# ===============================================
# Commands to identify, inspect, and analyze file contents.
# ===============================================
 
# -------------------------------
# 1. FILE — Identify File Type
# -------------------------------
 
file filename                    # Identify the file type (text, binary, executable, etc.)
file /bin/ls                     # Example: Inspect system executable
 
# -------------------------------
# 2. HEAD — View Beginning of a File
# -------------------------------
 
head filename                    # Show first 10 lines of a file
head -n 20 filename              # Show first 20 lines
 
# -------------------------------
# 3. TAIL — View End of a File
# -------------------------------
 
tail filename                    # Show last 10 lines of a file
tail -n 50 filename              # Show last 50 lines
tail -f filename                 # Follow live updates (e.g., logs in real time)
 
# -------------------------------
# 4. OBJDUMP — Inspect Binary Executables
# -------------------------------
 
objdump -d filename              # Disassemble a binary executable
objdump -x filename              # Show all headers and metadata
objdump -d /bin/ls | head -n 10  # Example: Disassemble first 10 lines of /bin/ls
 
# -------------------------------
# 5. OD — Dump File in Octal/Hex/Binary
# -------------------------------
 
od -c filename                   # Show file contents as ASCII characters
od -x filename                   # Show contents in hexadecimal
od -b filename                   # Show contents in octal
# Example:
# od -c notes.txt                # See text + hidden characters
 
# -------------------------------
# 6. OTHER USEFUL TOOLS
# -------------------------------
 
strings /bin/ls                  # Extract readable ASCII strings from a binary file
xxd filename                     # Create a hex dump of a file
xxd -r filename.hex              # Reverse hex dump back into original file
less filename                    # Scroll through a file interactively
cat filename                     # Display entire file contents in terminal
 
# ===============================================
# Finding Files in Linux
# ===============================================
# Tools and commands to locate files and directories efficiently.
# ===============================================
 
# -------------------------------
# 1. FIND — Search for Files and Directories
# -------------------------------
 
find directory -name targetfile -print             # Search for a file by name in a given directory tree
find /home -name "*.txt" -print 2>/dev/null        # Find all .txt files under /home, hide permission errors
 
# Useful find options:
find . -type f                                     # Find only regular files
find . -type d                                     # Find only directories
find . -perm o=r                                   # Find files readable by others
find . -size +10M                                  # Find files larger than 10 MB
find . -iname "*.jpg"                              # Case-insensitive search for .jpg files
 
# Execute a command for each match:
find . -name "*.txt" -exec wc -l '{}' ';'          # Count lines in every .txt file found
 
# -------------------------------
# 2. WHICH — Locate Executable Files
# -------------------------------
 
which ls                                           # Show path to the binary for a command (e.g., /bin/ls)
type ls                                            # Show if a command is built-in, alias, or binary
command -v ls                                      # POSIX-compliant way to find command location
 
# -------------------------------
# 3. LOCATE — Fast Filename Search Using Index
# -------------------------------
 
locate ".txt"                                      # Find all files with ".txt" in their path
sudo updatedb                                      # Manually update the locate database index
 
# Notes:
# - locate is very fast but may show outdated results.
# - Cannot search by size, type, or permissions like find can.
 
# -------------------------------
# 4. OTHER USEFUL TOOLS
# -------------------------------
 
whereis ls                                         # Show binary, source, and man page locations for a command
fd pattern                                         # Modern, faster replacement for find (if installed)
grep -rl pattern directory/                        # Find files containing specific text recursively
 
# ===============================================
# UNIX File Compression & Backup Guide
# ===============================================
# Tools for archiving, compressing, and backing up files in UNIX/Linux.
# Includes tar, cpio, compress, gzip, and modern alternatives.
# ===============================================
 
# -------------------------------
# 1. TAR — Tape Archiver
# -------------------------------
 
tar -cvf backup.tar /home/user/documents          # Create an archive file (no compression)
tar -czvf backup.tar.gz /home/user/documents      # Create and compress archive with gzip
tar -cjvf backup.tar.bz2 /home/user/documents     # Create and compress archive with bzip2
tar -cJvf backup.tar.xz /home/user/documents      # Create and compress archive with xz
tar -tvf backup.tar                               # List contents of an archive
tar -xvf backup.tar                               # Extract archive contents
tar -xvf backup.tar -C /tmp/restore               # Extract archive into a specific directory
tar --exclude="*.tmp" -czvf backup.tar.gz dir/    # Exclude files matching pattern from archive
 
# Common tar options:
# -c : Create archive
# -x : Extract archive
# -t : List contents
# -v : Verbose (show files as processed)
# -f : Specify archive filename
# -z : Use gzip compression
# -j : Use bzip2 compression
# -J : Use xz compression
# --exclude=PATTERN : Skip files matching pattern
 
# -------------------------------
# 2. CPIO — Copy In/Out Archiver
# -------------------------------
 
find . -depth -print | cpio -ov -Htar > backup.tar   # Create a tar-format archive using cpio
cpio -tv < backup.tar                                # List contents of a cpio archive
cpio -idv < backup.tar                               # Extract files from a cpio archive
cpio -iduv < backup.tar                              # Extract and overwrite existing files
 
# Common cpio options:
# -o : Create archive (copy-out mode)
# -i : Extract archive (copy-in mode)
# -t : List contents
# -v : Verbose output
# -d : Create directories as needed
# -u : Overwrite existing files
# -H : Set archive format (tar, crc, odc, etc.)
 
# -------------------------------
# 3. COMPRESS & GZIP — File Compression Tools
# -------------------------------
 
compress file.txt                                    # Compress file to file.txt.Z
compress -d file.txt.Z                               # Decompress .Z file
 
gzip file.txt                                        # Compress file to file.txt.gz
gzip -d file.txt.gz                                  # Decompress .gz file
zcat file.txt.gz                                     # View compressed file contents without decompressing
 
# -------------------------------
# 4. MODERN ALTERNATIVES & IMPROVEMENTS
# -------------------------------
 
tar -cjvf backup.tar.bz2 dir/                        # Use bzip2 for better compression than gzip
tar -cJvf backup.tar.xz dir/                         # Use xz for maximum compression
zip -r backup.zip dir/                               # Create a cross-platform .zip archive
rsync -avz /source user@host:/backup                 # Perform incremental backups over network
 
# ------------------------------
# DHCP
# ------------------------------
 
# ------------------------------
# SERVER: Identify network interfaces
# ------------------------------
echo "Listing all network interfaces..."
ip link show
ip addr show
 
# ------------------------------
# SERVER: Assign static IP to Host-only adapter (ens4 example)
# Adjust 'ens4' to your actual interface name
# ------------------------------
echo "Configuring static IP for ens4..."
sudo tee /etc/netplan/99-static-dhcp-server.yaml > /dev/null <<EOF
network:
  version: 2
  ethernets:
    ens4:
      dhcp4: no
      addresses: [192.168.100.1/24]
      nameservers:
        addresses: [1.1.1.1, 8.8.8.8]
EOF
 
sudo netplan generate
sudo netplan apply
ip addr show dev ens4
 
 
# ------------------------------
# SERVER: Create lease database & set permissions
# ------------------------------
echo "Creating DHCP lease file and setting permissions..."
sudo mkdir -p /var/lib/dhcp
sudo touch /var/lib/dhcp/dhcpd.leases
sudo chown root:dhcpd /var/lib/dhcp /var/lib/dhcp/dhcpd.leases
sudo chmod 0644 /var/lib/dhcp/dhcpd.leases
 
# ------------------------------
# SERVER: Configure DHCP server
# ------------------------------
echo "Configuring /etc/dhcp/dhcpd.conf..."
sudo tee /etc/dhcp/dhcpd.conf > /dev/null <<EOF
authoritative;
ddns-update-style none;
default-lease-time 600;
max-lease-time 7200;
option domain-name "lab.local";
option domain-name-servers 1.1.1.1, 8.8.8.8;
 
subnet 192.168.100.0 netmask 255.255.255.0 {
  range 192.168.100.100 192.168.100.200;
  option routers 192.168.100.1;
  option broadcast-address 192.168.100.255;
}
EOF
 
# ------------------------------
# SERVER: Specify listening interface
# Replace 'ens4' with your interface
# ------------------------------
echo "Setting DHCP server interface..."
sudo sed -i 's/^INTERFACESv4=.*/INTERFACESv4="ens4"/' /etc/default/isc-dhcp-server
sudo sed -i 's/^INTERFACESv6=.*/INTERFACESv6=""/' /etc/default/isc-dhcp-server
 
# ------------------------------
# SERVER: Start and enable DHCP service
# ------------------------------
echo "Starting and enabling DHCP server..."
sudo systemctl daemon-reload
sudo systemctl enable --now isc-dhcp-server
sudo systemctl status isc-dhcp-server --no-pager
 
# Watch DHCP logs in real time (press Ctrl+C to exit)...
sudo journalctl -u isc-dhcp-server -f
 
# ------------------------------
# CLIENT: Install DHCP client and request lease
# ------------------------------
# Installing DHCP client (if missing)...
sudo apt install isc-dhcp-client -y
 
# Requesting DHCP lease on interface ens4...
sudo dhclient -v ens4
 
# ------------------------------
# CLIENT/SERVER: Capture DORA exchange
# ------------------------------
#To capture DHCP packets, run the following command:
sudo tcpdump -n -i ens4 udp and \(port 67 or port 68\) -vv
 
# ------------------------------
# CLIENT: Verify IP assignment
# ------------------------------
#Check assigned IP and routing:
ip addr show dev ens4
ip route show
ping -c3 1.1.1.1
ping -c3 google.com
 
# ------------------------------
# SERVER: View lease file
# ------------------------------
#Check DHCP leases on server:
sudo cat /var/lib/dhcp/dhcpd.leases
 
# ------------------------------
# DNS
# ------------------------------
 
# ------------------------------
# 1) Update & install BIND9
# ------------------------------
#Updating packages and installing BIND9...
sudo apt update
sudo apt install -y bind9 bind9-utils
 
# Start & enable BIND9 service
sudo systemctl start bind9
sudo systemctl enable bind9
sudo systemctl status bind9 --no-pager
 
# ------------------------------
# 2) Configure Forward Zone
# ------------------------------
FORWARD_ZONE="mynetwork.local"
FORWARD_DB="/etc/bind/db.${FORWARD_ZONE}"
 
# Add zone to named.conf.local if not already present
grep -q "${FORWARD_ZONE}" /etc/bind/named.conf.local || sudo tee -a /etc/bind/named.conf.local > /dev/null <<EOF
 
zone "${FORWARD_ZONE}" {
    type master;
    file "${FORWARD_DB}";
};
EOF
 
# Copy template to create zone file
sudo cp /etc/bind/db.local ${FORWARD_DB}
 
# Replace zone file with example content
sudo tee ${FORWARD_DB} > /dev/null <<EOF
\$TTL    604800
@       IN      SOA     ns1.${FORWARD_ZONE}. admin.${FORWARD_ZONE}. (
                              2         ; Serial
                         604800         ; Refresh
                          86400         ; Retry
                        2419200         ; Expire
                         604800 )       ; Negative Cache TTL
 
; Name server
@       IN      NS      ns1.${FORWARD_ZONE}.
 
; A records
ns1     IN      A       192.168.56.10
www     IN      A       192.168.56.20
www2    IN      A       192.168.56.21
 
; MX records
@       IN      MX 10   mail.${FORWARD_ZONE}.
@       IN      MX 20   mail2.${FORWARD_ZONE}.
 
mail    IN      A       192.168.56.30
mail2   IN      A       192.168.56.31
EOF
 
# ------------------------------
# 3) Configure Reverse Zone
# ------------------------------
REVERSE_ZONE="56.168.192.in-addr.arpa"
REVERSE_DB="/etc/bind/db.192.168.56"
 
grep -q "${REVERSE_ZONE}" /etc/bind/named.conf.local || sudo tee -a /etc/bind/named.conf.local > /dev/null <<EOF
 
zone "${REVERSE_ZONE}" {
    type master;
    file "${REVERSE_DB}";
};
EOF
 
sudo cp /etc/bind/db.127 ${REVERSE_DB}
 
sudo tee ${REVERSE_DB} > /dev/null <<EOF
\$TTL    604800
@       IN      SOA     ns1.${FORWARD_ZONE}. admin.${FORWARD_ZONE}. (
                              2         ; Serial
                         604800         ; Refresh
                          86400         ; Retry
                        2419200         ; Expire
                         604800 )       ; Negative Cache TTL
 
; Name server
@       IN      NS      ns1.${FORWARD_ZONE}.
 
; PTR records
10      IN      PTR     ns1.${FORWARD_ZONE}.
20      IN      PTR     www.${FORWARD_ZONE}.
21      IN      PTR     www2.${FORWARD_ZONE}.
30      IN      PTR     mail.${FORWARD_ZONE}.
EOF
 
# ------------------------------
# 4) Check configuration
# ------------------------------
sudo named-checkconf
sudo named-checkzone ${FORWARD_ZONE} ${FORWARD_DB}
sudo named-checkzone ${REVERSE_ZONE} ${REVERSE_DB}
 
# Restart BIND9
sudo systemctl restart bind9
 
# ------------------------------
# 5) Optional: Configure DDNS (Dynamic DNS)
# ------------------------------
echo "Setting up optional DDNS integration..."
 
# Generate TSIG key for DHCP <-> DNS
# NOTE: Replace "XXXXXXXXXXXXXXXXXXXX==" with your actual secret
KEY_NAME="dhcpupdate"
SECRET="XXXXXXXXXXXXXXXXXXXX=="
 
sudo tee /etc/bind/keys.conf > /dev/null <<EOF
key "${KEY_NAME}" {
    algorithm hmac-sha256;
    secret "${SECRET}";
};
EOF
 
# Include DDNS key in forward/reverse zones
sudo sed -i "/zone \"${FORWARD_ZONE}\" {/a \    allow-update { key ${KEY_NAME}; };" /etc/bind/named.conf.local
sudo sed -i "/zone \"${REVERSE_ZONE}\" {/a \    allow-update { key ${KEY_NAME}; };" /etc/bind/named.conf.local
 
# Restart BIND to apply DDNS
sudo systemctl restart bind9
 
# ------------------------------
# 6) Testing DNS from server
# ------------------------------
# Testing forward lookups...
dig @127.0.0.1 www.${FORWARD_ZONE}
dig @127.0.0.1 www2.${FORWARD_ZONE}
dig @127.0.0.1 mail.${FORWARD_ZONE}
 
# Testing reverse lookup...
dig -x 192.168.56.20

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