Linux: Commands Reference Guide

This comprehensive guide covers essential Linux commands with their options and practical examples for everyday system administration and development tasks.

File System Navigation

pwd (Print Working Directory)

Displays the current working directory path.

Syntax:

pwd [OPTIONS]

Common Options:

  • -L - Print the logical path (follows symbolic links)

  • -P - Print the physical path (resolves symbolic links)

Examples:

# Show current directory
pwd

# Show physical path (without symlinks)
pwd -P

ls (List Directory Contents)

Lists files and directories in the specified location.

Syntax:

ls [OPTIONS] [FILE/DIRECTORY]

Common Options:

  • -a - Show all files including hidden files (starting with .)

  • -l - Long format with detailed information

  • -h - Human-readable file sizes (with -l)

  • -t - Sort by modification time, newest first

  • -r - Reverse order

  • -R - Recursive listing of subdirectories

  • -S - Sort by file size, largest first

  • -d - List directories themselves, not their contents

  • -i - Print inode number of each file

  • -1 - List one file per line

Examples:

# Basic listing
ls

# List all files including hidden
ls -a

# Long format with human-readable sizes
ls -lh

# Sort by modification time
ls -lt

# List files recursively
ls -R

# List only directories
ls -d */

# Combination: detailed, all files, human-readable
ls -lah

# Sort by size, largest first
ls -lhS

cd (Change Directory)

Changes the current working directory.

Syntax:

cd [DIRECTORY]

Special Paths:

  • ~ - Home directory

  • - - Previous directory

  • .. - Parent directory

  • . - Current directory

  • / - Root directory

Examples:

# Go to home directory
cd
cd ~

# Go to specific directory
cd /var/www/html

# Go to parent directory
cd ..

# Go back to previous directory
cd -

# Go up two levels
cd ../..

# Relative path
cd documents/work

tree

Displays directory structure in a tree format.

Syntax:

tree [OPTIONS] [DIRECTORY]

Common Options:

  • -a - Show all files including hidden

  • -d - List directories only

  • -L [level] - Maximum display depth

  • -h - Print file sizes in human-readable format

  • -p - Print file permissions

  • -u - Print file owner/group

Examples:

# Show tree of current directory
tree

# Show only 2 levels deep
tree -L 2

# Show directories only
tree -d

# Show with file sizes and permissions
tree -hp

Note

If tree is not installed, install it using: sudo apt-get install tree

File Operations

cp (Copy)

Copies files and directories.

Syntax:

cp [OPTIONS] SOURCE DESTINATION

Common Options:

  • -r or -R - Copy directories recursively

  • -i - Interactive mode (prompt before overwrite)

  • -v - Verbose mode

  • -u - Copy only when source is newer

  • -p - Preserve file attributes

  • -a - Archive mode (preserve all attributes)

  • -n - No overwrite

Examples:

# Copy file
cp file.txt backup.txt

# Copy directory recursively
cp -r /source/dir /destination/dir

# Copy with confirmation
cp -i file.txt backup.txt

# Copy preserving attributes
cp -p file.txt backup.txt

# Copy multiple files to directory
cp file1.txt file2.txt file3.txt /destination/

# Archive mode (preserve everything)
cp -a /source/ /backup/

mv (Move)

Moves or renames files and directories.

Syntax:

mv [OPTIONS] SOURCE DESTINATION

Common Options:

  • -i - Interactive mode (prompt before overwrite)

  • -v - Verbose mode

  • -u - Move only when source is newer

  • -n - No overwrite

  • -f - Force overwrite

Examples:

# Rename file
mv oldname.txt newname.txt

# Move file to directory
mv file.txt /destination/

# Move multiple files
mv file1.txt file2.txt /destination/

# Move with confirmation
mv -i file.txt /destination/

# Rename directory
mv old_directory new_directory

rm (Remove)

Removes files and directories.

Syntax:

rm [OPTIONS] FILE/DIRECTORY

Common Options:

  • -r or -R - Remove directories recursively

  • -i - Interactive mode (prompt before removal)

  • -f - Force removal without prompts

  • -v - Verbose mode

  • -d - Remove empty directories

Examples:

# Remove file
rm file.txt

# Remove directory recursively
rm -r directory/

# Remove with confirmation
rm -i file.txt

# Force remove without confirmation
rm -f file.txt

# Remove multiple files
rm file1.txt file2.txt file3.txt

# Remove all .log files
rm *.log

Warning

Use rm -rf with extreme caution. This will permanently delete files without confirmation.

mkdir (Make Directory)

Creates new directories.

Syntax:

mkdir [OPTIONS] DIRECTORY

Common Options:

  • -p - Create parent directories as needed

  • -v - Verbose mode

  • -m - Set permissions

Examples:

# Create single directory
mkdir new_folder

# Create nested directories
mkdir -p path/to/new/folder

# Create with specific permissions
mkdir -m 755 new_folder

# Create multiple directories
mkdir folder1 folder2 folder3

touch

Creates empty files or updates file timestamps.

Syntax:

touch [OPTIONS] FILE

Common Options:

  • -a - Change access time only

  • -m - Change modification time only

  • -c - Do not create file if it doesn’t exist

  • -t - Use specified time instead of current

Examples:

# Create empty file
touch newfile.txt

# Create multiple files
touch file1.txt file2.txt file3.txt

# Update timestamp
touch existing_file.txt

# Create file with specific time
touch -t 202301011200 file.txt

File Viewing and Editing

cat (Concatenate)

Displays file contents, concatenates files, and creates files.

Syntax:

cat [OPTIONS] [FILE]

Common Options:

  • -n - Number all output lines

  • -b - Number non-empty lines

  • -s - Squeeze multiple blank lines

  • -A - Show all characters including non-printing

  • -E - Display $ at end of each line

  • -T - Display tabs as ^I

Examples:

# Display file contents
cat file.txt

# Display multiple files
cat file1.txt file2.txt

# Display with line numbers
cat -n file.txt

# Concatenate files into new file
cat file1.txt file2.txt > combined.txt

# Append to file
cat file1.txt >> existing.txt

# Create new file (Ctrl+D to save)
cat > newfile.txt

# Show non-printing characters
cat -A file.txt

less

Views file contents one page at a time with backward navigation.

Syntax:

less [OPTIONS] FILE

Common Options:

  • -N - Show line numbers

  • -S - Chop long lines

  • -i - Case-insensitive search

  • -X - Don’t clear screen on exit

  • -F - Quit if entire file fits on screen

Navigation Keys:

  • Space or f - Forward one page

  • b - Backward one page

  • d - Forward half page

  • u - Backward half page

  • g - Go to start of file

  • G - Go to end of file

  • /pattern - Search forward

  • ?pattern - Search backward

  • n - Next search match

  • N - Previous search match

  • q - Quit

Examples:

# View file
less file.txt

# View with line numbers
less -N file.txt

# View without line wrapping
less -S file.txt

# Case-insensitive search
less -i file.txt

# View command output
ls -la | less

tail

Displays the last part of files.

Syntax:

tail [OPTIONS] [FILE]

Common Options:

  • -n [NUM] - Print last NUM lines (default 10)

  • -c [NUM] - Print last NUM bytes

  • -f - Follow file (monitor for changes)

  • -F - Follow file with retry

  • -q - Quiet mode (no headers)

  • --pid=[PID] - With -f, terminate after process PID dies

Examples:

# Show last 10 lines (default)
tail file.txt

# Show last 20 lines
tail -n 20 file.txt

# Short form
tail -20 file.txt

# Monitor file in real-time
tail -f /var/log/apache2/error.log

# Follow with retry (useful for log rotation)
tail -F /var/log/syslog

# Show last 100 bytes
tail -c 100 file.txt

# Show all lines from line 50 onwards
tail -n +50 file.txt

Text Processing

grep (Global Regular Expression Print)

Searches for patterns in files.

Syntax:

grep [OPTIONS] PATTERN [FILE]

Common Options:

  • -i - Case-insensitive search

  • -v - Invert match (show non-matching lines)

  • -r or -R - Recursive search

  • -n - Show line numbers

  • -c - Count matching lines

  • -l - List filenames only

  • -w - Match whole words

  • -A [NUM] - Show NUM lines after match

  • -B [NUM] - Show NUM lines before match

  • -C [NUM] - Show NUM lines before and after match

  • -E - Extended regex (same as egrep)

  • -o - Print only matched parts

  • --color - Highlight matches

Examples:

# Basic search
grep "error" logfile.txt

# Case-insensitive
grep -i "error" logfile.txt

# Search recursively in directory
grep -r "function" /var/www/html/

# Show line numbers
grep -n "error" logfile.txt

# Count matches
grep -c "error" logfile.txt

# Invert match
grep -v "success" logfile.txt

# Whole word match
grep -w "log" file.txt

# Multiple patterns
grep -e "error" -e "warning" logfile.txt

# Show context (2 lines before and after)
grep -C 2 "error" logfile.txt

# Search in multiple files
grep "pattern" *.txt

# With color highlighting
grep --color "error" logfile.txt

# List only filenames
grep -l "pattern" *.txt

sed (Stream Editor)

Performs text transformations on files.

Syntax:

sed [OPTIONS] 'COMMAND' FILE

Common Options:

  • -i - Edit files in-place

  • -e - Add script to commands

  • -n - Suppress automatic printing

  • -r or -E - Extended regex

Common Commands:

  • s/pattern/replacement/ - Substitute

  • d - Delete

  • p - Print

  • a - Append

  • i - Insert

Examples:

# Replace first occurrence in each line
sed 's/old/new/' file.txt

# Replace all occurrences (global)
sed 's/old/new/g' file.txt

# Replace in-place (modify original file)
sed -i 's/old/new/g' file.txt

# Delete lines containing pattern
sed '/pattern/d' file.txt

# Delete empty lines
sed '/^$/d' file.txt

# Delete lines 5-10
sed '5,10d' file.txt

# Print only lines 5-10
sed -n '5,10p' file.txt

# Replace only on specific lines
sed '5,10s/old/new/g' file.txt

# Multiple operations
sed -e 's/old/new/g' -e 's/foo/bar/g' file.txt

# Case-insensitive replace
sed 's/old/new/gi' file.txt

# Add line after match
sed '/pattern/a New line text' file.txt

# Insert line before match
sed '/pattern/i New line text' file.txt

# Replace with backreference
sed 's/\([0-9]*\)/Number: \1/' file.txt

Compression and Archives

tar

Archives and extracts files.

Syntax:

tar [OPTIONS] FILE

Common Options:

  • -c - Create archive

  • -x - Extract archive

  • -v - Verbose mode

  • -f - Specify filename

  • -z - Compress with gzip (.tar.gz)

  • -j - Compress with bzip2 (.tar.bz2)

  • -J - Compress with xz (.tar.xz)

  • -t - List archive contents

  • -r - Append to archive

  • -u - Update archive

  • -C - Change to directory

Examples:

# Create tar archive
tar -cvf archive.tar directory/

# Create gzip compressed archive
tar -cvzf archive.tar.gz directory/

# Create bzip2 compressed archive
tar -cvjf archive.tar.bz2 directory/

# Extract tar archive
tar -xvf archive.tar

# Extract gzip archive
tar -xvzf archive.tar.gz

# Extract to specific directory
tar -xvzf archive.tar.gz -C /destination/path/

# List archive contents
tar -tvf archive.tar

# Extract specific file
tar -xvf archive.tar file.txt

# Append files to existing archive
tar -rvf archive.tar newfile.txt

# Extract single file from archive
tar -xvzf archive.tar.gz path/to/file

gzip

Compresses or decompresses files.

Syntax:

gzip [OPTIONS] FILE

Common Options:

  • -d - Decompress (same as gunzip)

  • -k - Keep original file

  • -r - Recursive compression

  • -v - Verbose mode

  • -[1-9] - Compression level (1=fastest, 9=best)

  • -l - List compressed file information

  • -c - Write to stdout

Examples:

# Compress file
gzip file.txt

# Compress and keep original
gzip -k file.txt

# Decompress file
gzip -d file.txt.gz
gunzip file.txt.gz

# Compress with best compression
gzip -9 file.txt

# Compress recursively
gzip -r directory/

# View compressed file info
gzip -l file.txt.gz

# Compress to stdout
gzip -c file.txt > file.txt.gz

zip/unzip

Creates and extracts ZIP archives.

zip Syntax:

zip [OPTIONS] archive.zip FILE

zip Common Options:

  • -r - Recursive

  • -e - Encrypt with password

  • -q - Quiet mode

  • -v - Verbose mode

  • -[0-9] - Compression level

  • -u - Update existing archive

  • -d - Delete files from archive

zip Examples:

# Create zip archive
zip archive.zip file.txt

# Create zip with multiple files
zip archive.zip file1.txt file2.txt

# Zip directory recursively
zip -r archive.zip directory/

# Zip with password protection
zip -e -r secure.zip directory/

# Add files to existing archive
zip -u archive.zip newfile.txt

# Best compression
zip -9 -r archive.zip directory/

unzip Syntax:

unzip [OPTIONS] archive.zip

unzip Common Options:

  • -l - List archive contents

  • -d - Extract to directory

  • -o - Overwrite without prompting

  • -q - Quiet mode

  • -v - Verbose mode

unzip Examples:

# Extract zip archive
unzip archive.zip

# Extract to specific directory
unzip archive.zip -d /destination/

# List archive contents
unzip -l archive.zip

# Extract specific file
unzip archive.zip file.txt

# Extract without prompting
unzip -o archive.zip

System Information

uname

Displays system information.

Syntax:

uname [OPTIONS]

Common Options:

  • -a - All information

  • -s - Kernel name

  • -n - Network node hostname

  • -r - Kernel release

  • -v - Kernel version

  • -m - Machine hardware name

  • -o - Operating system

Examples:

# Show all information
uname -a

# Show kernel version
uname -r

# Show operating system
uname -o

# Show machine hardware
uname -m

top

Displays real-time system processes and resource usage.

Syntax:

top [OPTIONS]

Interactive Commands:

  • q - Quit

  • k - Kill process

  • r - Renice process

  • h - Help

  • M - Sort by memory usage

  • P - Sort by CPU usage

  • 1 - Show individual CPU cores

  • c - Show full command path

Common Options:

  • -d [SEC] - Update delay in seconds

  • -u [USER] - Show specific user

  • -p [PID] - Monitor specific process

  • -n [NUM] - Number of iterations

Examples:

# Start top
top

# Update every 2 seconds
top -d 2

# Show processes for specific user
top -u username

# Monitor specific process
top -p 1234

# Run 5 iterations then exit
top -n 5

htop

Interactive process viewer (enhanced top).

Syntax:

htop [OPTIONS]

Interactive Commands:

  • F1 - Help

  • F2 - Setup

  • F3 - Search

  • F4 - Filter

  • F5 - Tree view

  • F6 - Sort by

  • F9 - Kill process

  • F10 - Quit

  • Space - Tag process

  • U - Show specific user

  • / - Search

Examples:

# Start htop
htop

# Show specific user processes
htop -u username

# Sort by memory
htop -s PERCENT_MEM

Note

If htop is not installed, install it using: sudo apt-get install htop

df (Disk Free)

Reports file system disk space usage.

Syntax:

df [OPTIONS] [FILE/DIRECTORY]

Common Options:

  • -h - Human-readable sizes

  • -a - Include all file systems

  • -T - Show file system type

  • -i - Show inode information

  • -t [TYPE] - Show only specified type

  • -x [TYPE] - Exclude specified type

  • --total - Show grand total

Examples:

# Show disk usage
df

# Human-readable format
df -h

# Show file system types
df -hT

# Show inode usage
df -i

# Show specific file system type
df -t ext4

# Exclude file system type
df -x tmpfs

# Show with total
df -h --total

# Check specific directory
df -h /var/www/html

du (Disk Usage)

Estimates file and directory space usage.

Syntax:

du [OPTIONS] [FILE/DIRECTORY]

Common Options:

  • -h - Human-readable sizes

  • -s - Summary (total size only)

  • -a - Include files

  • -c - Show grand total

  • -d [NUM] - Max depth

  • --max-depth=[NUM] - Max depth

  • -x - Skip different file systems

  • --exclude=[PATTERN] - Exclude pattern

Examples:

# Show current directory usage
du -h

# Show summary only
du -sh

# Show with grand total
du -ch

# Limit depth to 1 level
du -h --max-depth=1

# Sort by size
du -h | sort -h

# Show top 10 largest directories
du -h | sort -rh | head -10

# Exclude pattern
du -h --exclude="*.log"

# Check specific directory
du -sh /var/www/html

# Show all files and directories
du -ah

free

Displays memory usage information.

Syntax:

free [OPTIONS]

Common Options:

  • -h - Human-readable format

  • -b - Bytes

  • -k - Kilobytes

  • -m - Megabytes

  • -g - Gigabytes

  • -s [SEC] - Update every SEC seconds

  • -c [NUM] - Number of updates

  • -t - Show total line

  • -w - Wide mode

Examples:

# Show memory usage
free

# Human-readable format
free -h

# Show in megabytes
free -m

# Show in gigabytes
free -g

# Update every 2 seconds
free -h -s 2

# Show 5 updates
free -h -s 2 -c 5

# Show with total line
free -ht

# Wide mode (separate buffers/cache)
free -hw

Search and Find

find

Searches for files in directory hierarchy.

Syntax:

find [PATH] [OPTIONS] [EXPRESSION]

Common Options:

  • -name [PATTERN] - Search by name

  • -iname [PATTERN] - Case-insensitive name

  • -type [TYPE] - File type (f=file, d=directory, l=link)

  • -size [SIZE] - File size

  • -mtime [DAYS] - Modified days ago

  • -user [USER] - Owner

  • -perm [MODE] - Permissions

  • -exec [CMD] {} \; - Execute command on results

  • -delete - Delete found files

  • -maxdepth [NUM] - Maximum depth

Examples:

# Find files by name
find /path -name "*.txt"

# Case-insensitive search
find /path -iname "*.TXT"

# Find directories only
find /path -type d

# Find files only
find /path -type f

# Find files larger than 100MB
find /path -size +100M

# Find files smaller than 10KB
find /path -size -10k

# Find files modified in last 7 days
find /path -mtime -7

# Find files modified more than 30 days ago
find /path -mtime +30

# Find and delete
find /path -name "*.tmp" -delete

# Find and execute command
find /path -name "*.log" -exec rm {} \;

# Find with permissions
find /path -perm 644

# Limit search depth
find /path -maxdepth 2 -name "*.txt"

# Find empty files
find /path -type f -empty

# Find by owner
find /path -user username

Permissions

chown (Change Owner)

Changes file owner and group.

Syntax:

chown [OPTIONS] USER[:GROUP] FILE

Common Options:

  • -R - Recursive

  • -v - Verbose

  • -c - Report only changes

  • --reference=[FILE] - Use reference file

Examples:

# Change owner
chown username file.txt

# Change owner and group
chown username:groupname file.txt

# Change recursively
chown -R username:groupname directory/

# Change group only
chown :groupname file.txt

# Verbose mode
chown -v username file.txt

# Use reference file
chown --reference=ref.txt file.txt

chmod (Change Mode)

Changes file permissions.

Syntax:

chmod [OPTIONS] MODE FILE

Permission Numbers:

  • 4 - Read (r)

  • 2 - Write (w)

  • 1 - Execute (x)

Permission Format: [owner][group][others]

Example: 755 = rwxr-xr-x

Common Options:

  • -R - Recursive

  • -v - Verbose

  • -c - Report only changes

Symbolic Mode:

  • u - User/owner

  • g - Group

  • o - Others

  • a - All

  • + - Add permission

  • - - Remove permission

  • = - Set exact permission

Examples:

# Numeric mode - full permissions
chmod 777 file.txt

# Numeric mode - read/write for owner, read for others
chmod 644 file.txt

# Numeric mode - executable
chmod 755 script.sh

# Recursive
chmod -R 755 directory/

# Symbolic - add execute
chmod +x script.sh

# Symbolic - remove write from others
chmod o-w file.txt

# Symbolic - set exact permissions
chmod u=rwx,g=rx,o=r file.txt

# Add read for all
chmod a+r file.txt

# Remove execute from group
chmod g-x file.txt

# Make directory and contents readable
chmod -R a+rX directory/

Networking

ping

Tests network connectivity.

Syntax:

ping [OPTIONS] DESTINATION

Common Options:

  • -c [NUM] - Number of packets to send

  • -i [SEC] - Interval between packets

  • -s [SIZE] - Packet size

  • -t [TTL] - Time to live

  • -W [SEC] - Timeout in seconds

  • -q - Quiet mode

Examples:

# Ping host
ping google.com

# Send 5 packets
ping -c 5 google.com

# Set interval to 2 seconds
ping -i 2 google.com

# Set packet size
ping -s 1024 google.com

# Ping IP address
ping 8.8.8.8

# Quiet mode (summary only)
ping -c 10 -q google.com

curl

Transfers data from or to a server.

Syntax:

curl [OPTIONS] URL

Common Options:

  • -X [METHOD] - HTTP method (GET, POST, PUT, DELETE)

  • -d [DATA] - HTTP POST data

  • -H [HEADER] - Custom header

  • -o [FILE] - Output to file

  • -O - Save with remote filename

  • -L - Follow redirects

  • -i - Include headers

  • -I - Show headers only

  • -u [USER:PASS] - Authentication

  • -s - Silent mode

  • -v - Verbose mode

Examples:

# Simple GET request
curl https://example.com

# Download file
curl -O https://example.com/file.zip

# Save to specific filename
curl -o output.html https://example.com

# Follow redirects
curl -L https://example.com

# POST request with data
curl -X POST -d "param1=value1&param2=value2" https://api.example.com

# POST JSON data
curl -X POST -H "Content-Type: application/json" -d '{"key":"value"}' https://api.example.com

# Custom headers
curl -H "Authorization: Bearer token" https://api.example.com

# Show response headers
curl -i https://example.com

# Show only headers
curl -I https://example.com

# Basic authentication
curl -u username:password https://example.com

# Upload file
curl -F "file=@/path/to/file" https://example.com/upload

wget

Downloads files from the web.

Syntax:

wget [OPTIONS] URL

Common Options:

  • -O [FILE] - Save to specific filename

  • -c - Continue partial download

  • -b - Background download

  • -q - Quiet mode

  • -v - Verbose mode

  • -r - Recursive download

  • -np - No parent directories

  • -l [NUM] - Maximum recursion depth

  • --limit-rate=[RATE] - Limit download speed

  • --user=[USER] - Username

  • --password=[PASS] - Password

Examples:

# Download file
wget https://example.com/file.zip

# Save with different name
wget -O newname.zip https://example.com/file.zip

# Continue interrupted download
wget -c https://example.com/largefile.iso

# Background download
wget -b https://example.com/file.zip

# Download multiple files
wget https://example.com/file1.zip https://example.com/file2.zip

# Recursive download
wget -r https://example.com/directory/

# Limit download speed
wget --limit-rate=500k https://example.com/file.zip

# Quiet mode
wget -q https://example.com/file.zip

# Authentication
wget --user=username --password=password https://example.com/file.zip

ssh (Secure Shell)

Connects to remote servers securely.

Syntax:

ssh [OPTIONS] [USER@]HOST [COMMAND]

Common Options:

  • -p [PORT] - Specify port

  • -i [KEY] - Identity file (private key)

  • -v - Verbose mode

  • -C - Enable compression

  • -L [PORT:HOST:HOSTPORT] - Local port forwarding

  • -R [PORT:HOST:HOSTPORT] - Remote port forwarding

  • -N - No remote command

  • -f - Background mode

Examples:

# Connect to server
ssh username@hostname

# Connect to specific port
ssh -p 2222 username@hostname

# Use specific key file
ssh -i ~/.ssh/id_rsa username@hostname

# Execute remote command
ssh username@hostname 'ls -la'

# Local port forwarding
ssh -L 8080:localhost:80 username@hostname

# Copy SSH key to server
ssh-copy-id username@hostname

# Verbose mode (debugging)
ssh -v username@hostname

scp (Secure Copy)

Securely copies files between hosts.

Syntax:

scp [OPTIONS] SOURCE DESTINATION

Common Options:

  • -r - Recursive (for directories)

  • -P [PORT] - Specify port

  • -i [KEY] - Identity file

  • -v - Verbose mode

  • -C - Enable compression

  • -p - Preserve file attributes

  • -q - Quiet mode

Examples:

# Copy file to remote server
scp file.txt username@hostname:/path/to/destination/

# Copy file from remote server
scp username@hostname:/path/to/file.txt /local/path/

# Copy directory recursively
scp -r directory/ username@hostname:/path/to/destination/

# Specify port
scp -P 2222 file.txt username@hostname:/path/

# Use specific key
scp -i ~/.ssh/id_rsa file.txt username@hostname:/path/

# Copy multiple files
scp file1.txt file2.txt username@hostname:/path/

# Copy between two remote hosts
scp user1@host1:/path/file.txt user2@host2:/path/

# Preserve file attributes
scp -p file.txt username@hostname:/path/

# With compression
scp -C largefile.zip username@hostname:/path/

Process Management

nohup

Runs commands immune to hangups, with output to a non-tty.

Syntax:

nohup COMMAND [ARGS] &

Examples:

# Run command in background
nohup ./script.sh &

# Run with custom output file
nohup ./script.sh > output.log 2>&1 &

# Run long process
nohup python long_script.py &

# Check process
jobs
ps aux | grep script.sh

# View nohup output
tail -f nohup.out

Note

By default, output is written to nohup.out in the current directory.

Package Management

dpkg

Debian package manager (low-level).

Syntax:

dpkg [OPTIONS] PACKAGE

Common Options:

  • -i - Install package

  • -r - Remove package

  • -P - Purge package

  • -l - List installed packages

  • -L - List files installed by package

  • -s - Show package status

  • -S - Search for file owner

Examples:

# Install package
sudo dpkg -i package.deb

# Remove package
sudo dpkg -r package-name

# Purge package (remove including config)
sudo dpkg -P package-name

# List all installed packages
dpkg -l

# List specific package
dpkg -l | grep package-name

# Show package information
dpkg -s package-name

# List files from package
dpkg -L package-name

# Find package that owns file
dpkg -S /path/to/file

# Reconfigure package
sudo dpkg-reconfigure package-name

snap

Universal Linux package manager.

Syntax:

snap [COMMAND] [OPTIONS]

Common Commands:

  • install - Install snap

  • remove - Remove snap

  • list - List installed snaps

  • find - Find snaps in store

  • refresh - Update snaps

  • info - Show snap information

Examples:

# Install snap
sudo snap install package-name

# Install from specific channel
sudo snap install package-name --channel=edge

# Install classic snap
sudo snap install package-name --classic

# Remove snap
sudo snap remove package-name

# List installed snaps
snap list

# Search for snaps
snap find package-name

# Update all snaps
sudo snap refresh

# Update specific snap
sudo snap refresh package-name

# Show snap info
snap info package-name

# Disable snap
sudo snap disable package-name

# Enable snap
sudo snap enable package-name

systemctl

Controls the systemd system and service manager.

Syntax:

systemctl [COMMAND] [SERVICE]

Common Commands:

  • start - Start service

  • stop - Stop service

  • restart - Restart service

  • reload - Reload configuration

  • enable - Enable service at boot

  • disable - Disable service at boot

  • status - Show service status

  • is-active - Check if active

  • is-enabled - Check if enabled

  • list-units - List units

Examples:

# Start service
sudo systemctl start apache2

# Stop service
sudo systemctl stop apache2

# Restart service
sudo systemctl restart apache2

# Reload configuration
sudo systemctl reload apache2

# Enable service at boot
sudo systemctl enable apache2

# Disable service at boot
sudo systemctl disable apache2

# Check service status
systemctl status apache2

# Check if service is active
systemctl is-active apache2

# Check if service is enabled
systemctl is-enabled apache2

# List all services
systemctl list-units --type=service

# List running services
systemctl list-units --type=service --state=running

# Show failed services
systemctl --failed

# Reboot system
sudo systemctl reboot

# Shutdown system
sudo systemctl poweroff

Database Operations

Create Database

Syntax:

GRANT ALL PRIVILEGES ON <database_name>.* TO '<mysql_username>'@'localhost';

Example:

Here, root is username of mysql:

mysql -uroot -p
CREATE DATABASE m2demo;
GRANT ALL PRIVILEGES ON m2demo.* TO 'root'@'localhost';
FLUSH PRIVILEGES;

Import Database

Examples:

# Method 1: Command line
mysql -u <username> -p <your_database_name> < database_file_name.sql

# Method 2: Inside MySQL
mysql -uroot -p
SET FOREIGN_KEY_CHECKS=0;
use database_name;
SOURCE database_file_name.sql;
SET FOREIGN_KEY_CHECKS=1;

Export Database

Example:

mysqldump -u <username> -p <your_database_name> > database_file_name.sql

# Export with compression
mysqldump -u <username> -p <your_database_name> | gzip > database_file_name.sql.gz

Remove Definer

Example:

# Find DEFINER statements
grep "DEFINER" db_bkp_01042022.sql -rsn

# Remove DEFINER from SQL file
sed -i 's/DEFINER=`[^`]*`@`[^`]*`//g' your_database_name.sql

# Alternative with find
find . -name "your_database_name.sql" -type f -exec sed -i 's/DEFINER=`root`@`localhost`/ /g' {} +

Additional Examples

Create .tar File

Examples:

# Create tar archive
tar -cvf archive.tar directory/

# Create compressed tar.gz
tar -cvzf code.tar.gz app/code/

# Create tar.bz2 (better compression)
tar -cvjf code.tar.bz2 app/code/

Extract .tar File

Examples:

# Extract tar
tar -xvf archive.tar

# Extract tar.gz
tar -xvzf code.tar.gz

# Extract to specific directory
tar -xvzf code.tar.gz -C /destination/path/

Create .zip File

Examples:

# Create zip file
zip archive.zip file.txt

# Create zip recursively
zip -r code.zip app/code/

# Create with password
zip -e -r secure.zip app/code/

Extract .zip File

Examples:

# Extract zip
unzip file_name.zip

# Extract to specific directory
unzip file_name.zip -d /destination/

# List contents without extracting
unzip -l file_name.zip

SCP File Transfer

Complete Example:

  1. Create tar files in Server A:

    cd /var/www/html/
tar -cvzf mage245.tar.gz mage245/

  2. Transfer file Server A to B

    Important

    Login Server B using SSH.

  3. Go to destination path where you want the copied file in Server B

    Example: cd /home/plugincardknox/public_html

  4. Execute below command in Server B:

    scp root@197.280.111.178:/var/www/html/mage245.tar.gz .

    Note

    The . at the end means current directory

  5. Extract the transferred file:

    tar -xvzf mage245.tar.gz

Quick Reference

File Operations:

pwd                    # Print working directory
ls -lah                # List all files with details
cd /path               # Change directory
cp -r source dest      # Copy recursively
mv old new             # Move/rename
rm -rf directory/      # Remove directory
mkdir -p path/to/dir   # Create nested directories
touch file.txt         # Create empty file

File Viewing:

cat file.txt           # Display file
less file.txt          # View file (paginated)
head -n 20 file.txt    # First 20 lines
tail -f log.txt        # Monitor file
grep "pattern" file    # Search in file

System Info:

uname -a               # System information
df -h                  # Disk usage
free -h                # Memory usage
top                    # Process monitor
htop                   # Better process monitor

Networking:

ping google.com        # Test connectivity
curl https://api.com   # HTTP request
wget url/file.zip      # Download file
ssh user@host          # Remote connection
scp file user@host:    # Secure copy

Permissions:

chmod 755 file         # Change permissions
chown user:group file  # Change owner
chmod +x script.sh     # Make executable

Archives:

tar -czf file.tar.gz dir/      # Create tar.gz
tar -xzf file.tar.gz           # Extract tar.gz
zip -r archive.zip dir/        # Create zip
unzip archive.zip              # Extract zip

System Services:

systemctl start service        # Start service
systemctl stop service         # Stop service
systemctl restart service      # Restart service
systemctl status service       # Check status
systemctl enable service       # Enable at boot

See Also

Tip

Use man <command> or <command> --help to get detailed help for any command.