Linux Commands Frequently Used by DevOps Engineers

Linux Commands Frequently Used by DevOps Engineers

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Table of Contents

1. Overview
2. System Information
3. File and Directory Management
4. Text Processing
5. Process Management
6. Networking
7. User and Permission Management
8. Disk and Storage
9. Compression and Archiving

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1. Overview

This document is a reference guide for Linux commands frequently used by DevOps engineers in server management, deployment, and monitoring tasks.
Commands are categorized by task type, with options and examples for immediate practical use.
This guide targets entry to intermediate level DevOps engineers who need quick reference material.

Chapters by Task Type

Chapter	Description
System Information	Query basic system information such as hostname, kernel version, and uptime. Used as the first step in troubleshooting and status checks.
File and Directory Management	Create, copy, move, delete, and search files/directories. Essential for deployment scripts and daily server maintenance.
Text Processing	Process text-based tasks including log analysis, config file editing, and data extraction. Powerful automation through pipeline combinations.
Process Management	Query, terminate running processes, and control system services. Core commands for resource monitoring and incident response.
Networking	Check network connectivity, download files, and test APIs. Essential for infrastructure configuration and debugging.
User and Permission Management	Manage user accounts and file permissions. Used for security policy enforcement and access control.
Disk and Storage	Check disk capacity, mount status, and partition information. Used for storage management and capacity monitoring.
Compression and Archiving	Compress, decompress, and create archives. Used for backup and deployment package creation.

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2. System Information

Commands for querying basic system status and information.
These are typically the first commands run during incident response, environment verification, and documentation.

Command List

Command	Function
hostname	Check or set system hostname
uname	Display kernel and system information
uptime	Check system uptime and load
date	Check or set system date/time
whoami	Display current logged-in user
id	Display user UID/GID information

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hostname

Check or set the system hostname.

Key Options

Option	Description
-f	Display FQDN (Fully Qualified Domain Name)
-i	Display host IP address
-s	Display short hostname

Examples

# Check current hostname
hostname

# Check FQDN
hostname -f

# Check IP address
hostname -i

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uname

Display kernel version, architecture, and other system information.

Key Options

Option	Description
-a	Display all system information
-r	Display kernel release version
-m	Display hardware architecture
-n	Display network node hostname

Examples

# Display all system information
uname -a

# Check kernel version only
uname -r

# Check architecture (x86_64, arm64, etc.)
uname -m

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uptime

Check system uptime, logged-in users, and system load.

Key Options

Option	Description
-p	Display uptime in human-readable format
-s	Display system start time

Examples

# Default output (uptime, users, load average)
uptime

# Human-readable uptime format
uptime -p

# Check system start time
uptime -s

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date

Check or set the system date and time.

Key Options

Option	Description
+FORMAT	Output in specified format
-u	Display UTC time
-d STRING	Parse specified date/time string

Common Formats

Format	Example Output
+%Y-%m-%d	2024-01-15
+%H:%M:%S	14:30:25
+%Y%m%d_%H%M%S	20240115_143025

Examples

# Check current date/time
date

# Output in ISO format
date +%Y-%m-%d

# Timestamp for log filenames
date +%Y%m%d_%H%M%S

# Check UTC time
date -u

# Calculate date (7 days ago)
date -d "7 days ago" +%Y-%m-%d

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whoami

Display the current logged-in username.

Examples

# Check current user
whoami

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id

Display UID, GID, and group information for current or specified user.

Key Options

Option	Description
-u	Display UID only
-g	Display primary GID only
-G	Display all group IDs
-n	Display names instead of numbers

Examples

# Full information for current user
id

# Check specific user information
id username

# Check UID only
id -u

# Check group names
id -nG

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3. File and Directory Management

Perform basic filesystem operations including create, copy, move, delete, and search files and directories.
Most frequently used commands in deployment scripts, backup tasks, and daily server management.

Command List

Command	Function
ls	List directory contents
cd	Change directory
pwd	Print working directory path
mkdir	Create directory
rm	Remove files/directories
cp	Copy files/directories
mv	Move or rename files/directories
find	Search for files/directories
ln	Create links (hard/symbolic)
touch	Create empty file or update timestamp

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ls

List directory contents.

Key Options

Option	Description
-l	Long format (permissions, owner, size, date)
-a	Show all including hidden files
-h	Human-readable file sizes
-R	Recursive listing of subdirectories
-t	Sort by modification time
-S	Sort by file size

Examples

# Long format listing
ls -l

# Include hidden files with readable sizes
ls -lah

# Sort by modification time
ls -lt

# List specific pattern files
ls -l *.log

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cd

Change the working directory.

Examples

# Go to home directory
cd ~
cd

# Go to parent directory
cd ..

# Go to absolute path
cd /var/log

# Go to previous directory
cd -

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pwd

Print the absolute path of the current working directory.

Key Options

Option	Description
-P	Print physical path (resolve symlinks)
-L	Print logical path (default)

Examples

# Check current path
pwd

# Physical path (resolve symlinks)
pwd -P

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mkdir

Create new directories.

Key Options

Option	Description
-p	Create parent directories as needed
-m MODE	Set permissions on creation
-v	Verbose output

Examples

# Create single directory
mkdir logs

# Create nested directories at once
mkdir -p /app/logs/2024/01

# Create with specific permissions
mkdir -m 755 scripts

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rm

Remove files or directories.

Key Options

Option	Description
-r	Recursive removal for directories
-f	Force removal (no confirmation)
-i	Prompt before removal
-v	Verbose output

Examples

# Remove file
rm file.txt

# Remove directory and all contents
rm -rf directory/

# Prompt before removal
rm -i important.txt

# Remove files matching pattern
rm -f *.tmp

⚠️ Warning: rm -rf / or critical paths are unrecoverable

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cp

Copy files or directories.

Key Options

Option	Description
-r	Recursive copy for directories
-p	Preserve permissions, ownership, timestamps
-a	Archive mode (-rp + preserve symlinks)
-v	Verbose output
-i	Prompt before overwrite

Examples

# Copy file
cp source.txt dest.txt

# Copy directory
cp -r source_dir/ dest_dir/

# Copy preserving attributes
cp -a /var/www/ /backup/www/

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

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mv

Move or rename files and directories.

Key Options

Option	Description
-i	Prompt before overwrite
-f	Force move (no confirmation)
-v	Verbose output
-n	Do not overwrite existing files

Examples

# Rename file
mv old_name.txt new_name.txt

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

# Move directory
mv source_dir/ /new/location/

# Move multiple files
mv *.log /var/log/archive/

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find

Search for files and directories matching conditions.

Key Options

Option	Description
-name PATTERN	Search by filename pattern
-type f/d	Specify file(f) or directory(d)
-mtime +/-N	Search by modification time (days)
-size +/-N	Search by file size
-exec CMD {} \;	Execute command on results
-perm MODE	Search by permissions

Examples

# Search by name
find /var/log -name "*.log"

# Find files older than 7 days
find /tmp -type f -mtime +7

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

# Search and delete
find /tmp -name "*.tmp" -exec rm {} \;

# Find empty directories
find /app -type d -empty

# Find files with 777 permissions
find / -type f -perm 0777

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ln

Create links to files.

Key Options

Option	Description
-s	Create symbolic link
-f	Overwrite existing link
-v	Verbose output

Examples

# Create symbolic link
ln -s /path/to/original /path/to/link

# Symbolic link for config file
ln -s /etc/nginx/sites-available/mysite /etc/nginx/sites-enabled/

# Create hard link
ln original.txt hardlink.txt

# Replace existing link
ln -sf /new/target /path/to/link

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touch

Create empty files or update existing file timestamps.

Key Options

Option	Description
-a	Change access time only
-m	Change modification time only
-t STAMP	Set specific timestamp
-d STRING	Set time from date string

Examples

# Create empty file
touch newfile.txt

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

# Update timestamp
touch existing_file.txt

# Set specific date
touch -d "2024-01-01 00:00:00" file.txt

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4. Text Processing

Process text-based tasks including log file analysis, config file editing, and data extraction/transformation.
Powerful data processing automation is possible through pipeline (|) combinations.

Command List

Command	Function
cat	Display file contents
head	Display beginning of file
tail	Display end of file
grep	Pattern search
awk	Text processing and data extraction
sed	Stream editor
cut	Extract fields
sort	Sort lines
uniq	Remove duplicates
wc	Count lines/words/characters

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cat

Display file contents or concatenate files.

Key Options

Option	Description
-n	Number all output lines
-b	Number non-empty lines only
-s	Squeeze multiple blank lines
-A	Show all control characters

Examples

# Display file contents
cat file.txt

# Display with line numbers
cat -n file.txt

# Concatenate files
cat file1.txt file2.txt > combined.txt

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

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head

Display the beginning of a file. Default is 10 lines.

Key Options

Option	Description
-n N	Display first N lines
-c N	Display first N bytes

Examples

# Display first 10 lines
head file.txt

# Display first 20 lines
head -n 20 file.txt

# Display first 100 bytes
head -c 100 file.txt

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tail

Display the end of a file. Essential for log monitoring.

Key Options

Option	Description
-n N	Display last N lines
-f	Follow file changes in real-time
-F	Follow + detect file recreation
-c N	Display last N bytes

Examples

# Display last 10 lines
tail file.txt

# Display last 50 lines
tail -n 50 file.txt

# Real-time log monitoring
tail -f /var/log/syslog

# Monitor with log rotation support
tail -F /var/log/nginx/access.log

# Monitor multiple files
tail -f /var/log/*.log

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grep

Search for patterns in files.

Key Options

Option	Description
-i	Case-insensitive search
-r	Recursive directory search
-n	Display line numbers
-v	Invert match (non-matching lines)
-c	Count matching lines
-l	Display only filenames with matches
-E	Extended regular expressions
-A N	Display N lines after match
-B N	Display N lines before match

Examples

# Basic search
grep "error" /var/log/syslog

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

# Recursive search in directory
grep -r "TODO" /app/src/

# Display with line numbers
grep -n "failed" app.log

# Exclude pattern
grep -v "DEBUG" app.log

# Multiple patterns
grep -E "error|warning|critical" app.log

# Display context (3 lines before and after)
grep -B 3 -A 3 "Exception" app.log

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awk

Process text field by field and extract data.

Basic Syntax

awk 'pattern { action }' file

Key Built-in Variables

Variable	Description
$0	Entire line
$1, $2, ...	1st, 2nd, ... field
NF	Number of fields
NR	Current line number
FS	Field separator

Examples

# Print specific fields
awk '{print $1, $3}' file.txt

# Specify field separator
awk -F: '{print $1}' /etc/passwd

# Conditional printing
awk '$3 > 100 {print $1, $3}' data.txt

# Print line numbers
awk '{print NR, $0}' file.txt

# Calculate sum
awk '{sum += $1} END {print sum}' numbers.txt

# Process lines matching pattern
awk '/error/ {print $0}' logfile.txt

# Print last field
awk '{print $NF}' file.txt

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sed

Stream editor for text substitution and transformation.

Basic Syntax

sed 's/pattern/replacement/flags' file

Key Options

Option	Description
-i	Edit file in-place
-e	Execute multiple commands
-n	Suppress automatic output
g	Global replacement (flag)

Examples

# String substitution (first occurrence)
sed 's/old/new/' file.txt

# Global substitution
sed 's/old/new/g' file.txt

# Edit file in-place
sed -i 's/old/new/g' file.txt

# Delete specific line
sed '5d' file.txt

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

# Print specific lines only
sed -n '10,20p' file.txt

# Multiple substitution commands
sed -e 's/foo/bar/g' -e 's/baz/qux/g' file.txt

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cut

Extract specific fields or character ranges from lines.

Key Options

Option	Description
-d DELIM	Specify field delimiter
-f N	Extract Nth field
-c N-M	Extract character positions N to M

Examples

# Extract first field (colon-delimited)
cut -d: -f1 /etc/passwd

# Extract multiple fields
cut -d, -f1,3,5 data.csv

# Extract by character position
cut -c1-10 file.txt

# Tab-delimited file (default)
cut -f2 data.tsv

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sort

Sort lines.

Key Options

Option	Description
-n	Numeric sort
-r	Reverse sort
-k N	Sort by Nth field
-t DELIM	Specify field delimiter
-u	Remove duplicates after sort

Examples

# Alphabetical sort
sort file.txt

# Numeric sort
sort -n numbers.txt

# Reverse sort
sort -r file.txt

# Sort by 2nd field
sort -t: -k2 data.txt

# Sort and remove duplicates
sort -u file.txt

# Sort by file size (ls output)
ls -l | sort -k5 -n

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uniq

Remove consecutive duplicate lines. Usually used with sort.

Key Options

Option	Description
-c	Display count of occurrences
-d	Display only duplicated lines
-u	Display only unique lines
-i	Case-insensitive comparison

Examples

# Remove duplicates (requires sorted input)
sort file.txt | uniq

# Count occurrences
sort file.txt | uniq -c

# Show only duplicated lines
sort file.txt | uniq -d

# Sort by count
sort file.txt | uniq -c | sort -rn

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wc

Count lines, words, and characters in files.

Key Options

Option	Description
-l	Line count
-w	Word count
-c	Byte count
-m	Character count

Examples

# Full statistics
wc file.txt

# Line count only
wc -l file.txt

# Multiple file statistics
wc -l *.log

# Use with pipe
cat file.txt | grep "error" | wc -l

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5. Process Management

Query running processes, monitor, terminate, and control system services.
Core commands for system resource management and incident response.

Command List

Command	Function
ps	Display process status
top	Real-time process monitoring
htop	Enhanced process monitoring
kill	Terminate process
pkill	Terminate process by name
systemctl	Manage system services
journalctl	Query system logs
nohup	Run process immune to hangups

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ps

Display information about running processes.

Key Options

Option	Description
aux	Detailed output of all processes
-ef	Full format of all processes
-u USER	Specific user's processes
--forest	Tree view

Examples

# Display all processes
ps aux

# Full format output
ps -ef

# Search specific process
ps aux | grep nginx

# Process tree view
ps aux --forest

# Specific user's processes
ps -u www-data

# Top CPU-consuming processes
ps aux --sort=-%cpu | head -10

# Top memory-consuming processes
ps aux --sort=-%mem | head -10

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top

Monitor system status and processes in real-time.

Runtime Shortcuts

Key	Function
q	Quit
k	Kill process
M	Sort by memory usage
P	Sort by CPU usage
1	Show per-CPU stats
c	Show full command path

Key Options

Option	Description
-d N	Refresh interval N seconds
-u USER	Specific user's processes only
-p PID	Monitor specific PID
-b	Batch mode (for file output)

Examples

# Default execution
top

# 2-second refresh interval
top -d 2

# Specific user's processes only
top -u nginx

# Batch mode for file output
top -b -n 1 > top_output.txt

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htop

Enhanced version of top. Colorful interface with mouse support.

Runtime Shortcuts

Key	Function
F5	Tree view
F6	Select sort column
F9	Kill process
F10	Quit
/	Search
\	Filter

Examples

# Default execution
htop

# Specific user's processes only
htop -u nginx

# Start with tree view
htop -t

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kill

Send signals to processes to terminate them.

Key Signals

Signal	Number	Description
SIGTERM	15	Graceful termination request (default)
SIGKILL	9	Force termination
SIGHUP	1	Restart/reload configuration
SIGSTOP	19	Pause process

Examples

# Graceful termination request
kill 1234

# Force termination
kill -9 1234
kill -SIGKILL 1234

# Terminate multiple processes
kill 1234 5678 9012

# Reload configuration (nginx, etc.)
kill -HUP 1234

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pkill

Terminate processes by name or conditions.

Key Options

Option	Description
-f	Search in full command line
-u USER	Specific user's processes
-signal	Specify signal to send

Examples

# Terminate by name
pkill nginx

# Force termination
pkill -9 nginx

# Search in full command
pkill -f "python app.py"

# Terminate specific user's processes
pkill -u www-data

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systemctl

Manage systemd-based services.

Key Commands

Command	Description
start	Start service
stop	Stop service
restart	Restart service
reload	Reload configuration
status	Check service status
enable	Enable auto-start on boot
disable	Disable auto-start on boot

Examples

# Check service status
systemctl status nginx

# Start service
sudo systemctl start nginx

# Stop service
sudo systemctl stop nginx

# Restart service
sudo systemctl restart nginx

# Reload configuration (without restart)
sudo systemctl reload nginx

# Enable auto-start on boot
sudo systemctl enable nginx

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

# List failed services
systemctl --failed

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journalctl

Query systemd journal logs.

Key Options

Option	Description
-u UNIT	Specific service logs
-f	Follow logs in real-time
-n N	Show last N lines
--since	Logs since specific time
-p LEVEL	Filter by log level
-b	Logs since current boot

Examples

# View all logs
journalctl

# Specific service logs
journalctl -u nginx

# Follow logs in real-time
journalctl -f -u nginx

# Last 100 lines
journalctl -n 100

# Today's logs only
journalctl --since today

# Error level and above only
journalctl -p err

# Specific time range
journalctl --since "2024-01-01" --until "2024-01-02"

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nohup

Run processes that continue after terminal closes.

Examples

# Run in background
nohup ./script.sh &

# Specify output file
nohup ./script.sh > output.log 2>&1 &

# Discard output
nohup ./script.sh > /dev/null 2>&1 &

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6. Networking

Perform network-related tasks including connectivity checks, file downloads, and API testing.
Essential commands for infrastructure configuration, troubleshooting, and external service integration.

Command List

Command	Function
ping	Test network connectivity
curl	Transfer data via URL and API testing
wget	Download files
ss	Socket statistics
netstat	Network connection status
ip	IP address and routing management
dig	DNS lookup
nslookup	DNS lookup (simple)
traceroute	Trace network path
scp	Copy files via SSH
rsync	File synchronization

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ping

Send ICMP packets to test network connectivity.

Key Options

Option	Description
-c N	Stop after N packets
-i N	Send interval N seconds
-W N	Timeout N seconds
-s SIZE	Specify packet size

Examples

# Basic ping
ping google.com

# Send 4 packets only
ping -c 4 google.com

# 1 second timeout
ping -c 4 -W 1 192.168.1.1

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curl

Transfer data via URLs. Essential for API testing.

Key Options

Option	Description
-X METHOD	Specify HTTP method
-H HEADER	Add header
-d DATA	POST data
-o FILE	Output to file
-O	Save with remote filename
-L	Follow redirects
-s	Silent mode
-v	Verbose output
-I	Headers only
-k	Skip SSL certificate verification

Examples

# GET request
curl https://api.example.com/users

# POST request (JSON)
curl -X POST https://api.example.com/users \
  -H "Content-Type: application/json" \
  -d '{"name": "John", "email": "john@example.com"}'

# Check headers
curl -I https://example.com

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

# Include auth token
curl -H "Authorization: Bearer TOKEN" https://api.example.com/data

# Verbose debug
curl -v https://example.com

# Output HTTP status code only
curl -s -o /dev/null -w "%{http_code}" https://example.com

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wget

Download files from URLs.

Key Options

Option	Description
-O FILE	Specify output filename
-q	Quiet mode
-c	Resume download
-r	Recursive download
-P DIR	Specify download directory
--limit-rate	Limit download speed

Examples

# Basic download
wget https://example.com/file.tar.gz

# Specify filename
wget -O myfile.tar.gz https://example.com/file.tar.gz

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

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

# Limit speed
wget --limit-rate=1m https://example.com/file.tar.gz

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ss

Display socket statistics. Modern replacement for netstat.

Key Options

Option	Description
-t	TCP sockets
-u	UDP sockets
-l	Listening sockets
-n	Numeric output (no DNS lookup)
-p	Show process information
-a	All sockets

Examples

# Listening TCP ports
ss -tlnp

# All TCP connections
ss -tan

# Check specific port
ss -tlnp | grep :80

# ESTABLISHED connections
ss -t state established

# Connections by process
ss -tp

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netstat

Display network connections, routing tables, and interface statistics.

Key Options

Option	Description
-t	TCP connections
-u	UDP connections
-l	Listening state
-n	Numeric output
-p	Process information
-r	Routing table

Examples

# Check listening ports
netstat -tlnp

# All connections
netstat -an

# Routing table
netstat -r

# Interface statistics
netstat -i

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ip

Manage IP addresses, routing, and network interfaces.

Key Subcommands

Command	Description
ip addr	IP address management
ip link	Network interface management
ip route	Routing table management
ip neigh	ARP table query

Examples

# Check IP addresses
ip addr
ip a

# Specific interface
ip addr show eth0

# Routing table
ip route
ip r

# Check default gateway
ip route | grep default

# Enable/disable interface
sudo ip link set eth0 up
sudo ip link set eth0 down

# Add IP address
sudo ip addr add 192.168.1.100/24 dev eth0

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dig

Query DNS records in detail.

Key Options

Option	Description
+short	Brief output
-t TYPE	Specify record type
@SERVER	Specify DNS server

Examples

# A record query
dig example.com

# Brief output
dig +short example.com

# MX record
dig -t MX example.com

# NS record
dig -t NS example.com

# Query specific DNS server
dig @8.8.8.8 example.com

# Reverse lookup
dig -x 8.8.8.8

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nslookup

Perform simple DNS lookups.

Examples

# Basic lookup
nslookup example.com

# Use specific DNS server
nslookup example.com 8.8.8.8

# Reverse lookup
nslookup 8.8.8.8

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traceroute

Trace the network path to a destination.

Key Options

Option	Description
-n	No DNS lookup
-m N	Maximum hops
-w N	Timeout seconds

Examples

# Basic trace
traceroute google.com

# Without DNS lookup
traceroute -n google.com

# Maximum 15 hops
traceroute -m 15 google.com

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scp

Copy files between hosts via SSH.

Key Options

Option	Description
-r	Recursive copy for directories
-P PORT	Specify SSH port
-i KEY	Specify SSH key file
-v	Verbose output

Examples

# Local → Remote
scp file.txt user@server:/path/to/dest/

# Remote → Local
scp user@server:/path/to/file.txt ./

# Copy directory
scp -r mydir/ user@server:/path/to/dest/

# Specify port
scp -P 2222 file.txt user@server:/path/

# Use key file
scp -i ~/.ssh/mykey.pem file.txt user@server:/path/

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rsync

Efficiently synchronize files. Transfers only changed parts.

Key Options

Option	Description
-a	Archive mode (preserve permissions, times)
-v	Verbose output
-z	Compress during transfer
--delete	Delete files not in source
--dry-run	Test run
-e	Specify remote shell
--progress	Show progress

Examples

# Local synchronization
rsync -av source/ dest/

# Sync to remote server
rsync -avz source/ user@server:/path/to/dest/

# Sync from remote to local
rsync -avz user@server:/path/to/source/ ./dest/

# Full sync with deletion
rsync -av --delete source/ dest/

# Test run
rsync -av --dry-run source/ dest/

# Specify SSH port
rsync -avz -e "ssh -p 2222" source/ user@server:/path/

# Show progress
rsync -av --progress source/ dest/

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7. User and Permission Management

Manage user accounts and file/directory permissions for access control.
Essential commands for security policy enforcement and system administration.

Command List

Command	Function
chmod	Change file permissions
chown	Change file ownership
useradd	Create user
usermod	Modify user
userdel	Delete user
passwd	Change password
sudo	Execute with admin privileges
su	Switch user
groups	Check group membership

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chmod

Change file or directory permissions.

Permission Notation

Symbol	Meaning
r (4)	Read
w (2)	Write
x (1)	Execute
u	Owner
g	Group
o	Others
a	All

Key Options

Option	Description
-R	Recursive
-v	Verbose

Examples

# Numeric mode
chmod 755 script.sh    # rwxr-xr-x
chmod 644 file.txt     # rw-r--r--
chmod 600 secret.key   # rw-------

# Symbolic mode
chmod +x script.sh     # Add execute permission
chmod u+w file.txt     # Add owner write permission
chmod go-rwx file.txt  # Remove all permissions for group/others

# Recursive for directory
chmod -R 755 /var/www/

# Directories 755, files 644
find /var/www -type d -exec chmod 755 {} \;
find /var/www -type f -exec chmod 644 {} \;

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chown

Change file or directory ownership and group.

Key Options

Option	Description
-R	Recursive
-v	Verbose

Examples

# Change owner
sudo chown user file.txt

# Change owner and group
sudo chown user:group file.txt

# Change group only
sudo chown :group file.txt

# Recursive
sudo chown -R www-data:www-data /var/www/

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useradd

Create a new user account.

Key Options

Option	Description
-m	Create home directory
-s SHELL	Specify default shell
-g GROUP	Specify primary group
-G GROUPS	Specify supplementary groups
-d DIR	Specify home directory path

Examples

# Create basic user
sudo useradd username

# Create with home directory
sudo useradd -m username

# Specify shell and groups
sudo useradd -m -s /bin/bash -G sudo,docker username

# Create system user (for services)
sudo useradd -r -s /usr/sbin/nologin serviceuser

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usermod

Modify existing user account.

Key Options

Option	Description
-aG GROUP	Add to group
-s SHELL	Change shell
-L	Lock account
-U	Unlock account
-l NAME	Change username

Examples

# Add to group
sudo usermod -aG docker username
sudo usermod -aG sudo username

# Change shell
sudo usermod -s /bin/zsh username

# Lock account
sudo usermod -L username

# Unlock account
sudo usermod -U username

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userdel

Delete a user account.

Key Options

Option	Description
-r	Remove home directory and mail spool
-f	Force deletion

Examples

# Delete user only
sudo userdel username

# Delete including home directory
sudo userdel -r username

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passwd

Set or change user password.

Key Options

Option	Description
-l	Lock account
-u	Unlock account
-d	Delete password
-e	Expire password (force change on next login)

Examples

# Change own password
passwd

# Change another user's password (root)
sudo passwd username

# Force password change on next login
sudo passwd -e username

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sudo

Execute commands with another user's privileges (typically root).

Key Options

Option	Description
-u USER	Execute as specific user
-i	Start login shell
-s	Start shell
-l	List allowed commands

Examples

# Execute with root privileges
sudo apt update

# Start root shell
sudo -i

# Execute as specific user
sudo -u www-data whoami

# Check allowed commands
sudo -l

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su

Switch to another user.

Key Options

Option	Description
-	Login shell
-c CMD	Execute command and return

Examples

# Switch to root
su -

# Switch to specific user
su - username

# Execute command and return
su -c "whoami" username

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groups

Display group membership.

Examples

# Current user's groups
groups

# Specific user's groups
groups username

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8. Disk and Storage

Check and manage disk capacity, partitions, and mount status.
Used for storage management, capacity monitoring, and system maintenance.

Command List

Command	Function
df	Display disk usage
du	Display directory size
lsblk	List block devices
mount	Mount filesystem
umount	Unmount filesystem
fdisk	Partition management
blkid	Display block device UUID

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df

Display filesystem disk usage.

Key Options

Option	Description
-h	Human-readable units (GB, MB)
-T	Show filesystem type
-i	Show inode usage

Examples

# Check disk usage
df -h

# Include filesystem type
df -hT

# Inode usage
df -i

# Specific path
df -h /var

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du

Display file and directory sizes.

Key Options

Option	Description
-h	Human-readable units
-s	Summary only
-d N	Depth N levels
--max-depth=N	Maximum depth
-a	Include files

Examples

# Current directory size
du -sh

# Subdirectory sizes
du -h --max-depth=1

# Find large directories
du -h --max-depth=1 | sort -hr | head -10

# Specific directory
du -sh /var/log/

# Top 10 largest files/directories
du -ah / 2>/dev/null | sort -hr | head -10

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lsblk

Display block device information in tree format.

Key Options

Option	Description
-f	Include filesystem information
-a	Include empty devices
-o COLUMNS	Specify output columns

Examples

# Basic output
lsblk

# Include filesystem information
lsblk -f

# Detailed information
lsblk -o NAME,SIZE,FSTYPE,MOUNTPOINT,UUID

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mount

Mount filesystems or check mount status.

Key Options

Option	Description
-t TYPE	Filesystem type
-o OPTIONS	Mount options
-a	Mount all entries in /etc/fstab

Examples

# Current mount status
mount

# Mount device
sudo mount /dev/sdb1 /mnt/data

# Specify type
sudo mount -t ext4 /dev/sdb1 /mnt/data

# Read-only mount
sudo mount -o ro /dev/sdb1 /mnt/data

# Mount ISO file
sudo mount -o loop image.iso /mnt/iso

# NFS mount
sudo mount -t nfs server:/share /mnt/nfs

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umount

Unmount mounted filesystems.

Key Options

Option	Description
-l	Lazy unmount
-f	Force unmount

Examples

# Unmount
sudo umount /mnt/data

# Unmount by device name
sudo umount /dev/sdb1

# Force unmount
sudo umount -f /mnt/stuck

# Lazy unmount (even if busy)
sudo umount -l /mnt/busy

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fdisk

Manage disk partitions.

Key Options

Option	Description
-l	List partitions

Examples

# List all disk partitions
sudo fdisk -l

# Partition specific disk
sudo fdisk /dev/sdb
# Interactive commands: n(new), d(delete), p(print), w(write)

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blkid

Display block device UUID and filesystem information.

Examples

# All device information
sudo blkid

# Specific device
sudo blkid /dev/sda1

# UUID only
sudo blkid -s UUID -o value /dev/sda1

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9. Compression and Archiving

Compress, decompress, and create archives.
Used for backup, deployment package creation, and log archiving.

Command List

Command	Function
tar	Create/extract archives
gzip	gzip compression/decompression
gunzip	gzip decompression
zip	zip compression
unzip	zip extraction
zcat	Display compressed file contents

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tar

Archive files or compress archives.

Key Options

Option	Description
-c	Create archive
-x	Extract archive
-v	Verbose output
-f FILE	Specify filename
-z	gzip compression
-j	bzip2 compression
-J	xz compression
-t	List contents
-C DIR	Specify target directory

Examples

# Create tar.gz
tar -czvf archive.tar.gz directory/

# Extract tar.gz
tar -xzvf archive.tar.gz

# Extract to specific directory
tar -xzvf archive.tar.gz -C /path/to/dest/

# List contents (without extraction)
tar -tzvf archive.tar.gz

# Create tar.bz2
tar -cjvf archive.tar.bz2 directory/

# Archive specific files
tar -czvf logs.tar.gz /var/log/*.log

# Exclude specific pattern
tar -czvf backup.tar.gz --exclude='*.tmp' directory/

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gzip

Compress files with gzip. Replaces original file.

Key Options

Option	Description
-d	Decompress
-k	Keep original file
-v	Verbose output
-1~-9	Compression level (9 is maximum)

Examples

# Compress
gzip file.txt           # → file.txt.gz

# Keep original
gzip -k file.txt

# Decompress
gzip -d file.txt.gz

# Maximum compression
gzip -9 largefile.txt

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gunzip

Decompress gzip files.

Examples

# Decompress
gunzip file.txt.gz

# Keep original
gunzip -k file.txt.gz

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zip

Compress files in zip format.

Key Options

Option	Description
-r	Include directories recursively
-e	Set password
-9	Maximum compression

Examples

# Compress files
zip archive.zip file1.txt file2.txt

# Compress directory
zip -r archive.zip directory/

# Set password
zip -e secure.zip file.txt

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

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unzip

Extract zip files.

Key Options

Option	Description
-d DIR	Specify target directory
-l	List contents
-o	Overwrite without prompt

Examples

# Extract
unzip archive.zip

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

# List contents
unzip -l archive.zip

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zcat

Display compressed file contents without extraction.

Examples

# Display gzip file contents
zcat file.txt.gz

# Use with pipe
zcat access.log.gz | grep "404"

# Multiple files
zcat *.gz | grep "error"

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References

• GNU Coreutils Manual
• Linux man pages
• TLDR pages

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Last updated: 2024