lsof (List Open Files) Command Guide

Table of Contents

• Overview
• Installation
• Basic Usage
• Network Connections
• Process Analysis
• File System Operations
• Advanced Options
• Troubleshooting
• Best Practices

Overview

lsof (List Open Files) is a powerful diagnostic tool that provides information about files opened by processes. It’s essential for system administration, debugging, and security analysis.

Key Features

• List open files
• Monitor network connections
• Track process file usage
• Identify deleted files
• Memory mapping analysis
• Port monitoring

Installation

Ubuntu (22.04/24.04)

sudo apt update
sudo apt install lsof

macOS

# Pre-installed on macOS
# Or install via Homebrew
brew install lsof

Basic Usage

List All Open Files

# List all open files
sudo lsof
 
# List files with statistics
sudo lsof -s
 
# Count number of open files
sudo lsof | wc -l

Filter by User

# Files opened by user
lsof -u username
 
# Files NOT opened by user
lsof -u ^username
 
# Files opened by multiple users
lsof -u user1,user2

Process Specific

# Files opened by PID
lsof -p 1234
 
# Files opened by multiple PIDs
lsof -p 1234,5678
 
# Exclude PIDs
lsof -p ^1234

Network Connections

Network Related Commands

# Show all network connections
sudo lsof -i
 
# Show TCP connections
sudo lsof -i tcp
 
# Show UDP connections
sudo lsof -i udp
 
# Show specific port
sudo lsof -i :80

Network Filters

# Specific IP address
sudo lsof [email protected]
 
# Specific port range
sudo lsof -i :1-1024
 
# Specific protocol and port
sudo lsof -i tcp:22
 
# Show listening ports
sudo lsof -i -sTCP:LISTEN

IPv4/IPv6

# Show only IPv4
sudo lsof -i 4
 
# Show only IPv6
sudo lsof -i 6
 
# Show both
sudo lsof -i 4 -i 6

Process Analysis

Process Specific Information

# Files opened by command name
lsof -c nginx
 
# Multiple command names
lsof -c nginx -c apache2
 
# Case insensitive search
lsof -c /nginx/i

Process States

# Show running processes
lsof -s TCP:ESTABLISHED
 
# Show listening processes
lsof -s TCP:LISTEN
 
# Show specific states
lsof -s TCP:CLOSE_WAIT

Process Tree

# Show parent processes
lsof -R
 
# Show process group ID
lsof -g
 
# Show process owner
lsof -l

File System Operations

File and Directory Operations

# Files in directory
lsof +D /path/to/directory
 
# Recursive directory search
lsof +d /path/to/directory
 
# Specific file
lsof /path/to/file

File Types

# Show regular files
lsof -F f
 
# Show directory files
lsof -F d
 
# Show character special files
lsof -F c

Deleted Files

# Show deleted files
lsof +L1
 
# Show files with link count 0
lsof +L0
 
# Files deleted but still open
lsof | grep deleted

Advanced Options

Output Format

# Custom field selection
lsof -F pcfn
 
# Field separator
lsof -F0
 
# Wide output
lsof -w

Time Options

# Repeat mode
lsof -r 5
 
# Repeat until no files
lsof +r 5
 
# Timeout
lsof -S 2

Memory Operations

# Show memory maps
lsof -m
 
# Show file sizes
lsof -s
 
# Show offset
lsof -o

Troubleshooting

Common Issues

1. Permission Issues

# Run with sudo
sudo lsof
 
# Check specific user permissions
sudo lsof -u username

2. Process Access

# Check blocked processes
sudo lsof -b
 
# Avoid blocking
sudo lsof -n

3. Network Issues

# Check specific port
sudo lsof -i :port_number
 
# Check all network activity
sudo lsof -i -n

Best Practices

Performance Optimization

# Avoid DNS lookups
lsof -n
 
# Avoid port name lookups
lsof -P
 
# Combine optimizations
lsof -nP

Security Monitoring

# Monitor suspicious ports
sudo lsof -i :22,80,443
 
# Check unknown processes
sudo lsof -i | grep ESTABLISHED
 
# Monitor file changes
watch -n 1 'lsof | wc -l'

Quick Reference

Essential Commands

# Basic listing
sudo lsof
 
# Network connections
sudo lsof -i
 
# Process files
lsof -p PID
 
# User files
lsof -u username
 
# Port check
sudo lsof -i :port

Common Options

-i    # Internet connections
-p    # Process ID
-u    # Username
-c    # Command name
+D    # Directory
-n    # No DNS lookup
-P    # No port names
-r    # Repeat mode

Example Use Cases

Web Server Monitoring

# Monitor web server ports
sudo lsof -i :80,443
 
# Check web server processes
sudo lsof -c apache2 -c nginx
 
# Monitor web log access
sudo lsof | grep access.log

System Troubleshooting

# Find largest open files
sudo lsof -s | sort -nr -k7 | head -10
 
# Check for deleted but open files
sudo lsof +L1
 
# Monitor specific application
watch -n 1 'lsof -c application_name'

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Remember:

• Use sudo when needed
• Consider performance impact
• Filter output appropriately
• Regular monitoring
• Document findings
• Use with other tools

For detailed information, consult the man pages (man lsof).