Essential Linux Commands — 50+ Commands for Daily Administration

# Navigate and list
pwd                    # Print working directory
ls -la                 # List all files with details
cd /var/log            # Change directory
mkdir -p a/b/c         # Create nested directories

# View files
cat file.txt           # Display entire file
less file.txt          # Scroll through file (q to quit)
head -n 20 file.txt    # First 20 lines
tail -f /var/log/syslog # Follow log in real time

# Create and manipulate
touch newfile.txt      # Create empty file
cp source dest         # Copy file
cp -r dir1 dir2        # Copy directory recursively
mv old new             # Move or rename
rm file.txt            # Delete file (permanent!)
rm -rf dir             # Delete directory and contents
ln -s target link      # Create symbolic link

# Change permissions (u=user, g=group, o=others, a=all)
chmod u+x script.sh    # Add execute for owner
chmod 755 script.sh    # rwxr-xr-x (numeric mode)
chown alice:admins file.txt  # Change owner and group
chgrp developers file.txt    # Change group only

# View permissions
umask                  # Show default permission mask
ls -l                  # permissions are the first column
stat file.txt          # Detailed file metadata

# View processes
ps aux                 # All processes (BSD style)
ps -ef                 # All processes (Unix style)
top                    # Interactive process viewer
htop                   # Enhanced version of top

# Process signals
kill 1234              # Terminate process by PID
kill -9 1234           # Force kill (SIGKILL)
kill -15 1234          # Graceful stop (SIGTERM)
pkill nginx            # Kill processes by name

# Background and foreground
command &              # Run in background
nohup command &        # Run immune to hangups
jobs                   # List background jobs
fg %1                  # Bring job 1 to foreground

# Connection monitoring
ss -tuln               # Show listening ports (modern netstat)
ss -tup                # Show active connections with processes
ping -c 4 8.8.8.8      # Test network connectivity

# DNS and routing
dig example.com        # DNS lookup
nslookup example.com   # Simplified DNS query
host example.com       # Quick host lookup
traceroute example.com # Trace network path (use mtr for better)

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

# SSH operations
ssh user@host          # Connect to remote host
scp file user@host:/path/  # Copy file over SSH
rsync -av src/ user@host:/dest/  # Sync directories

# Search in files
grep "error" /var/log/syslog           # Find lines containing "error"
grep -i "warning" log.txt              # Case-insensitive search
grep -r "TODO" /home/projects/         # Recursive search
grep -c "failed" auth.log              # Count matches
grep -E "err(or|atic)" log.txt         # Extended regex

# Print specific columns
awk '{print $1, $3}' /var/log/syslog   # First and third columns

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

# Conditional processing
awk '$3 > 50 {print $1, $3}' data.txt  # Print if column 3 > 50

# Find and replace
sed 's/old/new/g' file.txt             # Replace all occurrences
sed -i 's/old/new/g' file.txt          # In-place edit

# Line operations
sed -n '5,10p' file.txt                # Print lines 5-10
sed '/^#/d' config.txt                 # Delete comment lines
sed 's/  */ /g' file.txt               # Collapse whitespace

# Count unique IPs in an Apache access log
awk '{print $1}' /var/log/apache2/access.log | sort | uniq -c | sort -nr | head -10

# Disk and memory
df -h                  # Disk usage (human-readable)
du -sh /var/log        # Directory size
free -h                # Memory usage

# System info
uname -a               # Kernel version and architecture
lscpu                  # CPU details
lsblk                  # Block devices and partitions
dmesg | tail           # Recent kernel messages
uptime                 # System uptime and load

# Resource-intensive processes
ps aux --sort=-%mem | head    # Top memory processes
ps aux --sort=-%cpu | head    # Top CPU processes

# tar (tape archive)
tar -czf archive.tar.gz /path/to/dir   # Create gzipped archive
tar -xzf archive.tar.gz                # Extract gzipped archive
tar -cjf archive.tar.bz2 /path         # Create bzipped archive
tar -tf archive.tar.gz                 # List contents

# zip
zip -r archive.zip /path/to/dir        # Create zip
unzip archive.zip                      # Extract zip

# Find the 5 largest files in a directory
find /var/log -type f -size +1M | xargs ls -lhS | head -5

# Find processes using more than 500MB of memory
ps aux | awk '$6 > 500000 {print $11, $6/1024 " MB"}'

# Monitor failed SSH login attempts in real time
tail -f /var/log/auth.log | grep "Failed password"

#!/bin/bash
# log_analyzer.sh — Analyze web server access log
# Usage: ./log_analyzer.sh /var/log/nginx/access.log

LOG_FILE="${1:-/var/log/nginx/access.log}"

if [ ! -f "$LOG_FILE" ]; then
    echo "Error: Log file not found: $LOG_FILE"
    exit 1
fi

echo "=== Web Server Log Analysis ==="
echo "File: $LOG_FILE"
echo ""

# Total requests
total_lines=$(wc -l < "$LOG_FILE")
echo "Total requests: $total_lines"

# Unique IPs
unique_ips=$(awk '{print $1}' "$LOG_FILE" | sort -u | wc -l)
echo "Unique IPs: $unique_ips"

# HTTP status code distribution
echo ""
echo "=== HTTP Status Codes ==="
awk '{print $9}' "$LOG_FILE" | sort | uniq -c | sort -rn

# Most requested pages
echo ""
echo "=== Most Requested Pages ==="
awk '{print $7}' "$LOG_FILE" | sort | uniq -c | sort -rn | head -10

# Busiest hour
echo ""
echo "=== Busiest Hour ==="
awk '{print $4}' "$LOG_FILE" | sed 's/\[//' | cut -d: -f2 | sort | uniq -c | sort -rn | head -5