Linux Interview Questions: Essential Commands for Developers

# Navigation
pwd                     # Print working directory
cd /path/to/dir         # Change directory
cd ..                   # Go up one level
cd ~                    # Go to home directory
cd -                    # Go to previous directory
 
# Listing files
ls                      # List files
ls -la                  # Long format, include hidden files
ls -lh                  # Human-readable sizes
ls -lt                  # Sort by modification time
 
# File info
file document.pdf       # Determine file type
stat file.txt          # Detailed file information
du -sh directory/       # Directory size
df -h                   # Disk space usage

# Create
touch file.txt              # Create empty file
mkdir directory             # Create directory
mkdir -p path/to/nested     # Create nested directories
 
# Copy
cp source dest              # Copy file
cp -r source/ dest/         # Copy directory recursively
cp -p source dest           # Preserve permissions/timestamps
 
# Move/rename
mv oldname newname          # Rename file
mv file /path/to/dest       # Move file
 
# Delete
rm file.txt                 # Delete file
rm -r directory/            # Delete directory recursively
rm -rf directory/           # Force delete (careful!)
rmdir empty_directory/      # Delete empty directory only

# DANGEROUS - if $DIR is empty, deletes everything!
rm -rf $DIR/*
 
# SAFER - quotes and checks
if [ -n "$DIR" ]; then
    rm -rf "$DIR"/*
fi

# View permissions
ls -la
# -rw-r--r-- 1 user group 1234 Jan 1 12:00 file.txt
# drwxr-xr-x 2 user group 4096 Jan 1 12:00 directory/

# Numeric notation
chmod 755 script.sh     # rwxr-xr-x (owner: full, others: read+execute)
chmod 644 file.txt      # rw-r--r-- (owner: read+write, others: read)
chmod 600 secrets.txt   # rw------- (owner only)
 
# Symbolic notation
chmod u+x script.sh     # Add execute for owner
chmod g-w file.txt      # Remove write for group
chmod o=r file.txt      # Set others to read only
chmod a+r file.txt      # Add read for all
 
# Change ownership
chown user:group file.txt
chown -R user:group directory/

chmod 4755 file         # SUID
chmod 2755 directory    # SGID
chmod 1777 directory    # Sticky bit

# Create soft link (symlink)
ln -s /path/to/original link_name
 
# Create hard link
ln /path/to/original link_name

# Check if files are hard linked (same inode)
ls -li file1 file2
# 123456 -rw-r--r-- 2 user group 100 Jan 1 12:00 file1
# 123456 -rw-r--r-- 2 user group 100 Jan 1 12:00 file2

grep "error" logfile.log           # Find lines containing "error"
grep -i "error" logfile.log        # Case insensitive
grep -r "TODO" ./src               # Recursive search in directory
grep -n "error" logfile.log        # Show line numbers
grep -v "debug" logfile.log        # Invert match (exclude debug)
grep -c "error" logfile.log        # Count matches
grep -E "error|warning" log.log    # Extended regex (OR)
grep -A 3 "error" log.log          # Show 3 lines after match
grep -B 2 "error" log.log          # Show 2 lines before match

# Replace first occurrence per line
sed 's/old/new/' file.txt
 
# Replace all occurrences
sed 's/old/new/g' file.txt
 
# Edit file in-place
sed -i 's/old/new/g' file.txt
 
# Delete lines matching pattern
sed '/pattern/d' file.txt
 
# Print specific lines
sed -n '5,10p' file.txt            # Lines 5-10

# Print specific columns
awk '{print $1, $3}' file.txt
 
# With delimiter
awk -F',' '{print $2}' data.csv
 
# Conditional printing
awk '$3 > 100 {print $1}' data.txt
 
# Sum a column
awk '{sum += $1} END {print sum}' numbers.txt
 
# Common log parsing
awk '{print $1}' access.log | sort | uniq -c | sort -rn | head
# ^ Count requests by IP, sorted by frequency

# View processes
ps aux                    # All processes, detailed
ps aux | grep node        # Find specific process
pgrep -a node            # Find by name (PID and command)
top                      # Interactive process viewer
htop                     # Better interactive viewer
 
# Process details
ps aux --sort=-%mem | head    # Top memory consumers
ps aux --sort=-%cpu | head    # Top CPU consumers
 
# Background/foreground
command &                 # Run in background
jobs                      # List background jobs
fg %1                     # Bring job 1 to foreground
bg %1                     # Resume job 1 in background
nohup command &           # Run immune to hangups

kill PID                  # SIGTERM (graceful shutdown)
kill -9 PID               # SIGKILL (force kill)
kill -15 PID              # SIGTERM (same as kill PID)
 
pkill process_name        # Kill by name
pkill -f "node server"    # Kill by full command match
killall node              # Kill all processes with name
 
# Find what's using a port
lsof -i :3000
# Kill it
kill $(lsof -t -i :3000)

# Test connectivity
ping google.com
ping -c 4 google.com      # Only 4 packets
 
# DNS lookup
nslookup example.com
dig example.com
 
# HTTP requests
curl https://api.example.com
curl -X POST -d '{"key":"value"}' -H "Content-Type: application/json" url
curl -I https://example.com    # Headers only
curl -o file.zip https://url   # Download to file
wget https://url/file.zip      # Download file
 
# View network connections
netstat -tulpn            # Listening ports with PIDs
ss -tulpn                 # Modern replacement for netstat
lsof -i :3000             # What's using port 3000
 
# Network interfaces
ip addr                   # Show IP addresses
ifconfig                  # Legacy command
ip route                  # Show routing table
 
# Trace route
traceroute google.com

# Check if port is open locally
nc -zv localhost 3000
 
# Check remote port
nc -zv example.com 443
 
# Common ports to know
# 22 - SSH
# 80 - HTTP
# 443 - HTTPS
# 3000 - Node.js default
# 5432 - PostgreSQL
# 3306 - MySQL
# 6379 - Redis
# 27017 - MongoDB

# View logs
cat /var/log/syslog             # Full file
tail /var/log/syslog            # Last 10 lines
tail -f /var/log/syslog         # Follow (live updates)
tail -n 100 app.log             # Last 100 lines
head -n 50 app.log              # First 50 lines
less /var/log/syslog            # Paginated viewing
 
# Search logs
grep "ERROR" app.log
grep -i "error\|warning" app.log
zgrep "ERROR" app.log.gz        # Search compressed logs
 
# Count occurrences
grep -c "ERROR" app.log
 
# Time-based filtering (if logs have timestamps)
awk '$0 >= "2024-01-15 10:00" && $0 <= "2024-01-15 11:00"' app.log
 
# Analyze log patterns
cut -d' ' -f1 access.log | sort | uniq -c | sort -rn | head
# ^ Count occurrences of first field (e.g., IPs in access log)

journalctl                       # All logs
journalctl -u nginx              # Logs for specific service
journalctl -f                    # Follow mode
journalctl --since "1 hour ago"
journalctl --since "2024-01-15" --until "2024-01-16"
journalctl -p err                # Only errors

# find - search in real-time
find /path -name "*.log"                    # By name pattern
find /path -name "*.log" -mtime -7          # Modified in last 7 days
find /path -size +100M                      # Larger than 100MB
find /path -type f -name "*.tmp" -delete    # Find and delete
find /path -type d -name "node_modules"     # Find directories
find /path -user john                       # By owner
find /path -perm 755                        # By permissions
 
# Execute command on results
find . -name "*.js" -exec grep "TODO" {} \;
find . -name "*.log" -exec rm {} \;
 
# Using xargs (often faster)
find . -name "*.js" | xargs grep "TODO"
find . -name "*.log" -print0 | xargs -0 rm  # Handle spaces in names
 
# locate - uses database (faster but may be outdated)
locate filename
updatedb                # Update the locate database

# View variables
env                         # All environment variables
echo $PATH                  # Specific variable
printenv HOME
 
# Set temporarily (current session)
export MY_VAR="value"
export PATH="$PATH:/new/path"
 
# Set for single command
MY_VAR="value" ./script.sh
 
# Persistent variables
# Add to ~/.bashrc or ~/.bash_profile:
export MY_VAR="value"
# Then reload:
source ~/.bashrc

#!/bin/bash
 
# Variables
NAME="World"
echo "Hello, $NAME"
 
# Command line arguments
echo "Script name: $0"
echo "First argument: $1"
echo "All arguments: $@"
echo "Number of arguments: $#"
 
# Conditionals
if [ -f "$1" ]; then
    echo "File exists"
elif [ -d "$1" ]; then
    echo "It's a directory"
else
    echo "Not found"
fi
 
# Loops
for file in *.txt; do
    echo "Processing $file"
done
 
for i in {1..5}; do
    echo "Number $i"
done
 
while read -r line; do
    echo "$line"
done < file.txt
 
# Functions
greet() {
    local name=$1
    echo "Hello, $name"
}
greet "Developer"
 
# Exit codes
command
if [ $? -eq 0 ]; then
    echo "Success"
else
    echo "Failed"
fi

# Output redirection
command > file.txt          # Redirect stdout (overwrite)
command >> file.txt         # Redirect stdout (append)
command 2> error.log        # Redirect stderr
command > out.log 2>&1      # Redirect both stdout and stderr
command &> all.log          # Shorthand for above (bash)
 
# Input redirection
command < input.txt         # Read from file
command << EOF              # Here document
line 1
line 2
EOF
 
# Pipes
command1 | command2         # Pipe stdout to next command
command1 |& command2        # Pipe stdout and stderr
 
# Common pipe patterns
cat file | grep "pattern" | sort | uniq -c
ps aux | grep node | awk '{print $2}' | xargs kill

# Discard output
command > /dev/null          # Discard stdout
command 2> /dev/null         # Discard stderr
command > /dev/null 2>&1     # Discard all output