Linux: Process Management

node server.js
# Error: listen EADDRINUSE: address already in use :::3000

# Find the process using port 3000
ss -tlnp | grep 3000
# LISTEN 0 128 *:3000 *:* users:(("node",pid=1234,fd=20))

# Terminate the process
kill 1234

# Restart the service
node server.js

# Core usage
ps aux               # Detailed info on all processes
ps aux | grep nginx  # Filter specific processes
ps -ef               # Standard format

# Common field meanings
# USER    PID  %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
# alice  1234  0.3  1.2 123456 54321 ?        Ssl  10:23   0:01 node server.js

# Custom output
ps -eo pid,user,%cpu,%mem,comm --sort=-%cpu | head -10

top                             # Start top
top -o %MEM                     # Sort by memory
top -u alice                    # Show only specific user processes
top -bn1 | head -20             # One-shot output

# Hotkeys while top is running
# P    Sort by CPU
# M    Sort by memory
# k    Kill process (prompts for PID)
# u    Filter by user
# q    Quit top

# htop is an enhanced version of top (nicer UI, mouse support)
htop

kill PID                        # Send SIGTERM (15), request termination
kill -2 PID                     # Send SIGINT (2), equivalent to Ctrl+C
kill -9 PID                     # Send SIGKILL (9), force termination
kill -1 PID                     # Send SIGHUP (1), reload config
kill -19 PID                    # Send SIGSTOP (19), pause process
kill -18 PID                    # Send SIGCONT (18), resume process

# Kill by name
pkill -f "node server.js"       # Match command line
killall nginx                   # Kill all processes with the same name

# & — Put in background
node server.js &                 # Returns terminal immediately after starting

> 💡 Tip: Appending `&` to a command runs it in the background, freeing the terminal immediately. However, `&` alone means the process will be terminated when the terminal session closes. To keep a process running after the terminal closes, combine with `nohup`: `nohup command &`.
sleep 30 &                       # Run sleep in background

# jobs/fg/bg — Manage background jobs
jobs                            # List background jobs
# [1]+  Running                 node server.js &
# [2]-  Running                 sleep 30 &
fg %1                          # Bring job 1 to foreground
bg %2                          # Resume paused job 2 in background

# Ctrl+Z — Suspend current foreground job and put in background
# Ctrl+C — Terminate foreground job

# nohup — Keep running after terminal closes
nohup node server.js &
nohup python app.py > app.log 2>&1 &

> ℹ️ Note: `nohup` + `&` is the standard combination for persistent background commands — `nohup` ignores the SIGHUP signal (sent when the terminal closes), and `&` puts it in the background. Output defaults to `nohup.out`; it's recommended to specify a log file manually: `nohup command > app.log 2>&1 &`.

# View all processes
ps aux | head -10

# View processes for a specific user
ps -u alice -o pid,%cpu,%mem,comm

# Top 10 processes by CPU usage
ps aux --sort=-%cpu | head -11

# View process tree
ps auxf | head -20

# Start top
top

# Operations inside top
# P → Sort by CPU
# M → Sort by memory
# k → Enter PID, press Enter, enter signal number (15 or 9), press Enter
# q → Quit

# Non-interactive mode
top -bn1 | head -15

# Start a background process
sleep 1000 &

# Find it
ps aux | grep "sleep 1000"
# alice  5678  0.0  0.0   1234   567 pts/0    S    10:23   0:00 sleep 1000

# Graceful termination
kill 5678

# If that doesn't work
kill -9 5678

# Start a long-running task
nohup python train_model.py > train.log 2>&1 &

# View the log
tail -f train.log

# Even after closing the terminal, train_model.py continues running
# After re-logging in, ps aux will show it's still running

# nice range: -20 (highest priority) to 19 (lowest priority)

# Start with lower priority
nice -n 10 ./heavy-task

# Adjust priority of an existing process
renice -n 5 -p 1234

# View process nice values
ps -eo pid,nice,comm | head -10

#!/bin/bash
# manage-server.sh - Manage a Node.js application

APP_NAME="server.js"
APP_DIR="/home/alice/app"
PID_FILE="/tmp/app.pid"

start() {
    if [ -f "$PID_FILE" ] && kill -0 $(cat "$PID_FILE") 2>/dev/null; then
        echo "App is already running (PID: $(cat $PID_FILE))"
        exit 1
    fi
    cd "$APP_DIR"
    nohup node "$APP_NAME" > app.log 2>&1 &
    echo $! > "$PID_FILE"
    echo "App started (PID: $!)"
}

stop() {
    if [ ! -f "$PID_FILE" ]; then
        echo "App is not running"
        exit 1
    fi
    PID=$(cat "$PID_FILE")
    echo "Stopping app (PID: $PID)..."
    kill "$PID" 2>/dev/null
    sleep 2
    if kill -0 "$PID" 2>/dev/null; then
        echo "Force killing..."
        kill -9 "$PID" 2>/dev/null
    fi
    rm -f "$PID_FILE"
    echo "Stopped"
}

status() {
    if [ -f "$PID_FILE" ] && kill -0 $(cat "$PID_FILE") 2>/dev/null; then
        echo "Running (PID: $(cat $PID_FILE))"
        ps -p $(cat "$PID_FILE") -o pid,%cpu,%mem,etime,comm
    else
        echo "Not running"
    fi
}

case "${1:-status}" in
    start) start ;;
    stop) stop ;;
    restart) stop; start ;;
    *) status ;;
esac