# Your processes only (minimal info)
ps
# Output:
#   PID TTY          TIME CMD
#  1234 pts/0    00:00:00 bash
#  1235 pts/0    00:00:00 ps
 
# All processes, BSD-style (most common on Linux)
ps aux
# 'a' = all users, 'u' = user-oriented format, 'x' = include processes without TTY
 
# All processes, UNIX-style
ps -ef
# '-e' = all processes, '-f' = full format
 
# Why two styles? Historical reasons (BSD vs. System V Unix)
# Use whichever you prefer; ps aux is more common on Linux
 

# Sort by memory usage (top 10)
ps aux --sort=-%mem | head -11
# --sort=-%mem means sort by memory, descending (- = reverse)
 
# Sort by CPU usage
ps aux --sort=-%cpu | head -11
 
# Sort by process start time
ps aux --sort=start_time
 
# Multiple sorts (CPU, then memory)
ps aux --sort=-%cpu,-%mem | head -11
 
# Custom columns
ps -eo pid,ppid,user,%cpu,%mem,cmd --sort=-%cpu | head -15
# -e = all processes
# -o = output format (specify columns)
# Available columns: pid, ppid, user, %cpu, %mem, vsz, rss, tty, stat, start, time, cmd, and many more
 
# Find specific processes
ps aux | grep nginx
ps -C nginx                      # By command name
ps -u alice                      # By user
ps -p 1234,5678                  # By PID
 
# Process tree (parent-child relationships)
ps auxf                          # 'f' = forest (ASCII tree)
ps -ejH                          # UNIX-style tree
 

# Show process tree
pstree
 
# With PIDs
pstree -p
 
# For a specific user
pstree alice
 
# For a specific process and its descendants
pstree -p 1234
 
# Example output:
# systemd─┬─ModemManager───2*[{ModemManager}]
#         ├─NetworkManager───2*[{NetworkManager}]
#         ├─accounts-daemon───2*[{accounts-daemon}]
#         ├─bash───pstree
#         ├─nginx───nginx
#         └─sshd───sshd───bash───python
 

top
 
# Key information in top:
# - Load average (1, 5, 15 minute averages)
# - Tasks: running, sleeping, stopped, zombie
# - %CPU: user processes, system, idle, I/O wait
# - Memory: total, used, free, buffers, cache
# - Process list: sorted by CPU usage by default
 

# Install htop
sudo apt install htop    # Debian/Ubuntu
sudo yum install htop    # CentOS/RHEL
sudo dnf install htop    # Fedora
 
# Run htop
htop
 

# Find PIDs by name
pgrep nginx
# Output: 890 891 892
 
# Show PID and name
pgrep -l nginx
# Output: 890 nginx
#         891 nginx
#         892 nginx
 
# Show PID and full command
pgrep -a nginx
# Output: 890 nginx: master process /usr/sbin/nginx
#         891 nginx: worker process
 
# Find by user
pgrep -u alice
pgrep -u alice python    # Alice's python processes
 
# Count matching processes
pgrep -c nginx
# Output: 3
 
# Most recent match
pgrep -n nginx
 
# Oldest match
pgrep -o nginx
 
# Exact match only
pgrep -x bash    # Matches "bash" but not "bash-script"
 
# Kill by name (careful!)
pkill nginx
pkill -u alice python    # Kill alice's python processes
pkill -f "python app.py" # Kill by full command line
 

# Run in foreground (default)
sleep 60
# Terminal blocked for 60 seconds
 
# Run in background (add & at the end)
sleep 60 &
# [1] 12345              ← Job number 1, PID 12345
# Terminal immediately available
 
# Multiple background jobs
sleep 30 &    # Job 1
sleep 40 &    # Job 2
sleep 50 &    # Job 3
 

# List jobs
jobs
# [1]   Running                 sleep 30 &
# [2]-  Running                 sleep 40 &
# [3]+  Running                 sleep 50 &
# + = most recent job, - = second most recent
 
# List with PIDs
jobs -l
# [1]  12345 Running             sleep 30 &
# [2]  12346 Running             sleep 40 &
 
# Bring a background job to foreground
fg %1                    # By job number
fg                       # Most recent job (the + one)
 
# Suspend a foreground process (pause it)
# Run: sleep 60
# Press: Ctrl+Z
# Output: [1]+  Stopped     sleep 60
 
# Resume a stopped job in the background
bg %1
# [1]+ sleep 60 &
 
# Resume in foreground
fg %1
 

# Start editing a file
vim large_file.txt
 
# Realize you need to run a command
# Press Ctrl+Z
# [1]+  Stopped                 vim large_file.txt
 
# Run your command
ls -l
 
# Return to editing
fg
 

# Run a command, then decide to background it
find / -name "*.log" 2>/dev/null    # Oops, this is slow!
# Press Ctrl+Z to suspend
# [1]+  Stopped     find / -name "*.log" 2>/dev/null
bg
# Now it runs in background while you continue working
 
# Problem: If you log out, background jobs are killed!
# Solution: nohup or disown
 

# Run a command that survives logout
nohup ./long_running_script.sh &
# Output appended to nohup.out by default
 
# Redirect output to a specific file
nohup ./script.sh > output.log 2>&1 &
# > output.log = redirect stdout
# 2>&1 = redirect stderr to same file as stdout
# & = run in background
 
# Check if it's running after logout/login
ps aux | grep script.sh
 

# Start a job
./long_script.sh &
# [1] 12345
 
# Disown it (shell forgets about it)
disown %1
 
# Or disown most recent job
disown
 
# Now you can close the terminal without killing the process
 
# Verify it's still running
ps -p 12345
 

# Polite termination (SIGTERM = 15, default)
kill 12345
kill -15 12345            # Explicit SIGTERM
kill -TERM 12345          # By name
 
# How SIGTERM works:
# 1. kernel sends SIGTERM to PID 12345
# 2. Process receives signal
# 3. Process cleanup code runs (close files, flush buffers, save state)
# 4. Process exits with cleanup complete
 
# Force kill (SIGKILL = 9, last resort!)
kill -9 12345
kill -SIGKILL 12345
 
# When to use kill -9:
# - Process doesn't respond to SIGTERM
# - Process is in uninterruptible sleep (state D)
# - Process is frozen/hung
# WARNING: No cleanup happens! Files may be corrupted!
 
# Reload configuration (common with daemons)
kill -HUP 12345
# Example: nginx, sshd reload config without disconnecting users
 
# Pause a process
kill -STOP 12345
# Resume it
kill -CONT 12345
 
# Send user-defined signals
kill -USR1 12345    # App-specific behavior
kill -USR2 12345    # App-specific behavior
 
# List all signals
kill -l
 

# Kill all processes with a specific name
killall nginx
killall -TERM nginx      # Explicit SIGTERM
killall -9 nginx         # Force kill all
 
# Kill all processes owned by a user
killall -u alice
 
# Interactive confirmation
killall -i nginx         # Prompts before each kill
 
# pkill — kill by pattern matching
pkill python             # All python processes
pkill -f "app.py"        # Match full command line
pkill -u alice python    # Alice's python processes
pkill -x python3         # Exact name match only
 
# pkill with signals
pkill -HUP nginx         # Reload nginx
pkill -TERM python       # Gracefully stop all python
 
# Caution: killall and pkill are dangerous!
# Always test with pgrep first:
pgrep -a nginx           # See what would be affected
# Then kill:
pkill nginx
 

#!/bin/bash
# trap 'commands' SIGNALS
 
# Clean up temp files when script exits
TEMP_FILE=$(mktemp)
trap "rm -f $TEMP_FILE; echo 'Cleaned up!'" EXIT
# EXIT is a pseudo-signal that triggers when script exits for any reason
 
# Handle Ctrl+C gracefully
trap "echo 'Interrupted! Exiting...'; exit 1" INT
# INT = SIGINT (Ctrl+C)
 
# Handle termination signal
trap "echo 'Terminated! Cleaning up...'; cleanup; exit 1" TERM
 
# Ignore a signal (process won't be affected by it)
trap '' HUP              # Ignore SIGHUP
trap '' INT              # Ignore Ctrl+C (dangerous!)
 
# Reset trap to default behavior
trap - INT               # Ctrl+C works normally again
 
# Multiple signals, one handler
trap cleanup EXIT INT TERM
 

#!/bin/bash
# A robust script with cleanup
 
LOCKFILE="/var/run/myapp.lock"
LOGFILE="/var/log/myapp.log"
TEMP_DIR=$(mktemp -d)
 
# Cleanup function
cleanup() {
    echo "[$(date)] Cleaning up..." >> "$LOGFILE"
    rm -f "$LOCKFILE"
    rm -rf "$TEMP_DIR"
    echo "Cleanup complete"
}
 
# Set traps
trap cleanup EXIT INT TERM
 
# Prevent multiple instances
if [[ -f "$LOCKFILE" ]]; then
    echo "Another instance is already running (PID: $(cat $LOCKFILE))"
    exit 1
fi
echo $$ > "$LOCKFILE"
 
# Main script work
echo "[$(date)] Started (PID: $$)" >> "$LOGFILE"
 
for i in {1..10}; do
    echo "Processing step $i/10..."
    # Simulate work
    echo "Data $i" > "$TEMP_DIR/file$i.txt"
    sleep 1
done
 
echo "Done!"
# cleanup() runs automatically due to EXIT trap