Pipes, Redirection & Processes

# Pipe — connect stdout of one command to stdin of next
cat access.log | grep "ERROR" | sort | uniq -c | sort -rn | head -20

# Redirection
echo "hello" > file.txt     # write (overwrite)
echo "world" >> file.txt    # append
command 2> errors.log       # redirect stderr
command > out.log 2>&1      # redirect both to file
command &> /dev/null        # discard all output

# Useful pipelines
# Find largest files
find / -type f -exec du -h {} + 2>/dev/null | sort -rh | head -20

# Extract unique IPs from logs
awk '{print $1}' access.log | sort -u | wc -l

# Replace in all files
grep -rl "old_text" ./src | xargs sed -i '' 's/old_text/new_text/g'

# Monitor file changes
watch -n 1 "wc -l app.log"

# Parallel execution
command1 & command2 & wait  # run both, wait for all

# Background & foreground
long_command &          # run in background
jobs                    # list background jobs
fg %1                   # bring job 1 to foreground
bg %1                   # resume stopped job in background

# Process info
ps aux                  # all processes
pgrep -f "node"         # find process by name
kill -15 <pid>          # graceful shutdown (SIGTERM)
kill -9 <pid>           # force kill (SIGKILL)
pkill -f "node server"  # kill by name pattern

# System resources
top / htop              # interactive process monitor
df -h                   # disk space
free -h                 # memory usage
uptime                  # system uptime & load average