Linux Processes

Also Known As

TL;DR

Explanation

Each process has a PID (process ID), parent PID (PPID), memory map, open file descriptors, and a signal mask. Key tools: ps aux (snapshot of all processes), top/htop (live view), kill -SIGNAL PID (send signal), strace (trace system calls), lsof (open files per process). Signals: SIGTERM (graceful shutdown request), SIGKILL (immediate termination, cannot be caught), SIGHUP (reload config), SIGUSR1/SIGUSR2 (custom). PHP-FPM responds to SIGTERM for graceful shutdown and SIGUSR2 for reload.

Common Misconception

Why It Matters

Common Mistakes

• Using kill -9 as the first option — SIGKILL prevents cleanup, open file handles stay open, transactions may be left incomplete.
• Not handling SIGTERM in queue workers — workers killed mid-job corrupt the queue state.
• ps aux grep includes the grep process itself in results — use ps aux | grep [p]rocess to exclude it.
• Not knowing that zombie processes are already dead — they are waiting for the parent to read their exit code; kill the parent or fix the parent to wait().

Code Examples

# Brutal kill — no cleanup, potential data loss:
kill -9 $(pgrep php-fpm)   # Immediately kills all FPM workers
# Active requests terminated mid-execution
# DB transactions left open
# Temp files not cleaned up

# Graceful PHP-FPM reload (zero-downtime):
kill -USR2 $(cat /var/run/php-fpm.pid)  # Reload config, finish active requests

# Graceful shutdown (waits for active requests):
kill -TERM $(cat /var/run/php-fpm.pid)  # SIGTERM — graceful

# Monitor:
watch -n1 'ps aux | grep php-fpm | grep -v grep'

# Queue worker: handle SIGTERM gracefully in PHP:
pcntl_async_signals(true);
pcntl_signal(SIGTERM, function() use (&$running) { $running = false; });

References

• ↗ https://man7.org/linux/man-pages/man7/signal.7.html

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