Processes

Your application is running in production. You need to restart it for a config change. Do you: A) kill -9 and hope for the best B) Send a signal it can handle gracefully If you picked A, you’ve probably lost data. Let’s fix that. The Essential Signals Signal Number Default Action Use Case SIGTERM 15 Terminate Graceful shutdown request SIGINT 2 Terminate Ctrl+C, interactive stop SIGHUP 1 Terminate Config reload (by convention) SIGKILL 9 Terminate Force kill (cannot be caught) SIGUSR1/2 10/12 Terminate Application-defined SIGCHLD 17 Ignore Child process state change SIGTERM is the polite ask. “Please shut down when convenient.” SIGKILL is the eviction notice. No cleanup, no saving state, immediate death. ...

Understanding processes is fundamental to Linux troubleshooting. These tools and techniques will help you find what’s running, what’s stuck, and what needs to die. Viewing Processes ps - Process Snapshot 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 # All processes (BSD style) ps aux # All processes (Unix style) ps -ef # Process tree ps auxf # Specific columns ps -eo pid,ppid,user,%cpu,%mem,stat,cmd # Find specific process ps aux | grep nginx # By exact name (no grep needed) ps -C nginx # By user ps -u www-data Understanding ps Output U r w S o w E o w R t - d a t a 1 P 2 I 3 D 1 4 % C 0 2 P . . U 0 5 % M 0 1 E . . M 1 2 1 4 6 5 9 6 V 9 7 S 3 8 Z 6 9 1 9 3 8 R 2 7 S 5 6 S 6 5 T ? ? T Y S S S T s l A T S F 1 T e 0 A b : R 2 0 T 4 0 T 0 5 I : : M 0 2 E 3 3 C / n O s g M b i M i n A n x N / : D i n w i o t r k e r PID: Process ID %CPU: CPU usage %MEM: Memory usage VSZ: Virtual memory size RSS: Resident set size (actual RAM) STAT: Process state TIME: CPU time consumed Process States (STAT) R S D Z T N s l R S S Z S H L S M F u l l o t i o e u o n e e m o g w s l r n e e b p h s t e i p p i p p i i g n i i e e p r o - r g n n d r i n t o g g i o h u o r l r n ( ( r i e e d i u i t a a n n t y d d p t i y e e r e n r d o r t c r e e u r s p r s t u i p b t l i e b ) l e , u s u a l l y I / O ) top - Real-time View 1 2 3 4 5 6 7 8 9 10 11 # Basic top # Sort by memory top -o %MEM # Specific user top -u www-data # Batch mode (for scripts) top -b -n 1 Inside top: ...