Process Management¶

A process is a running program with a PID, state, priority, and resource allocation. This entry covers viewing, filtering, and controlling processes - essential for system administration and debugging.

Key Facts¶

Every process has a PID (Process ID) and PPID (Parent Process ID)
PID 1 is the init/systemd process - parent of all others
Daemon = background process (system service)
Thread = unit of execution within a process, shares memory space

Process States¶

State	Symbol	Description
Running	R	Actively executing or ready
Sleeping (interruptible)	S	Waiting for event, can be interrupted
Sleeping (uninterruptible)	D	Waiting for I/O, cannot be interrupted
Stopped	T	Paused (Ctrl+Z or signal)
Zombie	Z	Terminated but parent hasn't collected exit status

ps - Process Snapshot¶

ps                          # processes in current terminal
ps aux                      # all processes, full info (BSD syntax)
ps -ef                      # all processes (UNIX syntax)
ps afx                      # process tree
ps -u username              # processes for specific user
ps -T -p PID                # show threads of a process
ps 1                        # info for PID 1

ps aux Columns¶

USER  PID  %CPU  %MEM  VSZ  RSS  TTY  STAT  START  TIME  COMMAND

VSZ: virtual memory size
RSS: resident set size (actual RAM)
STAT: process state

top - Interactive Monitor¶

top                     # default (sort by CPU%)
top -o PID              # sort by PID
top -u username         # filter by user
top -p PID1,PID2        # monitor specific PIDs

Inside top¶

Key	Action
q	Quit
k	Kill a process
r	Renice (change priority)
M	Sort by memory
P	Sort by CPU
1	Toggle per-CPU stats
u	Filter by user

Load Average¶

Shown in top header: load average: 0.15, 0.20, 0.18 (1, 5, 15 min)

Value < CPU cores = not overloaded
Value = cores = at capacity
Value > cores = overloaded

htop - Enhanced Monitor¶

sudo apt install htop
htop                    # visual CPU/memory bars, mouse support
htop -u username        # filter by user

Finding Processes¶

pidof firefox           # PID by exact name
pgrep pattern           # PIDs matching pattern
pgrep -la nginx         # PIDs + full command lines

Signals¶

kill PID                # SIGTERM (graceful, default)
kill -9 PID             # SIGKILL (force, immediate)
kill -15 PID            # SIGTERM explicitly
kill -1 PID             # SIGHUP (reload config)
killall processname     # kill all by name
pkill pattern           # kill by pattern
kill -l                 # list all signals

Signal	Number	Description
SIGHUP	1	Hangup / reload config
SIGINT	2	Interrupt (Ctrl+C)
SIGQUIT	3	Quit with core dump
SIGKILL	9	Force kill (cannot be caught)
SIGTERM	15	Graceful terminate
SIGSTOP	19	Pause (cannot be caught)
SIGCONT	18	Resume paused process

Process Priority (nice)¶

Range: -20 (highest priority) to +19 (lowest).

nice -n 10 command      # start with lower priority
renice -n 5 -p PID      # change running process priority

Process Creation¶

Name assigned to process
Added to process list
Priority determined
Process control block (PCB) formed
Resources allocated

Process Attributes¶

Attribute	Description
`pid`	Process ID
`ppid`	Parent Process ID
`fd`	Open file descriptors
`cwd`	Current working directory
`environ`	Environment variables
`nice`	Priority value

Gotchas¶

kill -9 should be last resort - process cannot clean up (temp files, locks, sockets)
Zombie processes can't be killed - they're already dead; kill their parent instead
top load average includes processes in D state (waiting for I/O)
killall on Solaris kills ALL processes - on Linux it kills by name (be careful on mixed environments)
Background processes with & still die when terminal closes - use nohup or disown