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Overview of linux processes

Definitions

A process is an instance of a program running in Linux. An application can use single process (simple commands like ls), or multiple processes (e.g. web browsers).

Process types
Interactive processes and jobs control
Execution Description
command, ./command Applications, services, user apps and libs
command & Runs in foreground
jobs System configuration files
Ctrl+Z Suspend (stop, but not quit)
Ctrl+C Interrupt (terminate and quit) a process running in the foreground.
bg (after Ctrl+Z) Reactivate a suspended program in the background.
fg Move to foreground
/tmp Storage for all variable files and temporary files created by users
Daemons

Daemons are server processes(services) that run continuously. Most of the time, they are initialized at system startup and then wait in the background until their service is required.

Init process

The first thing the kernel does is to execute init program. Init is the root/parent of all processes executing on Linux. Based on the appropriate run-level, scripts are executed to start various processes to run the system and make it functional. /sbin/init /usr/lib/systemd/systemd

Execution of a program from shell:
  • relative to current path: ./command
  • if exists in $PATH, can be located: ping
Essential process characteristics:
  • Process Identifier (PID)
  • Parent Process Identifier (PPID)
  • User and group Identifiers (UID and GID)
[training@0715631312e8 bin]$ ll | grep ping
-rwxr-xr-x 1 root root   44896 Nov 20  2015 ping
BUT==> 
[training@0715631312e8 bin]$ ps aux | grep ping
training    51  0.3  0.0  10812   828 ?        S    10:53   0:00 ping 127.0.0.1
  • Environment
  • When terminated - Exit Code
When a process exits - it returns its exit status to the parent.
Exit statuses are used to control process execution flow.
Process exit code can be checked in shell with $?

Checking processes information

  • top
Flags:
top -u
top -n 1

Commands:
Shift + O 
c # Command line
k # Kill
  • ps
ps aux | grep NAME
  • htop
Other useful:
  • htop
  • pstree

Checking process data in /proc

cat /proc/PID/environ cat /proc/PID/maps ls -lah /proc/963

Controlling processes state

  • kill with TERM, INT, KILL, HUP
man signal / man 7 signal
  • pkill
  • killall
  • renice

Scheduling processes

  • at
at tomorrow <RETURN>
Ctrl+D