Linux administration

• The kernel considers each program running on your system to be a process
• A process 'lives' as it executes, with a lifetime that may be short or long
• A process is said to 'die' when it terminates
• The kernel identifies each process by a number known as a process id, or pid
• The kernel keeps track of various properties of each process

• A process has a user id (uid) and a group id (gid) which together specify what permissions it has

• A process has a parent process id (ppid) — the pid of the process which created it

  • The kernel starts an init process with pid 1 at boot-up
  • Every other process is a descendant of pid 1

• Each process has its own working directory, initially inherited from its parent process

• There is an environment for each process — a collection of named environment variables and their associated values

  • A process's environment is normally inherited from its parent process

• The init process is the ancestor of all other processes:

• (Apache starts many child processes so that they can serve HTTP requests at the same time)

• The ps command gives a snapshot of the processes running on a system at a given moment in time

• Very flexible in what it shows, and how:

  • Normally shows a fairly brief summary of each process
  • Normally shows only processes which are both owned by the current user and attached to a terminal

• Unfortunately, it doesn't use standard option syntax

• Instead it uses a mixture of options with one of three syntaxes:

  • Traditional BSD ps: a single letter with no hyphen
  • Unix98 ps: a single letter preceded by a hyphen
  • GNU: a word or phrase preceded by two hyphens

• ps has many options. Some of the most commonly used are:

• Displays a snapshot of running processes
• Always uses a tree-like display, like ps -f
  • But by default shows only the name of each command
• Normally shows all processes
  • Specify a pid as an argument to show a specific process and its descendants
  • Specify a user name as an argument to show process trees owned by that user

• Shows full-screen, continuously-updated snapshots of process activity
  • Waits a short period of time between each snapshot to give the illusion of real-time monitoring
• Processes are displayed in descending order of how much processor time they're using
• Also displays system uptime, load average, CPU status, and memory information

• A process can be sent a signal by the kernel or by another process
• Each signal is a very simple message:
  • A small whole number
  • With a mnemonic name
• Signal names are all-capitals, like INT
  • They are often written with SIG as part of the name: SIGINT
• Some signals are treated specially by the kernel; others have a conventional meaning
• There are about 30 signals available, not all of which are very useful

• The command kill -l lists all signals
• The following are the most commonly used:

• The kill command is used to send a signal to a process
  • Not just to terminate a running process!
• It is a normal executable command, but many shells also provide it as a built-in
• Use kill -HUP pid or kill -s HUP pid to send a SIGHUP to the process with that pid
• If you miss out the signal name, kill will send a SIGTERM
• You can specify more than one pid to signal all those processes

• On Unix systems, long-lived processes that provide some service are often referred to as daemons
• Daemons typically have a configuration file (usually under /etc) which affects their behaviour
• Many daemons read their configuration file only at startup
• If the configuration changes, you have to explicitly tell the daemon by sending it a SIGHUP signal
• You can sometimes use pidof to find the daemon's pid; for example, to tell the inetd daemon to reload its configuration, run:

$ kill -HUP $(pidof /usr/sbin/inetd)

as root

• free: Display memory in different units

free -m

##               total        used        free      shared  buff/cache   available
## Mem:          28088        5375       13660         123        9052       24157
## Swap:          8191           0        8191

• vmstat: Report virtual memory statistics

vmstat

## procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
##  r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
##  1  1      0 13988544 750252 8519600    0    0   115   185  292  162 17  7 74  2  0

• htop: Text, interactive menu interface

• a. Use top to show the processes running on your machine.
• b. Make top sort by memory usage, so that the most memory-hungry processes appear at the top.
• c. Restrict the display to show only processes owned by you.
• d. Try killing one of your processes (make sure it's nothing important).
• e. Display a list of all the processes running on the machine using ps (displaying the full command line for them).
• f. Get the same listing as a tree, using both ps and pstree.
• g. Have ps sort the output by system time used.