View source for COMP 3000 2012 Week 3 Notes

== Shell ==

* useful command line programs
** bc -l
*** command line calculator
** cal
*** command line calendar
** uniq
*** filters out duplicate lines
** sort
*** takes input on STDIN, sorts lines, prints to STDOUT
* vmlinux is the linux kernel with virtual memory {{ref|vmlinux}}
** vmlinuz is the same but compressed
* bash runs on vmlinux
** most programs run on bash
** ls/cd also run on vmlinux
** ls runs as a child process of bash
* to get rid of a misbehaving child
** the parent must kill it
** waits for child to die
** an unreaped child becomes a zombie
** zombies can't be killed, must be reaped
** to kill a zombie, must kill parent
** orphans become a child of init (ward of state)
** init is good at reaping children

== job control ==

* job is a process
** term comes from batch processing
* Useful control sequences
** Ctrl-C
*** terminate job
** Ctrl-Z
*** stop job
* To start a stopped job in the foreground
** fg %n
*** where n is the job number
* To start a stopped job in the background
** bg %n
* To determine jobs that are running
** jobs
* Who manages jobs?
** the shell
** built on kernel functionality

== Why we're studying  <span style="text-decoration:line-through;">unix</span> Linux based operating systems==

* all operating systems have roughly the same architecture
* windows is just a variation on unix. Built and designed very similarily.
* "Like a different accent. Same everything, but renamed"
* Slight differences in the way things are done
** Example: On Unix, configs reside in flat text files. In Windows, they live in an opaque key/value store called the registry.

Despite the similarities between OS families, Unix systems tend to be built much more transparently, which makes them a good vehicle to study OS topics

* windows comes from vms
** increment(vms) -> wnt

see [http://www.team.net/mjb/hawg.html Unix - The Hole Hawg].

== Init and Shell Scripts ==

Guest mini-lecture by Ann Fry

* /etc/init.d/
* initialization scripts
* examples
** /etc/init.d/networking [start|stop]
** /etc/init.d/networking [start|stop]
'''Note:''' ''command'' [ ''keyword1'' | ''keyword2'' | ... ] means to execute a command with an optional choice of two or more keywords.
* shell scripts
* start with
** #!/bin/bash
** #!/bin/csh
** #!/bin/ksh


''Enter'' Anil Somayaji.

''[http://en.wikipedia.org/wiki/Exeunt Exeunt]'' Ann ''stage left''.


* Init scripts
* Traditional init scripts run sequentially
* Modern init scripts run in parallel
* executables
* ELF
* A file starting with #!
** next comes the path to an interpreter for the rest of the file
=== Common Directories ===
see [http://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard Filesystem Hierarchy Standard] and [http://www.pathname.com/fhs/pub/fhs-2.3.html FHS 2.3]
* /etc
** pronounced ett-see
** system wide configuration files
** different subdirectories and directory structures
** Competing Unix cultures apparent in the naming conventions and directory structures
** /etc/<prog>.conf
** /etc/<prog>/
** /etc/<prog>.d/
* /bin
** system binaries
** often this is local
** Bare minimum of what's needed to boot.
** Important for diskless systems (see /usr/) or think clients. 
** Historically, we kept this as small as possible
* /lib
** system libraries
* /home
** user directories
* /usr
** often this is shared
** Historically, the /usr/ directory was a remote file system mounted after the system had fully booted. Servers had more disk space than terminals/thin clients.
** /usr/bin
** user binaries
** /usr/lib
** user libraries

== History of Unix ==

* In the beginning, there was AT&T's Unix
* AT&T gave site licenses with all the source
* Berkeley developed networking (at least TCP/IP) called BSD
** licensed under permissive BSD license
* AT&T forked System V
** Not permissively licensed
* Sun created hardware to run unix and forked SunOS from BSD
* Sun changed from SunOS (BSD) to Solaris (System V)
** people disliked this
* POSIX standard was an outcome of the fragmentation
* Free Software Foundation came out of the fragmentation
** founded by rms ([http://xkcd.com/225/ Richard] M. [http://www.youtube.com/watch?v=1sxNy0qQDkU Stallman])
** not initially a very open development environment
** built all the components of unix called GNU (GNU's Not Unix)
** yacc -> bison
** more -> less
** cc -> gcc
* Anil was sysadmin for a crystalography lab
** SGI IRX boxes
** ran IRIX
* 80386 came out
** capable of running a real unix because of virtual memory
** legal status of BSD kernel became an issue because of lawsuit
** GNU started Hurd
** Linus Torvalds writes Linux kernel
** uses GNU license
** allowed everyone to make contributions
* Eventually BSD legal stuff got figured out
** 386BSD
** FreeBSD
** NetBSD
*** OpenBSD
* Anil's opinion
* Linux has the best driver support of any OS including Windows

== Back to Init ==

* Why do we care?
* Unix is a culture
* the culture shows up all over, like in /etc, /etc/init.d
* Now there are 3 major replacements for system V init scripts
* system V init scripts are just shell scripts run sequentially
* it's desirable to run the initialization in parallel
** oh noes, race conditions
** init scripts have dependencies!!
* init scripts were ordered by convention
** had to figure out all the dependencies
** still not completely figured out
* everything on linux is an open source project
* look around for a good project to document


== References ==

{{note|vmlinux}}: https://en.wikipedia.org/wiki/Vmlinux