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 ^[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 unix 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 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.

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 Filesystem Hierarchy Standard and 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 (Richard M. 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

^ : https://en.wikipedia.org/wiki/Vmlinux