Intro to Unix for Smart People Part II

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Intro to Unix for Smart People Part II

• Joe Morrison, Lab49

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Unix graphics architecture

• The X Window System
• Or simply X
• But not X Windows

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X Window System Architecture
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Cut and paste in X

• Open protocol (ICCCM)
• When user does a CUT
• App sends a message to X My Window now owns
  selection PRIMARY
• When user does a PASTE
• App sends a message to X Please paste selection
  PRIMARY into my attribute foo
• X sends owning app a SelectionRequest event
• Owning app sends a ChangeProperty request to X to
  push data
• Owning app sends a SelectNotify request to X to
  forward to requestor
• Requestor can now read the data by sending X a
  GetProperty request
• Benefits
• Works cleanly with apps and X server on different
  network nodes
• Allows format negotiation
• Works even if apps cannot communicate with each
  other directly
• Allows interesting enhancements (xclipboard app
  can watch for events and take long-term ownership
  of selected items)

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Display manager simple scenario

• Simple scenario one computer with graphics
  capability
• Display manager runs first
• Starts X server
• Runs a graphical login program
• Graphical equivalent of getty

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Display manager simple scenario
Pretend this is an X terminal
/etc/inittab
fork/exec
startx
xdm
fork/exec (respawn)
init (pid 1)
wait for X server to start
display login / password prompt
xlogin
exec
User-specified X apps to start (xterm, etc.)
Start other required processes
xdm plays same role as getty (but graphical)
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Display manager complex scenario

• More complex scenario multiple X terminals, one
  host
• X terminal broadcasts manage me!
• Display manager responds by forking a process to
  run graphical login over the network
• Graphical equivalent of telnet daemon
• Support hordes of developers with dumb terminals
  and one giant server

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Display manager complex scenario
xdm plays same role as telnetd (but graphical)
Broadcast Can someone manage me?
xdm listener
for each request, fork/exec xlogin
xlogin
display login / password prompt
exec
User-specified X apps to start (xterm, etc.)
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Other scenarios

• Multiple hosts, each running xdm
• Poor mans load balancing
• Large group of X terminals serviced by a small
  number of hosts
• Each X terminal is managed by whichever xdm
  answers first (usually the least loaded)
• INDIRECT mode
• X terminals do not broadcast
• Instead they contact a primary xdm
• Primary xdm delegates to secondary xdm according
  to desired configuration

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Window managers

• Ordinary processes running in background
• Register with X server to receive notifications
  when applications start, open windows, etc.
• Get to meddle
• Relocate new application windows
• Draw ornamentation around windows (resize grips,
  title bar)
• Handle visual interaction for resizing, moving,
  minimizing, maximizing windows, etc.
• Send application resize/move instructions
• Get to decide
• Start menu? Icons? Docks? Button bars?
• Can be changed on the fly!
• You can write one!

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twm window manager (the original!)
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IceWM (themeable)
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Metacity desktop (Ubuntu)
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Session managers

• Responsible for saving/restoring application
  state
• Standard protocol for communication between
  window manager and session manager
• Enables window manager to offer services like
  logout and save session

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Unix desktop

• Display manager
• Window manager
• Session manager

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SSH (poor mans VPN)

• Secure replacement for telnet
• Also port forwarding

ssh -l 123410.0.0.780 rasputin.lab49.com
10.0.0.7
Encrypted communication (compressed too!)
RASPUTIN
ssh client in addition to telnet session
accepts connections on port 1234
ssh server in addition to telnet daemon
relays client port 1234 to port 80 on 10.0.0.7
ssh provides a secure, encrypted TCP/IP proxy to
any computer reachable from the ssh server
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X over SSH

• If you can SSH to a Unix machine, you can run any
  X application on that machine and have it display
  on yours (regardless of firewalls)

ssh -X rasputin.lab49.com
Encrypted communication (compressed too!)
RASPUTIN
ssh client in addition to telnet session acts
as a proxy X client for all apps started on
Rasputin
ssh server in addition to telnet daemon
creates a fake local X server and sets it to be
the default requests are encrypted, compressed,
and tunnelled back to the client
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Package management with RPM(Redhat Package
Manager)

• Both a Unix app (manages all installed packages)
  and a file format
• Maintains a database of installed apps
• Knows which RPM each file belongs to
• Knows what the md5sum of each file should be
• To install an application
• rpm ivh myapp-1.3.2.i386.rpm
• Checks dependencies, installs all files, runs
  postinstall scripts, etc.
• To remove
• rpm e myapp
• The RPM system remembers which files belong to
  myapp always removes cleanly
• Application does not need to cooperate (badly
  written app cannot screw up RPM)
• Removing an application never results in a
  different application being uninstallable
• To list files in an RPM
• rpm ql myapp
• To get general info about an RPM
• rpm qi myapp
• To find out which RPM owns a particular file
• rpm qf /usr/bin/emacs
• To check whether an application has been damaged

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Package management with APT (Advanced Package
Tool)

• Rather than operating on individual files, APT is
  based on repositories so it knows about all
  packages and their dependencies
• Like RPM but more holistic view of your system
• If you install an RPM and dont include all
  necessary dependencies, it will refuse to install
• But if you install using APT, it will quietly
  install everything needed

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Package management with YUM (Yellowdog Updater
Modified)

• Provides an APT-like experience on top of RPM

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About /proc

• The /proc pseudo filesystem is a real time,
  memory resident file system that tracks the
  processes running on your machine and the state
  of your system
• The contents of the /proc filesystem areused by
  many utilities which grab the data from the
  particular /proc directory and display it
• You can also write to the /proc filesystem, e.g.
  writing to the /proc/sys directory enables you to
  change kernel parameters
• echo 1 gt /proc/sys/net/ipv4/ip_forward
• Examples
• cat /proc/vmstat
• cat /proc/1234/cmdline
• cat /proc/meminfo
• The contents of /proc need to be standardized
  too many utilities count on the format of /proc
  files

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/proc/vmstat
nr_dirty 8 nr_writeback 0 nr_unstable
0 nr_page_table_pages 1040 nr_mapped
53684 nr_slab 11199 pgpgin 1436240 pgpgout
29582742 pswpin 23 pswpout 40 pgalloc_high
2789161 pgalloc_normal 53175006 pgalloc_dma
11 pgfree 56056660
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The comforts of home

• Xscreensaver
• XMMS (like Winamp)
• Xine / MPlayer (play videos)
• GRIP (rip CDs)
• Mozilla/Firefox/Opera
• Emacs
• Eclipse
• VNC (remote desktop)

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Closing thoughts

• In general Unix desktops are primitive compared
  to other popular operating systems
• (NeXT and Mac being notable exceptions)
• But its not just about features, ease of use,
  and stability
• (This better be good)
• Its also about internal interfaces
• Make OS reusable in more contexts
• Create a breeding ground for improvements
• For example
• Accessibility (rewrite the window manager)
• Embedded systems (replace /etc/inittab)

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Unix versus Windows

• Windows Best platform for creating end-user
  applications
• Unix Best set of building blocks for general
  purpose, secure, multi-user, multi-process
  computing

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Final closing thoughts (really)

• Windows
• Intelligent design
• Unix
• Mutation and natural selection