UNIX

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UNIX

• Unix is an operating system developed at Bell
  Labs in 1969.
• Like all operating systems, unix manages the
  resources of the computer and provides an
  interface for users to access system functions.
• Unix is a general purpose op. sys. It is used,
  for example, by Sun to develop java applications.
  It is not suited to real time applications.
• It was designed to provide a superior sw
  development environment to be simple, powerful,
  and compact.
• It was originally written in pdp-7 (DEC) assembly
  language by Ken Thompson so he could run his
  program space travel on the pdp.

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UNIX more history

• The uni- part refers to unix being developed for
  single users originally.
• Unixs history is intertwined with the
  development of C, also at Bell labs. Thompson
  developed a language called B which was used to
  help transport Unix to the Pdp-11 in 1971.
• Unix became the o.s. of choice for the pdp-11
  computer, preferable to DECs own o.s.
• Dennis Ritchie, also at Bell Labs, refined the
  B language to create C.
• Unix was itself rewritten in C in 1973.
• Unix was adapted to a 16 bit microcomputer by a
  small company, Onyx, in 1980.
• There are many versions out there Xenix, Ultrix
  are a couple.

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Key Features

1. Portability
2. Portable applications sw
3. Multiuser operation (it can support multiuser
   mainframe environments)
4. Background processing
5. Hierarchical file system
6. UNIX shell- a command interpreter
7. Pipes- allows several prgrams to be combined so
   that the output from one becomes input to the
   next
8. Utilities
9. Text-processing tools
10. Sw development tools
11. maturity

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Structure of UNIX

• The kernel is the core of the system controlling
  hardware and performing lowlevel functions.
• The shell is the command interpreter (user
  interface).
• Two hundred utility programs provide functions
  like file copy, text edit, sw compilation and so
  on.
• User programs logically occupy directory space
  analogous to the utility programs.

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The kernel

• The kernel interacts directly with system
  hardware, implements the file system, manages
  memory, enforces security, performs I/o. If the
  system is muti-user, the kernel provides the
  system timesharing capability.

apps
utilities
shell
kernel
Hardware peripherals
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The kernel

• The kernel also maintains system logs.
• Since the kernel interacts directly with the
  system hw, the kernel is custom written for each
  processor type that unix runs on.

File mgmt security
Date/time svces
I/o svces
kernel
Process scheduling
Ints and Error handling
System accounting
Memory mgmt
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Call interface

• Utilities and applications call the kernel to
  provide services.
• The call interface is the same for unix no matter
  what hw it runs on.

User program wants service
Call Request service Type Details data
Return Service complete Status data
UNIX kernel provides services
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The file system features

• Hierarchical
• Dynamic file sizes grow as needed
• Structureless no internal structure is imposed
• Security files can be protected from access by
  other users
• File device independence files and I/o
  devices are regarded the same by the o.s. So the
  same utilities used to process files can be used
  to process data from the terminal or as it is
  sent to the printer.

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Files directories

• File names can be up to 14 characters.
• A directory can be used to organize files. A
  directory is really just a file with a list of
  entries (file names) in it as well as information
  on where to find those files in the system (in a
  data structure called the inode).
• Users typically have their own directories.
• Files can have multiple names and multiple links
  (from different directories) Below, the
  filenames People and XYZ refer to the same file.
• Directory A Directory B
• Results 0011 - XYZ 1122
• People 1122
• Data 0123
• Files file 0123 file 0011 file 1122

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Hierarchical systems and path names

• Typical file systems have many levels / (root)
  in unix is at the top.
• Path names are separated by forward slashes
  (division sign) with a slash first if the
  pathname starts at the root directory
• /usr/higgins/csci201/unixnotes
• Such a path name is called fully qualified
  because the complete path from the root to the
  file is identified.
• Partial pathnames can be used, these are
  relative, indicating a path from the current
  working directory. If I am in /usr/higgins then
  the name csci201/unixnotes identifies the same
  file.
• Unix maintains information on each file
  location, size, link count, ownership, security
  setting, type, creation date, most recent
  modification, last access.

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devices

• Typical unix devices are
• /dev/lp (line printer)
• /dev/dk0 (disks)
• /dev/dk1 (etc)
• /dev/rdk0 (disk as raw I/o device)
• /dev/tty0 (terminals)
• /dev/null
• To copy a file to another file location you would
  type
• cp fname othername
• To copy the same file to the printer you would
  type
• cp fname /dev/lp

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directories

• /bin utilities
• /dev special files (I/o devices)
• /etc administration
• /lib libraries used by the language processors
• /tmp temporary
• /usr/bin overflow for /bin
• /adm/bin adminstration files
• /usr/games
• /usr/include include files for C
• /usr/lib archive libraries and textprocessing
  macros
• /usr/mail
• /usr/spool spool files for printing
• Usr/src source files

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Hierarchy chart / is the root
/
usr
lib
tmp
etc
dev
bin
bin
tmp
higgins
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File management utilities

• pwd prints name of the current working
  directory
• cd change pwd
• ls list directory contents
• cat concatenate files
• mv move/rename file
• ln create a new link for a file
• cp copy files
• mkdir make a dir
• rm remove a file
• rmdir
• du display disk utilization
• df display tree blocks for mounted systems
• touch update time of last mod for a file
• find find files

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examples

• pwd show present working directory
• /usr/higgins/courses/csci201
• cd /usr/higgins/courses/csci311 change to
  directory specified
• ls /usr/higgins/courses/csci201 list directory
  contents
• assignments
• sampletests
• classlist
• cat classlist show file contents on terminal
• bob
• joe
• sue
• mary
• rudolf
• xenophon

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examples- continued

• most common use of cat is to show file contents
• mv changes a filename
• mv classlist students
• mv can similarly be used to rename a directory
• ls- l means list long, and gives more details
  about files in the directory.
• ln is used to create additional links (not
  additional copies) of a file
• ln students classlist
• cp makes a duplicate file
• cp classlist list
• mkdir creates a new directory
• mkdir /usr/higgins/courses/csci116

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examples- continued

• rm is used to remove a file
• rm list
• rm can be used to delete directories and their
  contents also if you use
• rm r /usr/higgins/courses
• rmdir can be used to remove (delete) empty
  directories.

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File storage Buffered disk access

• Whatever block size the system uses the location
  of the first ten blocks of a file are stored in
  the inode. If the file is larger than 10 blocks,
  then the address of an indirect block is stored
  in the inode as well, which has the addresses of
  the files subsequent blocks. This technique is
  applied up to 3 times (triple indirection). The
  inode stores the location of a triple indirect
  block which stores the addresses of double
  indirect blocks, each of which stores the
  addresses of sngle indirect blocks, which point
  to file blocks on the disk.
• As with most modern operating systems, i/o is
  buffered in unix This means that some disk
  blocks are duplicated in main memory. A read
  request first checks if the required data is
  already available. If not, it can be brought
  into the buffer. When the buffer is full and a
  read requests new data, some must be swapped out
  of the buffer. Similarly, write requests write
  to a buffer until the buffer is full or is
  flushed.

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the unix shell

• The shell provides an interface between the user
  and unix o.s. services.
• Some features of this command interpreter are
• interactive the user has a dialog with the
  computer
• background processing time consuming tasks can
  be run in the background
• i/o redirection program i/o can be redirected
  from/to other files, the terminal, the printer,
  etc.
• simple programs can be piped together to perform
  more complex operations
• wild-card matching a pattern can be used to
  select files for processing so that a number of
  files can be processed with a single command
• shell scripts commonly used sequences of shell
  command can be stred in files and executed.
• shell variables can be used to control the shell
  and other programs

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