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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.
UNIX –more history
• The uni- part refers to unix being developed for single users originally.
• Unix’s 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
DEC’s 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.
Key Features
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Portability
Portable applications sw
Multiuser operation (it can support multiuser mainframe environments)
Background processing
Hierarchical file system
UNIX shell- a command interpreter
Pipes- allows several prgrams to be combined so that the output from one
becomes input to the next
Utilities
Text-processing tools
Sw development tools
maturity
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.
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
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
Date/time &security I/o svces
svces
kernel Process scheduling
Ints and
Error
System
Memory
handling
accounti mgmt
ng
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.
Call:
Request service:
Type
Details
data
User program
wants service
Return:
Service complete
Status
data
UNIX kernel
provides services
The file system: features
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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.
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
– People 1122
– Data
0123
• Files:
- XYZ
file 0123 file 0011 file 1122
1122
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.
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
directories
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/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
Hierarchy chart: / is the root
/
bin
dev
etc
usr
higgins
bin
lib
tmp
tmp
File management utilities
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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
examples
$ pwd
/usr/higgins/courses/csci201
$ cd /usr/higgins/courses/csci311
$ ls /usr/higgins/courses/csci201
assignments
sampletests
classlist
$cat classlist
bob
joe
sue
mary
rudolf
xenophon
show present working directory
change to directory specified
list directory contents
show file contents on terminal
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
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.
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.
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