This document provides an overview of the Unix operating system, including its design principles, programmer interface, shells, commands, file system, and shell programming. Key points include that Unix was designed to be a time-sharing system that supports multiple processes and simplicity, with a programmable shell and utilities for programmers. It also covers the various Unix shells, commands, features of the file system like hierarchy and security permissions, and programming constructs in shells like variables, conditionals, loops, filters and redirection.

1. Operating Systems
Part II: Introduction to
the Unix Operating
System
(Utilities and Shell
Programming)
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2. Unix Design Principles
 Time-sharing system
 Supports multiple processes
 Simplicity is key -> kernel provides small set of
functionality
 Source code provided with O/S
 Designed by programmers for programmers
(i.e. programmable shell, make, SCCS)
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3. Unix Design Principles
 Unix later used for networking, graphics, real-
time operation (not part of the original
programming objective)
 New functionality required large amount of
code (networking, GUI doubled the size of the
system)
 Continued strength -> even with new
development, system still remained Unix
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4. Programmer Interface
(The users)
shells and commands
compilers and interpreters
system libraries
system interface to the kernel
swapping demand paging
disk and tape drivers
virtual memory
kernel file system
page replacement
CPU scheduling signals
kernel interface to the hardware
hardware
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5. The Unix Shell
 Command interpreter
– Most common user interface
– Normally executes user-written and system
programs
– Called shell in Unix because it surrounds the kernel
 In Unix, users can write their own shell
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6. The Unix Shell
 Most popular Unix shells:
– Bourne shell (Steve Bourne, prompt is ‘$’)
– C shell (Bill Joy, most popular on BSD systems,
prompt is ‘%’)
– Korn shell (Dave Korn, became popular because it
combines features of C and Bourne shells, prompt
is ‘$’)
 Unix GUIs: X, OpenView, Motif
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7. The Unix Shell
 Users can create programs using shell scripts
(equivalent to batch files in MS-DOS)
 Most popular Unix shells are also programming
languages complete with variables and control
constructs (loops, conditional, etc.)
 Shell programming can be used to create
another shell
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8. Unix Commands
 Commands all have the same structure
 Case-sensitive! (cd <> CD, unlike in MS-DOS)
 The number of commands kept small but each
command is extended through
options/switches (preceded by ‘-’)
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9. Unix Commands
 Example
– Basic directory command: ls (lists all contents of a
directory)
– Extended command: ls -l (lists contents plus
other file information)
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10. The Unix File System
 Features of the Unix File System
– Hierarchical structure (similar to MS-DOS)
– Files are expandable (may grow are required)
– Security rights associated w/ each file/ directory
 Incorporates three-tiered structure on file access
 9 bits (User-Group-Others, rwxrwxrwx)
 Access types (r - Read, w - Write, x - Execute, ‘-’ means
no access)
-rwx---r-x 1 aam faculty 37 May 06 7:35 test
– Files may be shared (concurrent access)
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11. The Unix File System
 Features of Unix File System (continued)
– Various information kept for each file (i.e. name,
location, size, owner, security, modified by, last
access by, date & time stamp, etc.)
– File links - mechanism that allows multiple file
names to refer to the same data on the storage
device
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12. The Unix File System
 Most often used file system commands:
– pwd (present working directory)
– cd (change directory)
– mkdir (create directory)
– rmdir (remove directory)
– ls (list directory contents)
– cat (concatenate files) / more (pauses after screen
is full)
– cp (copy file)
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13. The Unix File System
 Most often used file system commands:
– vi (edit a file)
– mv (move a file)
– rm (remove file)
– ln (create link)
– chmod (change file access mode)
– chown (change file owner -> can be done only by
“root”/file owner
– cmp / diff (compare 2 files)
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14. Shell Basics
 Shell variables
– Similar to environment variables in MS-DOS
– Syntax: varname=value
– Preceded by $ when printing value (e.g.
echo $x, where x is a variable)
– Special variable PS1
Variable that contains string for command line
prompt
May be replaced
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15. Shell Basics
 Input/Output commands
– read - get input from keyboard and assign it to a
variable
– echo - send stream of characters to screen
– pr - send output to printer
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16. Shell Basics
 Command substitution
– The output is taken instead of the command itself
– Command is enclosed in `` (e.g. x=`date`)
– Common uses:
 Variable
assignment
 Command line arguments
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17. Shell Basics
 Wildcards
? Matches any single character except a leading ‘.’
(dot)
* Matches zero or more characters except a
leading dot
[] Defines a class of characters
- Defines an inclusive ranges (e.g. [0-9])
! Negates the defined class
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18. Shell Basics
 Quoting characters
Developed because of certain characters have special
meaning (e.g. $, *, <, >, #, etc.)
Removes meaning of the next character
’ Removes meaning of all characters
” Removes meaning of all characters except , $,
and ”
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19. Shell Basics
 Exercise for quoting characters
What’s the difference between...
echo ’$abc’
echo ”$abc”
echo $abc
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20. Shell Basics
 Redirection
Every time a shell is started, three files (devices) are
automatically opened:
– stdin (standard input) - device from w/c the program
reads the input (keyboard), file descriptor 0
– stdout (standard output) - device to w/c the
program writes the output (monitor), file descriptor 1
– stderr (standard error) - device to w/c the program
writes the errors (monitor), file descriptor 2
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21. Shell Basics
 Redirection (cont’d)
– Redirection - output is written to or read from
another file instead of the standard files/devices
– Syntax: command symbol filename
– Input redirection (<) - any command that reads its
input from stdin can have it read from a file
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22. Shell Basics
 Redirection (cont’d)
– Output redirection (> or >>) - any command that
writes its output to stdout can be redirected to
create, overwrite, or append to another file
– Error redirection (2> or 2>>) - error messages are
redirected to create, overwrite, or append to another
file
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23. Shell Basics
 Redirection (cont’d)
Special redirection commands:
2>&1 - redirects standard errors on same stream as
stdout
1>&2 - redirects outputs to stdout on same stream as
stderr
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24. Shell Basics
 Piping
– Sends output of one command as input of another
command
– Syntax: command1 | command2
– Eliminates temporary files using redirection
commands, for example
$ date > tmpfile
$ wc tmpfile
$ rm tmpfile
Can be written instead as date | wc
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25. Shell Basics
 Filters
– Program that reads input, performs translation,
produces output
– Commonly used filters:
 grep (looks for occurrences of words/phrases in files)
 sort (sorts input and writes to the output)
 sed (reads lines from input file, applies edit commands,
and outputs to stdout)
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26. Shell Basics
 Filters (cont’d)
– Commonly used filters
 awk
– designed by Al Aho, Peter Weinberger, and Brian Kernighan
– much more flexible and addresses limitations of sed
– allows programming constructs to be used (looping,
variables, arithmetic, etc.)
– processing language based on C
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27. Shell Programming
 Command line arguments
– Shellscript command parameters are represented
by $number
– Number indicates position of command argument
– Example:
myprog word1 bin
 $1 = word1
 $2 = bin
 $0 = myprog
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28. Shell Programming
 Command line arguments
– Other important shell variables related to command
parameters
$# the number of arguments
$* all arguments
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29. Shell Programming
 Decision statements
– IF command
if command
then
commands if condition is true
else
commands if condition is false
fi
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30. Shell Programming
 Decision statements
– CASE command
case word in
pattern) commands;;
pattern) commands;;
...
esac
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31. Shell Programming
 Looping constructs
– FOR loop
for var in list of words
do
loop body , $var set to successive elements of the list
done
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32. Shell Programming
 Looping constructs (cont’d)
– WHILE loop
while command
do
loop body executed as long as command is true
done
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33. Shell Programming
 Looping constructs (cont’d)
– UNTIL loop
until command
do
loop body executed as long as command is true
done
– The symbol ‘:’ is a shell built-in that does nothing
but return true
– The command true can also be used
– ‘:’ is more efficient since it is not a command
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