Unix day2 v1.3

Tech Mahindra Limited confidential© Tech Mahindra Limited 2008
UNIX
Day 2

CONFIDENTIAL© Copyright 2008
2
Objectives
 At the end of this session, you will be able to:
 Understand regular expressions
 Understand the grep family of commands
 Understand UNIX Shell features and its environment
 Read/Write basic shell scripts using decision making constructs

3
Agenda: Day 2
 Regular Expressions and grep
 Unix Shell
 Unix Shell Environment
 Unix Shell Scripting

Regular Expressions and Grep

5
Regular Expression
 Often called a pattern, is an expression that describes a set
of strings
 Example,
a regular expression “amit” may match “amit”,
“amita”,”amitabh”, “namit” etc…
 Many UNIX tools, primarily grep,sed & awk make use of
regular expressions in text processing

6
Regular Expressions (contd.)
 Regular Expressions can be divided into:
 Basic regular expressions (BRE)
 Supported by grep
 Extended regular expressions (ERE)
 Supported by grep –E or egrep

7
Basic Regular Expression
Quote the next metacharacter
^ Match the beginning of the line
. Match any character
$ Match the end of the line
[] Character class
Special Operators

8
Extended Regular Expression
| Alternation
() Grouping
Special Operators

9
Examples
 “a.g” matches aag, abg, a1g, etc
 “a[pmt]g” matches apg, amg or atg
 “a[^pmt]g” matches aag, abg but not apg or amg or atg
 “^ftp” matches ftp at the beginning of a line
 “tle$” matches tle at the end of a line
 “^$” matches a line with nothing in it
 “jelly|cream” matches either jelly or cream
 “(eg|pe)gs” matches either eggs or pegs

10
Quantifiers: number of repeats of the previous
character
* Match 0 or more times
+ Match 1 or more times
? Match 1 or 0 times
BRE
ERE

11
Examples
 “adg*” ad followed by zero or more g characters
 “.*” Any character, any number of times
 “[qjk]” Either q or j or k
 “[^qjk]” Neither q nor j nor k
 “[a-z]” Anything from a to z inclusive
 “[^a-z]” No lower case letters
 “[a-zA-Z]” Any letter
 “[a-z]+” Any non-zero sequence of lower case letters
 “(da)+” Either da or dada or dadada or...

12
grep family
 fgrep: fast searching for fixed strings
 $fgrep string file(s)
 It handles fixed character strings as text patterns
 Does not use regular expressions
 Faster than grep and egrep for searching text strings
 Examples
fgrep “Ramesh” datalist
 If found, lists the line(s) containing Ramesh

13
grep family
 grep: is called as a global regular expression printer
 It searches for a given pattern in a file(s)
 $ grep -[cvnl] [pattern] [files]
Option Description
-c counts the total no of lines containing the pattern
-v displays all the lines not containing the pattern
-n displays lines with line number
-l displays file names containing the pattern
 Example:
grep “Agg*[ra][ra]wal” datalist
 It lists all lines where the pattern matches some text
 The possible matches for the text are:
Agrawal, Agarwal, Aggarwal, Aggrawal
(and many more combinations possible)

14
grep family
 egrep: extended grep, supports both BRE as well as ERE
 grep –E can also be used in the place of egrep
 Examples:
 $ egrep ‘ (John|Johnathon) Smith ‘ employee.txt
 This will search for John Smith as well as for Johnathon Smith
 grep –E “(S|Sh)arma datalist
 Matches Sarma or Sharma in the text from datalist

Unix Shell

16
Shell Features
 Command Prompt
 Command Interpretation and Shell Meta Characters
 I/O Redirection
 Pipes
 Shell Programming Constructs
 Job Control
 Command History

17
Shell as a Command Interpreter
Shell prints the prompt
on screen
User enters the
command
Shell interprets the
command
Shell waits till the
command finishes
execution

18
Popular Unix Shells
 Bourne Shell
 Korn Shell
 C Shell
 Bourne Again Shell

19
Bourne Shell
 The Bourne shell is the original UNIX shell program
 It is very widely used
 Invoked using sh or /bin/sh at the command prompt
 The Bourne shell supports conditional branching in the form
of if/then/else statements
 In addition, the Bourne shell supports case statements and
loops (for, while, and until)
 The Bourne shell uses the $ as a prompt

20
Korn Shell
 The Korn shell is a newer variation of the Bourne shell.
 It supports everything the Bourne shell does, and adds
features not available in the Bourne shell.
 Invoked using ksh or /bin/ksh at the command shell prompt.
 The Korn shell was originally written by David Korn and is
copyrighted by AT&T.
 The programming structure of the Korn shell is very similar
to that of the Bourne shell. The Korn shell, however, is more
interactive.

21
C Shell
 The C shell is a very commonly used shell
 Its programming structure closely resembles that of the
programming language "C."
 The C shell uses the "%" as a prompt
 The C shell supports all of the features that the Bourne shell
supports, and has a more natural syntax for programming
 The C shell is more interactive than the Bourne shell, with
additional features that are not available in older shells
 The configuration of the C shell is controlled by the .rc and
the .login files

22
Bourne-Again Shell
 The Bourne-Again shell is a variation of the Bourne shell
 It is commonly used in Linux, but is widely available in other
standard UNIX distributions
 The Bourne Again shell is another modification of the Bourne
shell, and uses the $ as a prompt
 To start the Bourne Again shell, type "bash" at the shell
prompt
 The behavior and environment of the Bourne Again shell is
controlled by the .bashrc file, which is a hidden file in your
home directory

Unix Shell Environment

24
Environmental Variables
 The Unix system is controlled by a number of shell variables
that are separately set by the system - some during boot
sequence, and some after logging in.
 These variables are called system variables or environment
variables.
 The set statement displays the complete list of all these
variables. These built-in variable names are defined in
uppercase.

25
Environmental Variables
 PATH: is a variable that instructs the shell about the route
it should follow to locate any executable command
 HOME: when you log in, UNIX normally places you in a
directory named after your login name
 MAIL: determines where all incoming mail addressed to the
user is to be stored
 PS1 and PS2: PS1 - your command prompt and PS2-Multi-
line command string
 SHELL: determines the type of shell that a user sees on
logging in
Note: There are other environmental variables also but we have discussed
the important ones.

26
Profile
 .profile (.bash_profile in Linux): the script executed during
login time
 The .profile must be located in your home directory, and it is
executed after /etc/profile, the universal profile for all users
 Universal environment settings are kept by the
administrator in /etc/profile so that they are available to all
users

27
Setting Environment Variables
 Assigning a value to a variable will set an environment
variable temporarily
 For example:
$ x=50
This example will set value 50 to the variable x

28
Setting Environment Variables
 The variable will not be available if you exit the shell. Add
this variable to configuration files e.g. “.profile” to set it
permanently
 If you no longer want a variable to be set, you can use the
unset command to erase its value
 For example:
$ unset x
This command will cause x to have no value set

29
export
 By default, the values stored in shell variables are local to
the shell, i.e., they are available only in the shell in which
they are defined. They are not passed on to a child shell.
 But the shell can also export those variables recursively to
all child processes so that, once defined, they are available
globally. This is done with the export command.
 $ x=hello
 $ export x
 $ sh
 $ echo $x
 hello

30
. (dot): Running commands from file
 If we have a list of commands in a file “sample”, we can execute it
using command:
 $ . sample
 It is like executing a shell script, but with following difference:
 Standard shell scripts cause a new sub shell to be created to
run the script.
 The dot command uses the same shell to execute the
commands.
 The dot command, however, creates no child process, so
any changes it produces apply to the original shell
 It also does not require the script to have executable
permission

Unix Shell Scripting

32
Shell Scripts
 Scripts are mostly written for developing interfaces viz. interfaces
to application servers or to database servers.
 Scripts can be client side scripts like JavaScript, VBScript etc. or
server side scripts like shell scripts, perl scripts etc.
 When a group of Unix commands has to be executed regularly, it is
stored in a file. All such files are called as shell scripts.
 There is no restrictions on extension of these files, but
conventionally extension .sh is used for a shell script.
 Unlike compiled programs, shell scripts are interpreted at run time.
 You can use the vi editor to create/edit the shell script.

33
Shell Scripts
 Shell scripting language provides following:
 Scalar and array variables
 Set of operators
 Flow, Loop and case statements
 Positional Parameters
 here document
 Functions

34
Executing shell script
 You can execute the shell scripts using either command sh
or by just typing shell script name at the prompt (make sure
that you have execute permission)
 $ sh test.sh
 $ test.sh
 $ ./test.sh

35
Shell Scripting
 User-created Shell Variables:
variable=value => assigns value to variable
$variable => refers value of the variable
To display a variable:
echo $var “$var” ‘$var’
the output:
hello hello $var

36
Shell Scripting
 read statement
To read a value in a variable form keyboard:
read var
To read a value in a variable from a file:
read var < file1
It reads the first line from the file into variable var
read var1 var2 < file1
It reads the first word into var1 and second word into var2 from file1

37
Shell Scripting
 expr command
This command is used to evaluates expressions
e.g.
expr 10 + 20
 Operators
 Arithmetic:
+ Add
- Subtract
/ Divide
* Multiply
% Modulo

38
Shell Meta Characters
 Wildcard substitution: * ? []
 Redirection: >, >>, <, 2>, <<
 Piping: |
 Command substitution: ` `
 Sequential commands: semicolon (;)
 Inserting comment: #
 Command grouping: () parenthesis

39
test command
 test command:
 used to conduct several tests on integers, strings, file
attributes
 It produces no output so used with if/while where its exit
status is used
 Examples:
test 15 –gt 10
This command compares 15 and 10 and gives a true exit status
but does not produce any output.

40
test command
 Comparing numbers:
 Operators:
 Relational
-lt less than
-le less than equal to
-gt greater than
-ge greater than equal to
–eq equal to
–ne not equal to
 Logical:
-a And
-o OR
! NOT

41
Test Command - Strings
 String comparison used by the command test:
String Comparison True if
string1 = string2 strings equal
string1 != string2 strings not
equal
-n string string not null
-z string string is null

42
Test Command - Files
 File tests used by the command test
Test True if
-d file file exists and is a directory
-e file file exists
-f file file exists and is a regular file
-r file file exists and is a readable
-s file file exists and has a size > 0
-w file file exists and is a writable
-x file file exists and is a executable

43
Conditional Statement: if-then-else
 The if statement takes two-way decisions depending on the condition
if condition
then
commands
fi
if condition
then
commands
else
commands
fi
if condition
then
commands
else if condition
then
commands
fi
fi
if condition
then
commands
elif condition
then
commands
else
commands
fi

44
Case Statement
 The statement matches an expression for more than one
alternative, and permits multi-way branching
case variable/expression/value in
value1) command1
command2
;;
value2) command3
;;
*) command4
esac

45
Example of Case Statement
echo “Enter the color”
read color
case $color in
Red | red) echo “You have selected red color”
;;
Blue | blue) echo “You have selected blue
color”
;;
*) echo “Sorry! Yet to add this color”
;;
esac

46
Positional Parameters (command line
arguments)
 One can pass the command line arguments to shell script
while execution
 When arguments are specified with a shell script, they are
assigned to variables called positional parameters
 The first argument is read by the shell into the parameter
$1, the second into the parameter $2, and so on
 The $# represents total number of arguments passed to the
script
 The command is assigned to a variable $0
 You can use these variables up to $9

47
Positional Parameters (command line
arguments)
 The $* indicates all arguments, in a single variable,
separated by the first character in the environment variable
IFS
 The $@ is same as $* except when enclosed in double
quotes
 The “$@” works with string input

48
set
 set command assigns values to positional parameters:
$ set 23 532
 The command assigns value 23 to the positional
parameter $1, and 532 to $2
 It also sets $#, $*

49
shift
 shift command shifts command line arguments to left
 The shift command copies the contents of a positional parameter
to its immediate lower numbered positional parameter. When
called once, contents of $2 are copied to $1, $3 to $2 and so on.
$ shift 2
 The command does two shifts i.e. $1=$3, $2=$4, and so on.
 Using shift command we can pass more than 9 command line
parameters to shell script

50
Summary
In this session, we have covered:
 Regular Expressions and grep
 Unix Shell
 Unix Shell Environment
 Unix Shell Scripting

Thank You

Unix day2 v1.3

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