This document provides an overview of advanced shell scripting concepts in Unix. It discusses functions, string operators, select statements, shift, integer variables and arithmetic, arrays, typeset, local variables in functions, string formatting, exec, and print commands. Functions allow defining reusable blocks of code. String operators help check and set variable values. Select generates menus, shift shifts command line arguments. Typeset defines variable types and attributes. Arrays and integers add data structure capabilities to shell scripts.

1. UNIX
Advance Shell Scripting
Presentation By
Nihar R Paital

2. Functions
 A function is sort of a script-within a-script
 Functions improve the shell's programmability significantly
 To define a function, you can use either one of two forms:
function functname {
shell commands
}
or:
functname () {
shell commands
}
 to delete a function definition issue command
unset -f functname.
 To find out what functions are defined in your login session
functions
Nihar R Paital

3. String Operators
string operators let you do the following:
 Ensure that variables exist (i.e., are defined and have non-null
values)
 Set default values for variables
 Catch errors that result from variables not being set
 Remove portions of variables' values that match patterns
Nihar R Paital

4. Syntax of String Operators
Operator Substitution
${varname:-word} If varname exists and isn't null, return its value;
otherwise return word.
Purpose Returning a default value if the variable is
undefined.
Example:
$ count=20
$ echo ${count:-0} evaluates to 0 if count is undefined.
${varname:=word} If varname exists and isn't null, return its
value; otherwise set it to word and then return
its value
Purpose: Setting a variable to a default value if it is
undefined.
Example:
$ count=
$ echo ${count:=0} sets count to 0 if it is undefined. R Paital
Nihar

5. Syntax of String Operators (Contd)
${varname:?message} If varname exists and isn't null, return its
value;otherwise print varname: followed
by message, and abort the current
command or script. Omitting message
produces the default message parameter
null or not set.
Purpose: Catching errors that result from variables
being undefined.
Example:
$ count=
$ echo ${count:?" undefined!" } prints "count: undefined!"
if count is undefined.
${varname:+word} If varname exists and isn't null, return word;
otherwise return null.
Purpose: Testing for the existence of a variable.
Example:
$ count=30
$ echo ${count:+1} returns 1 (which could mean "true") if
count is defined. Else nothing will be displayed.
Nihar R Paital

6. select
 select allows you to generate simple menus easily
 Syntax:-
select name [in list]
do
statements that can use $name...
done
what select does:
 Generates a menu of each item in list, formatted with numbers
for each choice
 Prompts the user for a number
 Stores the selected choice in the variable name and the
selected number in the built-in variable REPLY
 Executes the statements in the body
Nihar R Paital
 Repeats the process forever

7. Example: select
PS3='Select an option and press Enter: '
select i in Date Host Users Quit
do
case $i in
Date) date;;
Host) hostname;;
Users) who;;
Quit) break;;
esac
done
When executed, this example will display the following:
1) Date
2) Host
3) Users
4) Quit
Select an option and press Enter:
If the user selects 1, the system date is displayed followed by the menu prompt:
1) Date
2) Host
3) Users
4) Quit
Select an option and press Enter: 1
Mon May 5 13:08:06 CDT 2003 Nihar R Paital
Select an option and press Enter:

8. shift
 Shift is used to shift position of positional parameter
 supply a numeric argument to shift, it will shift the arguments
that many times over
For example, shift 4 has the effect:
File :testshift.ksh
echo $1
shift 4
echo $1
Run the file as testshift.ksh as
$ testshift.ksh 10 20 30 40 50 60
Output:
10
50 Nihar R Paital

9. Integer Variables and Arithmetic
 The shell interprets words surrounded by $(( and )) as
arithmetic expressions. Variables in arithmetic expressions do
not need to be preceded by dollar signs
 Korn shell arithmetic expressions are equivalent to their
counterparts in the C language
 Table shows the arithmetic operators that are supported. There
is no need to backslash-escape them, because they are within
the $((...)) syntax.
 The assignment forms of these operators are also permitted.
For example, $((x += 2)) adds 2 to x and stores the result back
in x.
Nihar R Paital

10. A r it h m e t ic P r e c e d e n c e
 1. Expressions within parentheses are evaluated first.
 2. *, %, and / have greater precedence than + and -.
 3. Everything else is evaluated left-to-right.
Nihar R Paital

11. Arithmetic Operators
 Operator Meaning
 + Plus
 - Minus
 * Times
 / Division (with truncation)
 % Remainder
 << Bit-shift left
 >> Bit-shift right
Nihar R Paital

12. Relational Operators
 Operator Meaning
 < Less than
 > Greater than
 <= Less than or equal
 >= Greater than or equal
 == Equal
 != Not equal
 && Logical and
 || Logical or
Value 1 is for true and 0 for false
Ex:- $((3 > 2)) has the value 1
$(( (3 > 2) || (4 <= 1) )) also has the value 1
Nihar R Paital

13. Arithmetic Variables and Assignment
The ((...)) construct can also be used to define integer variables
and assign values to them. The statement:
(( intvar=expression ))
The shell provides a better equivalent: the built-in command
let.
let intvar=expression
There must not be any space on either side of the equal sign
(=).
Nihar R Paital

14. Arrays
 The two types of variables: character strings and integers. The
third type of variable the Korn shell supports is an array.
 Arrays in shell scripting are only one dimensional
 Arrays elements starts from 0 to max. 1024
 An array is like a list of things
 There are two ways to assign values to elements of an array.
 The first is
nicknames[2]=shell nicknames[3]=program
 The second way to assign values to an array is with a variant of the
set statement,
set -A aname val1 val2 val3 ...
creates the array aname (if it doesn't already exist) and assigns
val1 to aname[0] , val2 to aname[1] , etc.
Nihar R Paital

15. Array (Contd)
 To extract a value from an array, use the syntax
${aname [ i ]}.
 Ex:-
1) ${nicknames[2]} has the value “shell”
2) print "${nicknames[*]}",
O/p :- shell program
3) echo ${#x[*]}
to get length of an array
Note: In bash shell to define array,
X=(10 20 30 40)
To access it,
echo ${x[1]}
Nihar R Paital

16. Typeset
 The kinds of values that variables can hold is the typeset
command.
 typeset is used to specify the type of a variable (integer, string,
etc.);
 the basic syntax is:
typeset -o varname[=value]
 Options can be combined , multiple varnames can be used. If
you leave out varname, the shell prints a list of variables for
which the given option is turned on.
Nihar R Paital

17. Local Variables in Functions
 typeset without options has an important meaning: if a typeset
statement is inside a function definition, then the variables
involved all become local to that function
 you just want to declare a variable local to a function, use
typeset without any options.
Nihar R Paital

18. String Formatting Options
Typeset String Formatting Options
 Option Operation
 -Ln Left-justify. Remove leading blanks; if n is given,
fill with blanks or truncate on right to length n.
 -Rn Right-justify. Remove trailing blanks; if n is given,
fill with blanks or truncate on left to length n.
 -Zn Same as above, except add leading 0's instead of
blanks if needed.
 -l Convert letters to lowercase.
 -u Convert letters to uppercase.
Nihar R Paital

19. typeset String Formatting Options
 Ex:-
alpha=" aBcDeFgHiJkLmNoPqRsTuVwXyZ "
Statement Value of v
typeset -L v=$alpha "aBcDeFgHiJkLmNoPqRsTuVwXyZ “
typeset -L10 v=$alpha "aBcDeFgHiJ“
typeset -R v=$alpha " aBcDeFgHiJkLmNoPqRsTuVwXyZ“
typeset -R16 v=$alpha "kLmNoPqRsTuVwXyZ“
typeset -l v=$alpha " abcdefghijklmnopqrstuvwxyz“
typeset -uR5 v=$alpha "VWXYZ“
typeset -Z8 v="123.50“ "00123.50“
 A typeset -u undoes a typeset -l, and vice versa.
 A typeset -R undoes a typeset -L, and vice versa.
 typeset -Z has no effect if typeset -L has been used.
 to turn off typeset options type typeset +o, where o is the option you
turned on before
Nihar R Paital

20. Typeset Type and Attribute Options
 Option Operation
 -i Represent the variable internally as an integer; improves
efficiency of arithmetic.
 -r Make the variable read-only: forbid assignment to it and
disallow it from being unset.
 -x Export; same as export command.
 -f Refer to function names only
 Ex:-
typeset -r PATH
typeset -i x=5.
Typeset Function Options
 The -f option has various suboptions, all of which relate to functions
 Option Operation
 -f With no arguments, prints all function definitions.
 -f fname Prints the definition of function fname.
 +f Prints all function names. Nihar R Paital

21. Exec Command
 If we precede any unix command with exec , the
command overwrites the current process , often the
shell
 $ exec date
 Exec : To create additional file descriptors
 Exec can create several streams apart from
( 0,1,2) ,each with its own file descriptor.
 exec > xyz
 exec 3> abc
 Echo "hi how r u" 1>&3
 Standard output stream has to be reassigned to the
terminal exec >/dev/tty
Nihar R Paital

22. print
print escape sequences
print accepts a number of options, as well as several escape sequences that start with a
backslash
 Sequence Character printed
 a ALERT or [CTRL-G]
 b BACKSPACE or [CTRL-H]
 c Omit final NEWLINE
 f FORMFEED or [CTRL-L]
 n NEWLINE (not at end of command) or [CTRL-J]
 r RETURN (ENTER) or [CTRL-M]
 t TAB or [CTRL-I]
 v VERTICAL TAB or [CTRL-K]
 Single backslash
Options to print
Option Function
 -n Omit the final newline (same as the c escape sequence)
 -r Raw; ignore the escape sequences listed above
 -s Print to command history file
Ex:-
 print -s PATH=$PATH Nihar R Paital

23. Thank You!
Nihar R Paital