UNIX - Class5 - Advance Shell Scripting-P2

UNIX

Advance Shell Scripting

Presentation By

Nihar R Paital

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

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

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

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

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

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:

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

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

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

Arithmetic Operators

 Operator Meaning
 + Plus
 - Minus
 * Times
 / Division (with truncation)
 % Remainder
 << Bit-shift left
 >> Bit-shift right

Nihar R Paital

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

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

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

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

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

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

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

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

Typeset Type and Attribute Options
 -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
 -f With no arguments, prints all function definitions.
 -f fname Prints the definition of function fname.
 +f Prints all function names. Nihar R Paital

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

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

Thank You!

Nihar R Paital

UNIX - Class5 - Advance Shell Scripting-P2

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