Programming in Unix

1

Programming in Unix


Control Structures


Filehandles and File Tests


Directory Operations


Manipulating Files and Directories


HTML

2

Control Structures


In an if control structure, the block of
code is executed only when the
conditional expression is true

if ($name=fred){ }


Unless is the opposite. Run the code
unless the expression is true

unless ($name=fred){ }

3

Control Structures


The
else

clause works with
unless

the
same way it does with an if statement

unless ($name=fred){


print “His name isn’t fred”;

} else {


print “His name is fred”;

}

4

Expression Modifiers


Perl offers some variations on how to write some
control structures

if ($name=“fred”){


print “His name is fred.
\
n”;

could be written as:

print “His name is fred.
\
n” if ($name eq“fred”);


The conditional expression is still evaluated first,
even though it’s written at the end


Perl is unable to handle multiple expression
modifiers i.e.; if something while something

5

Naked Block


A “naked” block is a control structure
without a keyword or condition, it just
executes the body of the loop once

while (condition) {


#
this works

body;

body;

}

{




#
this is a “naked” block

body;

body;

}

6

Nested if


Use
elsif
when you have a nested if statement

if ($name eq “fred”) {


print “His name is fred
\
n”;

} elsif ( $name eq “barney
\
n”) {


print “His name is barney
\
n”;

} else {


print “I don’t know his name
\
n”;

}

7

for Control Structure


The Perl
for

control structure works the same
was as it does in C. It is usually used for
making computed iterations

for ($i=1; $i <= 10; $i++) {


body;

}

Does the same as:

initialization;

while (test){

body;

increment;

}

8

for Control Structure


Perl will let you leave any of the three
control parts empty, but you still need the
semicolons


for ( ; ; ){

print “This will leave you in an infinite loop”;

}

9

for Control Structure


Another way to create an infinite loop is:


while (1) {


print “This is another infinite loop”;

}



Make sure you have a way to exit the loop;

10

foreach Control Structure


Perl sees for and foreach the same.


If you have the two semicolons then it’s a
computed for loop


If you don’t it’s a foreach loop

for (1..10) { # works like a foreach


print “I can count to $_
\
n”;



}

11

loop blocks


Perl is a structured programming
language


Which means there is only one entrance
to any block of code, which is the top of
that block


There are five kinds of loop blocks: for,
foreach, while, until, or the naked block

12

loop control operators


The
last Operator


Perl’s equivalent for getting out of a loop
early


If condition is true then
last

forces the
loop to break out of the innermost
enclosing loop block causing execution to
continue with the statement immediately
following the block (page 138)


13

loop control operators


The
next Operator


next

alters the ordinary sequence flow of
execution.


Next causes execution to skip past the
rest of the innermost enclosing looping
block without terminating the block

14

loop control operators


The
redo Statement


Causes a jump to the beginning of the
current block


Works well when you need something
initialized before the first test


Note: last, next and redo are used to
jump
out

of a block not
in

15

Logical Operators


&& and || as Control Structures

if (this) {that;}

#or

that if this;


#or

this && that;

#logical and


If
this

is true, then the value of the entire
expression is still not known, so

that

has to be
evaluated


If
this

is false, there’s no point in looking at
that


16

The Ternary Operator ?:


Works like an if
-
then
-
else test

expression ? if_true_expr : if_false_expr;

$location = &is_weekend($day) ? “home” : “work”;


You can nest it

my $size =


($width < 10) ? “small” :


($width < 20) ? “medium” :


($width < 50) ? “large” : “extra
-
large”;

17

Directory Operations


The working directory is inherited by all
processes that Perl starts


chdir changes the working directory


chdir (“/etc”) || die “cannot cd to /etc”;




18

Globbing


Is a method of expanding filename
patterns into the matching patterns


Globbing
-

*



@a = glob “*”; #equals all files in your directory


Or put the pattern between angle brackets
which is the old way (prior to version 5.6)

@a = </etc/host*>;

19

Directory Handles


Directory Handles are not the same as file
handles


Always opened read only


Value is true if directory can be opened,
false if it cannot


opendir (ETC,”/etc”) || die “Cannot opendir /etc:” ;


closedir (ETC);

20

Directory Operations


Reading a directory handle


Each invocation of readdir is a scalar
context returns the next filename


When no more to read returns undef

opendir (ETC, “/etc”)|| die “no etc?”;

while ($name = readdir (ETC) {


print “$name
\
n”;

}

closedir (ETC);

21

Manipulating Files and
Directories


To delete a file in Perl you use the unlink
operator



unlink “filename”;


To rename a file use rename



rename “oldname” “newname”;

22

Manipulating Files and
Directories


To create a directory use mkdir



mkdir “directoryname”;


or

mkdir “directoryname”, 0700; #octal


or

$name=“CS345”;



$permissions = “0755”; #needs



leading zero or becomes 1363 octal



mkdir $name, oct($permissions);

23

Manipulating Files and
Directories


To remove a directory use rmdir



rmdir “dir_name”;


To change file or directory permissions
use chmod



chmod 0755, “filename”;

24

Manipulating Files and
Directories


When working with a file it’s good to test the file
for certain characteristics such as does it exits,
or is it an ordinary file

$file = “data.txt”;


if (
-
e $file)

{

print “File exists
\
n”;

}

else

{


die “File does not exist
\
n”;

}

25

Test Operators



-A
Days since the file last accessed
-C
Days since the file’s
inode was
last changed
-d
1 if the directory exists
-e
1 if the file exists
-M
Days since the file was modified
-r
1 if the file is readable by the
owner of the Perl script
-s
Size of file in bytes
26

Using Perl to Generate a
Web Page


To create a Perl script that will generate a Web
page type



#!followed by the location of the Perl


interpreter


The source can be viewed but not the .pl code


To inform a Web browser that the script
contains HTML code type



print “Content
-
type: text/html
\
n
\
n”;

27

Using Perl to Generate a
Web Page


To enter HTML code type print ;



print “<html><head>”;


Repeat until you have entered all the Perl
and HTML code


Save the script to your Web server


To access type in the URL of the page in
the Web browser window.

28

Using Perl to Generate a
Web Page

#!/usr/local/bin/perl


print “Content
-
type: text/html
\
n
\
n”;


print “<html><head>”;

print “<title>A Perl Web Page</title>”;

print “</head><body>”;

print “<h2> A Perl generated Web page </h2>”;

print “Welcome”;

print “</body></html>”;

29

Using Perl to Generate a
Web Page


To create a Perl script that will generate a Web
page, you must make sure you create or save
the script in the correct directory


The directory used to store scripts is usually
named cgi
-
bin


Contact the system administrator to verify that
you are saving scripts in the correct directory


Make sure the directory, any subdirectories, and
your files have permissions to read and execute