Scripts

Perl is a script language, which is compiled each time before running.
That unix knows that it is a perl script there must be the following header
at the topline of every perl script: #!/usr/bin/perl
where the path to perl has to be correct and the line must not exeed
32 charachters.

Comments and Commands

After the header line: #!/usr/bin/perl there
are either empty lines with no effect or commandlines or commentary lines.
Everything from and behind a "#" up
to the end of the line is comment and has no effect on the program. Commands
start with the first non space charachter on a line and end with a ";".
So one can continue a command over several lines and terminates it only
with the semicolon.

Direct commands and soubroutines

Normal commands are executed in the order written in the script. But soubroutines
can be placed anywhere and will only be evaluated when called from a normal
commandline. Perl knows it's a soubroutine if it the code is preceeded
with a "sub" and enclosed in a block like: sub
name { command;}

Other special lines

Perl can include other programming code with: require
something or with use something.

Quotations

Single quote: '' or: q//

Double quote: "" or: qq//

Quote for execution: `` or: qx//

Quote a list of words: ('term1','term2','term3')
or: qw/term1 term2 term3/

Quote a quoted string: qq/"$name" is $name/;

Quote something wich contains "/": qq!/usr/bin/$file
is readdy!;

Scalar and list context

That perl distinguishes between scalar and list context is the big feature,
which makes it uniqe and more usful then most other script languages.

A soubroutine can return lists and not only scalars like in C. Or an array
gives the number of elements in a scalar context and the elements itself
in a list context.

The enormous value of that feature should be evident.

Variables and Operators

General

There are scalar variables, one and two dimensional arrays and associative
arrays. Instead of declaring a variable one preceeds it with a spcial charachter.
$variable is a normal scalar variable.
@variable is an array and %variable

is an associative array. The user of perl does not have to distinguish
between a number and a string in a variable. Perl switches the type if
neccessary.

Scalars

Fill in a scalar with: $price = 300; $name = "JOHN";
Calculate with it like: $price *= 2; $price
= $oldprice * 4; $count++; $worth--; Print out the value of
a scalar with: print $price,"\n";

Arrays

Fill in a value: $arr[0] = "Fred"; $arr[1] = "John";
Print out this array: print join(' ',@arr),"\n";

If two dimensional: $arr[0][0] = 5; $arr[0][1]
= 7;

Hashes (Associative Arrays)

Fill in a single element with: $hash{'fred'} =
"USA"; $hash{'john'} = "CANADA";

Fill in the entire hash:

%a = (
   'r1',  'this is val of r1',
   'r2',  'this is val of r2',
   'r3',  'this is val of r3',
);

or with:

%a = (

   r1 => 'this is val of r1',
   r2 => 'this is val of r2',
   r3 => 'this is val of r3',
);

Assignements

Put something into a variable with a "="
or with some combined operator which assignes and and does something at
the same time:

$var = "string"; Puts the string into
$var

$var = 5; Puts a number into $var

$var .= "string"; Appends string to
$var

$var += 5; Adds number to $var

$var *= 5; Multipliy with 5

$var ||= 5; If $var is 0 make it 5

$var x= 3; Make $var to three times
$var as string: from a to aaa

Modify and assigne with:

($new = $old) =~ s/pattern/replacement/;

Comparisons

Compare strings with: eq ne like
in: $name eq "mary".

Compare numbers with: == != >= <= <=>

like in: $price == 400.

And/Or/Not

Acct on success or failure of an expression: $yes
or die; means exit if $yes is not set.

For AND we have: && and "and"
and for OR we have: || or "or".
Not is "!" or "not".

AND,OR and NOT are regularly used in if() statements:

if($first && $second){....;}

if($first || $second){....;}

if($first && ! $second{....;}
means that $first must be non zero but $second must not be so.

But many NOT's can be handled more reasonable with the unless() statement.
Instead:

print if ! $noway; one uses: print
unless $noway;

Branching

if

if(condition){
   command;
}elsif(condition){
   command;
}else{
   command;
}

command if condition;

unless (just the opposite of if)

unless(condition){
   command;
}else{
   command;
}

command unless condition;

Looping

while

while(condition){
   command;
}


# Go prematurely to the next iteration
while(condition){
   command;
   next if condition;
   command;
}

# Prematureley abort the loop with last
while(condition){
   command;
   last if condition;
}

# Prematureley continue the loop but do continue{} in any case
while(condition){
   command;

   continue if condition;
   command;
}continue{
   command;
}

# Redo the loop without evaluating while(condtion)
while(condtion){
   command;
   redo if condition;
}

command while condition;

until (just the opposite of while)

until(condition){
   command;
}

until(condition){
   command;
   next if condition;
   command;
}

until(condition){
   command;
   last if condition;
}

until(condition){
   command;
   continue if condition;

   command;
}continue{
   command;
}

command until condtion;

for (=foreach)

# Iterate over @data and have each value in $_
for(@data){
   print $_,"\n";
}

# Get each value into $info iteratively
for $info (@data){
   print $info,"\n";
}


# Iterate over a range of numbers
for $num (1..100){
   next if $num % 2;
   print $num,"\n";
}

# Eternal loop with (;;)
for (;;){
   $num++;
   last if $num > 100;
}

map

# syntax
map (command,list);
map {comm1;comm2;comm3;} list;

# example
map (rename($_,lc($_),<*>);

File Test Operators

File test operators check for the status of a file: Some examples:

-f $file	It's a plain file
-d $file	It's a directory
-r $file	Readable file
-x $file	Executable file
-w $file	Writable file
-o $file	We are owner
-l $file	File is a link
-e $file	File exists
-z $file	File has zero size, but exists
-s $file	File is greater than zero
-t FILEHANDLE	This filehandle is connetcted to a tty
-T $file	Textfile
-B $file	Binary file
-M $file	Returns the day number of last modification time

Regular Expressions

What it is

A regular expression is an abstract formulation of a string. Usually one
has a search pattern and a match which is the found string. There is also
a replacement for the match, if a substitution is made.

Patterns

A pattern stands for either one, any number, several, a particular number
or none cases of a character or a charachter-set given literaly, abstractly
or octaly.

PATTERN	MATCH
.	any character (dot)
.*	any number on any character (dot asterix)
a*	the maximum of consecutive a's
*a?**	the minimum of consecutive a's
.?	one or none of any characters
.+	one or more of any character
.{3,7}	three up to seven of any characters, but as many as possible
.{3,7}?	three up to seven, but the fewest number possible
.{3,}	at least 3 of any charachter
.{3}	exactly 3 times any character
[ab]	a or b
[^ab]	not a and also not b
[a-z]	any of a through z
^a \Aa	a at begining of string
a$ a\Z	a at end of string
A\|bb\|CCC	A or bb or CCC
tele(f\|ph)one	telefone or telephone
\w	A-Z or a-z or _
\W	none of the above
\d	0-9
\D	none of 0-9
\s	space or \t or \n (white space)
\S	non space
\t	tabulator
\n	newline
\r	carridge return
\b	word boundary
\bkey	matches key but not housekey
(?#.......)	Comment
(?i)	Case insensitive match. This can be inside a pattern variable.
(?:a\|b\|c)	a or b or c, but without string in $n
(?=.....)	Match ..... but do not store in $&
(?!.....)	Anything but ..... and do not store in $&

Substitututions

One can replace found matches with a replacement with the s/pattern/replacement/;
statement.

The "s" is the command. Then there follow three delimiters with first a
search pattern and second a replacement between them. If there are "/"
withing the pattern or the replacement then one chooses another delimiter
than "/" for instance a "!".

To change the content of a variable do: $var =~
s/pattern/replacement/;

To put the changed value into another variable, without distorting the
original variable do:

($name = $line) =~ s/^(\w+).*$/$1/;

COMMAND	WHAT it DOES
s/A/B/;	substitute the first a in a string with B
s/A/B/g;	substitute every a with a B
s/A+/A/g;	substitute any number of a with one A
s/^#//;	substitute a leading # with nothing. i.e remove it
s/^/#/;	prepend a # to the string
s/A(\d+)/B$1/g;	substitute a followed by a number with b followed by the same number
*s/(\d+)/$13/e;**	substitute the found number with 3 times it's value
Use two "e" for to get an eval effect:
perl -e '$aa = 4; $bb = '$aa'; $bb =~ s/(\$\w+)/$1/ee; print $bb,"\n";'
s/here goes date/$date/g;	substitute "here goes date" with the value of $date
s/(Masumi) (Nakatomi)/$2 $1/g;	switch the two terms
s/\000//g;	remove null charachters
s/$/\033/;	append a ^M to make it readable for dos

Input and Output

Output a value from a variable

print $var,"\n";

Output a formated string

printf("%-20s%10d",$user,$wage);

Read in a value into a variable and remove the newline

chomp() (perl5) removes a newline if one is there. The chop() (perl4) removes
any last character.

chomp($var = <STDIN>);

Read in a file an process it linewise

open(IN,"<filename") || die "Cannot open filename for input\n";
while(<IN>){

   command;
}
close IN;

Read a file into an array

open(AAA,"<infile") || die "Cannot open infile\n";
@bigarray = <AAA>;
close AAA;

Output into a file

open(OUT,">file") || die "Cannot oben file for output\n";
while(condition){
   print OUT $mystuff;
}
close OUT;

Check, whether open file would yield something (eof)

open(IN,"<file") || die "Cannot open file\n";
if(eof(IN)){
   print "File is empty\n";
}else{
   while(<IN>){
       print;

   }
}
close IN;

Process Files mentioned on the Commandline

The empty filehandle "<>" reads in each file iteratively. The name of
the current processed file is in $ARGV. For example print each line of
several files prepended with it's filename:

while(<>){
   $file = $ARGV;

   print $file,"\t",$_;
   open(IN,"<$file") or warn "Cannot open $file\n";
   ....commands for this file....
   close(IN);
}

Get Filenames

Get current directory at once

@dir = <*>;

Use current directory iteratively

while(<*>){

   ...commands...
}

Select files with <>

@files = </longpath/*.c>;

Select files with glob()

This is the official way of globbing:

@files = glob("$mypatch/*$suffix");

Readdir()

Perl can also read a directory itself, without a globbing shell. This is
faster and more controllable, but one has to use opendir() and closedir().

opendir(DIR,".") or die "Cannot open dir.\n";
while(readdir DIR){
   rename $_,lc($_);
}
closedir(DIR);

Pipe Input and Output from/to Unix
Commands

Process Data from a Unix Pipe

open(IN,"unixcommand|") || die "Could not execute unixcommand\n";
while(<IN>){
   command;
}
close IN;

Output Data into a Unix Pipe

open(OUT,"|more") || die "Could not open the pipe to more\n";
for $name (@names){
   $length = length($name);
   print OUT "The name $name consists of $lenght characters\n";
}
close OUT;

Execute Unix Commands

Execute a Unix Command and forget about the Output

system("someprog -auexe -fv $filename");

Execute a Unix Command an store the Output into a Variable

If it's just one line or a string:

chomp($date = qx!/usr/bin/date!); The
chomp() (perl5) removes the trailing "\n". $date gets the date.

If it gives a series of lines one put's the output into an array:

chomp(@alllines = qx!/usr/bin/who!);

Replace the whole perl program by a unix program

exec anotherprog; But then the perl
program is gone.

The Perl builtin Functions

String Functions

Get all upper case with:	$name = uc($name);
Get only first letter uppercase:	$name = ucfirst($name);
Get all lowercase:	$name = lc($name);
Get only first letter lowercase:	$name = lcfirst($name);
Get the length of a string:	$size = length($string);
Extract 5-th to 10-th characters from a string:	$part = substr($whole,4,5);
Remove line ending:	chomp($var);
Remove last character:	chop($var);
Crypt a string:	$code = crypt($word,$salt);
Execute a string as perl code:	eval $var;
Show position of substring in string:	$pos = index($string,$substring);
Show position of last substring in string:	$pos = rindex($string,$substring);
Quote all metacharachters:	$quote = quotemeta($string);

Array Functions

Get expressions for which a command returned true:	@found = grep(/[Jj]ohn/,@users);
Applay a command to each element of an array:	@new = map(lc($_),@start);
Put all array elements into a single string:	$string = join(' ',@arr);
Split a string and make an array out of it:	@data = split(/&/,$ENV{'QUERY_STRING'};
Sort an array:	sort(@salery);
Reverse an array:	reverse(@salery);
Get the keys of a hash(associative array):	keys(%hash);
Get the values of a hash:	values(%hash);
Get key and value of a hash iteratively:	each(%hash);
Delete an array:	@arr = ();
Delete an element of a hash:	delete $hash{$key};
Check for a hash key:	if(exists $hash{$key}){;}
Check wether a hash has elements:	scalar %hash;
Cut of last element of an array and return it:	$last = pop(@IQ_list);
Cut of first element of an array and return it:	$first = shift(@topguy);
Append an array element at the end:	push(@waiting,$name);
Prepend an array element to the front:	unshift(@nowait,$name);
Remove first 2 chars an replace them with $var:	splice(@arr,0,2,$var);
Get the number of elements of an array:	scalar @arr;
Get the last index of an array:	$lastindex = $#arr;

File Functions

Open a file for input:	open(IN,"</path/file") \|\| die "Cannot open file\n";
Open a file for output:	open(OUT,">/path/file") \|\| die "Cannot open file\n";
Open for appending:	open(OUT,">>$file") \|\| &myerr("Couldn't open $file");
Close a file:	close OUT;
Set permissions:	chmod 0755, $file;
Delete a file:	unlink $file;
Rename a file:	rename $file, $newname;
Make a hard link:	link $existing_file, $link_name;
Make a symbolic link:	symlink $existing_file, $link_name;
Make a directory:	mkdir $dirname, 0755;
Delete a directory:	rmdir $dirname;
Reduce a file's size:	truncate $file, $size;
Change owner- and group-ID:	chown $uid, $gid;
Find the real file of a symlink:	$file = readlink $linkfile;
Get all the file infos:	@stat = stat $file;

Conversions Functions

Number to character:	chr $num;
Charachter to number:	ord($char);
Hex to decimal:	hex(0x4F);
Octal to decimal:	oct(0700);
Get localtime from time:	localtime(time);
Get greenwich meantime:	gmtime(time);
Pack variables into string:	$string = pack("C4",split(/\./,$IP));
Unpack the above string:	@arr = unpack("C4",$string);

Subroutines (=functions in C++)

Define a Subroutine

sub mysub {
   command;
}

Example:

sub myerr {
   print "The following error occured:\n";
   print $_[0],"\n";
   &cleanup;

   exit(1);
}

Call a Subroutine

&mysub;

Give Arguments to a Subroutine

&mysub(@data);

Receive Arguments in the Subroutine

As global variables:

sub mysub {
   @myarr = @_;
}

sub mysub {
   ($dat1,$dat2,$dat3) = @_;
}

As local variables:

sub mysub {
   local($dat1,$dat2,$dat3) = @_;
}

Some of the Special Variables

*SYNTAX*	*MEANING*
$_	String from current loop. e.g. for(@arr){ $field = $_ . " ok"; }
$.	Line number from current file processed with: while(<XX>){
$0	Programname
$$	Process id of current program
$<	The real uid of current program
$>	Effektive uid of current program
$\|	For flushing output: select XXX; $\| = 1;
$&	The match of the last patternsearch
$1....	The ()-embraced matches of the last patternsearch
$`	The string to the left of the last match
$'	The string to the right of the last match

Forking

Forking is very easy! Just fork. One puts the fork in a three way if(){}
to separately the parent, the child and the error.

if($pid = fork){
   # Parent
   command;
}elsif($pid == 0){
   # Child
   command;
   # The child must end with an exit!!
   exit;
}else{
   # Error
   die "Fork did not work\n";
}

Building Pipes for forked Children

Building a Pipe

pipe(READHANDLE,WRITEHANDLE);

Flushing the Pipe

select(WRITEHANDLE); $| = 1; select(STDOUT);

Setting up two Pipes between the Parent and a Child

pipe(FROMCHILD,TOCHILD);  select(TOCHILD); $| = 1; select(STDOUT);
pipe(FROMPARENT,TOPARENT);select(TOPARENT);$| = 1; select(STDOUT);

if($pid = fork){
   # Parent

   close FROMPARENT;
   close TOPARENT;
   command;
}elsif($pid == 0){
   # Child
   close FROMCHILD;
   close TOCHILD;
   command;
   exit;
}else{
   # Error

   command;
   exit;
}

Building a Socket Connection to another
Computer

# Somwhere at the beginning of the script
require 5.002;
use Socket;
use sigtrap;


# Prepare infos
$port   = 80;
$remote = 'remotehost.domain';
$iaddr  = inet_aton($remote);
$paddr  = sockaddr_in($port,$iaddr);

# Socket
socket(S,AF_INET,SOCK_STREAM,$proto) or die $!;

# Flush socket
select(S); $| = 1; select(STDOUT);

# Connect
connect(S,$paddr) or die $!;

# Print to socket
print S "something\n";

# Read from socket

$gotit = <S>;

# Or read a single character only
read(S,$char,1);

# Close the socket
close(S);

Get Unix User and Network Informations

Get the password entry for a particular user with: @entry
= getpwnam("$user");

Or with bye user ID: @entry = getpwuid("$UID");

One can informations for group, host, network, services, protocols in the
above way with the commands: getgrnam, getgrid, gethostbyname,
gethostbyaddr, getnetbyname, getnetbyaddr, getservbyname, getservbyport,
getprotobyname, getprotobynumber.

If one wants to get all the entries of a particular categorie one can loop
through them by:

setpwent;

while(@he = getpwent){

commands...

}

entpwent;

For example: Get a list of all users with their homedirectories:

setpwent;
while(@he = getpwent){
   printf("%-20s%-30s\n",$he[0],$he[7]);
}
endpwent;

The same principle works for all the above data categories. But most of
them need a "stayopen" behind the set command.

Arithmetics

Addition: +

Subtraction: -

Multiplication: *

Division: /

Rise to the power of: **

Rise e to the pwoer of: exp()

Modulus: %

Square root: sqrt()

Absolut value: abs()

Tangens: atan2()

Sinus: sin()

Cosine: cos()

Random number: rand()

Formatting Output with "format"

This should be simplification of the printf formatting. One formats once
only and then it will be used for every write to a specified filehandle.
Prepare a format somwhere in the program:

format filehandle =

@<<<<<<<<<<@###.#####@>>>>>>>>>>@||||||||||

$var1, $var3, $var4

.

Now use write to print into that filhandle according to the format:

write FILEHANDLE;

The @<<< does left adjustment, the

@>>> right adjustment, @##.##
is for numericals and @||| centers.

Command line Switches

Show the version number of perl:	perl -v;
Check a new program without runing it:	perl -wc <file>;
Have an editing command on the command line:	perl -e 'command';
Automatically print while precessing lines:	perl -pe 'command' <file>;
Remove line endings and add them again:	perl -lpe 'command' <file>;
Edit a file in place:	perl -i -pe 'command' <file>;
Autosplit the lines while editing:	perl -a -e 'print if $F[3] =~ /ETH/;' <file>;
Have an input loop without printing:	perl -ne 'command' <file>;

Sunday, July 31, 2011

PERL Basics