Script Programming Systems Skills in C and Unix

Transcription

1 Script Programming with Perl II Systems Skills in C and Unix

2 Subroutines sub sum { return $a + $b; } So we can call this as: $a = 12; $b = 10; $sum = sum(); print the sum is $sum\n ;

3 Passing Arguments Passing Arguments A perlsubroutine can be called with a list in parenthesis. Example: sub add { $tmp= 0; # this defines a global variable foreach$_ (@_){ $tmp+= $_; } return $tmp; }

4 Local variables Local variables Perl subroutines can define local private variables. Example sub product { my ($x);# defines the local variable x foreach$_ (@_) { $x *= $_;} return $x; } You can have a list of local variables by simply expanding the list as: my ($x,

5 Command line arguments Command Line Arguments in Perl A Perl program can take command line arguments. One or more command line arguments can be passed when calling a perlprogram. perl program.pl infile.txt outfile.txt The number of command line arguments is given by $#ARGV + 1 and command line arguments are named $ARGV[0], $ARGV[1], etc

6 LWP Library for www in Perl LWP contains a collection of Perl modules use LWP::Simple; $_ = get($url); print $_; Good reference at wp.html

7 Getopt The Getopt::Long module implements an extended getopt function called GetOptions(). Command line arguments are given as -n 20 or num 20 -n 20 -t test use Getopt::Long; $images_to_get = 20; $directory = "."; GetOptions("n=i" => \$images_to_get,"t=s" => \$directory); References:

8 Hashes or associative arrays %hash = (); # initializes a hash to empty set. We can add elements later $hash{ guna } = aa ; $hash{ neil } = ab ; $hash{ george }= ac ;

9 each function Each function allows us to extract both value and key from a hash table example %table = { guna, 10, me, 20}; while (($key, $value) = each(%table) ){ print "$key => $value\n"; }

10 References Creating a Reference to a scalar $num = 10; $numref = \$num; Creating a Reference to an = (guna, me, neil); $arrayref = \@array; Creating a Reference to a hashtable %hash = {guna, aa, me, bb, him, cc}; $hashref = \%hash;

11 Dereferences Dereferencing a Reference to a scalar $num = 10; $numref = \$num; print $$numref; # prints the value 10 Dereferencing a Reference to an array using -> = (guna, me, neil); $arrayref = \@array; print $arrayref->[0]; # prints guna Dereferencing a Reference to an array using -> [guna, me, [blue, red]]; $arrayref = \@array; print $arrayref->[2][1]; # prints red

12 Systems programming opendir(dir,"."); foreach $file (readdir(dir)){ print "$file\n"; } close(dir);

13 Examples print "which directory to change to :"; chomp($dir = <STDIN>); if (chdir $dir){ print "we are now in $dir \n"; } else { print "we could not change to $dir \n"; }

14 Modifying Permissions foreach $file ( guna.c, temp.o ){ unless chmod(o666, $file) { warn could not chmodthe file $file \n ; } }

15 Renaming a file Rename($file1, $file2); Exercise: Write a perlscript that will take a folder as command line argument and rename all.txt files to.htmfiles

16 Copying a file use File:Copy; copy($file1, $file2); Exercise: Write a perlscript that will create a duplicate folder given a folder. Name the new folder, dup_folder

17 Running a perlscript from another #!/usr/local/bin/perl system 'perl mkdir.pl file.txt';

18 Encoding pages Example PAGE: for ($page=1; ; $page++){ $url = } last PAGE if ($count == $maxcount);

19 Shell Programming Systems Skills in C and Unix

20 The Shell A command line interpreter that provides the interface to Unix OS.

21 What Shell are we on? echo $SHELL Most unix systems have Bourne shell (sh) No command history Kornshell (ksh) Shell functions C shell (csh) History, no shell functions More details at unix.com

22 A Shell Script #!/bin/sh --above line should always be the first line in your script # A simple script who am I Date Execute with: sh first.sh

23 Things to do with shell scripts Remove all empty folders Remove all duplicate lines from a file Send if the assignment is not submitted Check output of a submitted program against sample output Given a roster file, extract ID s and create folders for each person Rename a folder that contains.txt files to a folder that contains all.htm files

24 Variables in shell System variables $SHELL $LOGNAME $PWD User defined variables name=guna echo $name

25 echo echo [options] [string, variables...] Options -n Do not output the trailing new line. -e enable interpretation escaped special characters \a alert (bell) \b backspace \c suppress trailing new line \n new line \r carriage return \t horizontal tab \\ backslash

26 Shell Variables echo $PATH an environment variable Environment variables can be changed PATH=$PATH:/usr/local/apache/bin:. Examples dir=pwd echo $dir subdir= lab1 abspath=$dir/$subdir

27 Command Line Arguments $# -represents the total number of arguments (much like argv) except command $0 -represents the name of the script, as invoked $1, $2, $3,.., $8, $9 -The first 9 command line arguments $* -all command line arguments OR $@ -all command line arguments

28 Using Quotes Shell scripting has three different styles of quoting --each with a different meaning: unquoted strings are normally interpreted "quoted strings are basically literals -- but $variables are evaluated 'quoted strings are absolutely literally interpreted `commands in quotes like this are executed, their output is then inserted as if it were assigned to a variable and then that variable was evaluated`

29 Examples day=`date cut -d" " -f1` printf"today is %s.\n" $day

30 Expressions Evaluating Expr sum=`expr$1 + $2` printf"%s + %s = %s\n" $1 $2 $sum Special Variables $? -the exit status of the last program to exit $$ -The shell's pid Examples test "$LOGNAME" = guna echo $?

31 expr Syntax: expr $var1 operator $var2 Operators +, -, /, %, \*

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33 Conditionals test -f somefile.txt or [ -f somefile.txt ] If statement if [ "$LOGNAME"="guna" ] then printf"%s is logged in" $LOGNAME else printf"intruder! Intruder!" fi

34 for varin do printf"%s\n" $var done for (( i= 1 ; i< 20 ; i++ )) do done The for loop

35 While loop ls sort while read file do echo $file done

36 File descriptors I/O Stdin(0), stdout(1), stderror(2) Input from stdin read data echo $data redirecting rmfilename 1>&2

37 Functions whologgedin() { echo hello $LOGNAME } Calling: > whologgedin

38 grep/sed/tr/s grep pattern file sed s/regex1/regex2/ sed tr/[a-z]/[a-z]/

39 Calling shell commands from perl #! /usr/local/perl `mv $file1 $file2`;

40 Things to do with shell scripts Remove all empty folders Remove all duplicate lines from a file Send if the assignment is not submitted Check output of a submitted program against sample output Given a roster file, extract ID s and create folders for each person Rename a folder that contains.txt files to a folder that contains all.htm files

41 Coding Examples