University:
Boston University
Course:
GRS CS 910 | Directed Study: Systems
Academic year:

2021
Views:

142

Pages:

16
Author:

Kolstrup96gf

Using Perl Command line Option Explained • Command format: perl -[v|p|e|i] “perl statement/expression” input • Options: (type “perl -h” for more options) -e # tell perl to execute some statements in what is quoted following -v # check current perl version -i[extension] # edit input files in place (makes backup if extension supplied) -n # assume "while (<>) { ... }" loop around program -p # assume loop like -n but print line also Command line Examples • perl -e 'print "Hello World\n"' - same result as run ‘codeEx_simplest.pl’ • perl -n -e 'print "$. - $_"' codeEx_simplest.pl - implicit loop, print code with line number • perl -p -n -e '$_="$. - $_"' codeEx_simplest.pl - implicit loop, implicit print, , using $_ new assignment • perl -ne 'print "$. - $_" unless /^#/' codeEx_simplest.pl - implicit loop, print code with line number • perl -ne 'print "$. - $_" if /^#/' codeEx_simplest.pl - print all lines that are starting with ‘#’ Good Programming Practices • Always starts with hash-bang line #!/usr/local/bin/perl • Using template/framework to standardize and simplify code tasks (see MyFramework.pl for explanation) • Learn to using Perl debugger tool rather than use ‘print’ • Start with minimum code required (isolate code) • Reduce interference by defining good interfaces through subroutines • Pay attention to format (especially with statement across multiple lines) • Many more … (refer to ‘Perl Best Practice’) Good Programming Practices Code Example Variable Scope • What is scope? The space that something is seen/valid • Two types of scope: Global vs. Lexical • Global variable – visible in the entire package, ‘our’ keyword • lexical variable – only visible in the context, with ‘my’ keyword • Override: Inside variable overrides(hides) the outside variable • Package independence - same variable name can be used in different packages, they are totally independent and won’t affect each other • Use namespace to provide specificity – use “package::variable” qualifier Variable Scope Example 1 Variable Scope Example 2 Variable Scope Example 3 Variable Scope Good Practice To avoid ambiguity – • avoid using same name for different variables unless you are sure they are meant to be same thing ; • use meaningful names for each variable Special Symbols • Also called ‘pre-defined variables’ in perldoc • Can be divided into five categories: • General Variables • Regular Expression Variables • Filehandle Variables • Error Variables • State Variables • Perl programming depends highly on using these special symbols (variables, more officially). So it is good to know about them. • Use ‘perldoc perlvar’ to read the help documentation Special Symbols - General $ARG/$_ – default input space @ARG/@_ – parameter array for subroutine $a – small number in sort(); $b – large number in sort() %ENV – environment variables %INC – the paths to be searched … Special Symbols – Regular Expression $1, $2, … - matching groups in the parentheses in pattern Output: Special Symbols – Regular Expression (2) • • • $&/${^MATCH} – last successful matching string $`/${^PREMATCH} – the string preceding the last matching string $’/${^POSTMATCH} – the string following the last matching string Special Symbols – File handlers • • • • • • • $AGRV – name of current file @ARGV – command line arguments ARGV – special file handle for command line filenames $. – current line number $/ - input line delimiter $\ - output line delimiter $% - current page number Special Symbols – File handlers • • • • $@ $! $^E $? Perl error string Error number from C, ‘errno’ Extended OS error info, such as ‘CDROM tray not closed’ Exit status from last process

Learning Perl Through Examples Part 3