If Then Else Logic Post: 302146772

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Top Forums UNIX for Dummies Questions & Answers If Then Else Logic Post 302146772 by jadionne on Thursday 22nd of November 2007 07:06:35 AM

11-22-2007

Registered User

Quote:

Originally Posted by bakunin

Your problem is you are looking in the wrong place. ;-)

The shell construct "if [ ...] ; then" is utilizing a command external to the shell: /usr/bin/test. In fact "[....]" is just an alternative way to write "test ...." and branch on the errorlevel (return value) of this command.

You have the following (basic) options in test, more to be found on the manpage of test:

(for the following we assume x=5 and y=10)
-gt "greater then" compare (integer) values: test $x -gt $y => returns FALSE
-ge "greater or equal"
-lt "lower then"
-le "lower equal"
-eq "equal"
-ne "not equal"

-a logical AND. Example: test "$x -gt 0 -a $y -ge x" => returns TRUE
-o logical OR. Analogous to above

$...$ grouping. Example: test "$ $x -gt 0 $ -a $ $y -gt $x -o $y -gt 0 $"

-n non-zero TRUE if a string is not empty
-z zero TRUE if a string is empty ("")

I hope this helps.

bakunin

This looks a bit more streight forward to me...but again do you have a detailed link that can clear up some questions for me.

For example

Code:

test $x -gt $y => returns FALSE

what is the => representing here?

jadionne

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sql::statement::term

SQL::Statement::Term(3pm)				User Contributed Perl Documentation				 SQL::Statement::Term(3pm)

NAME

       SQL::Statement::Term - base class for all terms

SYNOPSIS

	 # create a term with an SQL::Statement object as owner
	 my $term = SQL::Statement::Term->new( $owner );
	 # access the value of that term
	 $term->value( $eval );

DESCRIPTION

       SQL::Statement::Term is an abstract base class providing the interface for all terms.

INHERITANCE

	 SQL::Statement::Term

METHODS

   new
       Instantiates new term and stores a weak reference to the owner.

   value
       Abstract method which will return the value of the term. Must be overridden by derived classes.

   DESTROY
       Destroys the term and undefines the weak reference to the owner.

NAME

       SQL::Statement::ConstantTerm - term for constant values

SYNOPSIS

	 # create a term with an SQL::Statement object as owner
	 my $term = SQL::Statement::ConstantTerm->new( $owner, 'foo' );
	 # access the value of that term - returns 'foo'
	 $term->value( $eval );

DESCRIPTION

       SQL::Statement::ConstantTerm implements a term which will always return the same constant value.

INHERITANCE

	 SQL::Statement::ConstantTerm
	 ISA SQL::Statement::Term

METHODS

   new
       Instantiates new term and stores the constant to deliver and a weak reference to the owner.

   value
       Returns the specified constant.

NAME

       SQL::Statement::ColumnValue - term for column values

SYNOPSIS

	 # create a term with an SQL::Statement object as owner
	 my $term = SQL::Statement::ColumnValue->new( $owner, 'id' );
	 # access the value of that term - returns the value of the column 'id'
	 # of the currently active row in $eval
	 $term->value( $eval );

DESCRIPTION

       SQL::Statement::ColumnValue implements a term which will return the specified column of the active row.

INHERITANCE

	 SQL::Statement::ColumnValue
	 ISA SQL::Statement::Term

METHODS

   new
       Instantiates new term and stores the column name to deliver and a weak reference to the owner.

   value
       Returns the specified column value.

AUTHOR AND COPYRIGHT

       Copyright (c) 2009,2010 by Jens Rehsack: rehsackATcpan.org

       All rights reserved.

       You may distribute this module under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl
       README file.

perl v5.10.1							    2011-02-01						 SQL::Statement::Term(3pm)

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sql::statement::term