Welcome to Linux Knowledge Base and Tutorial
"The place where you learn linux"
Traveller''s Lunchbox

 Create an AccountHome | Submit News | Your Account  

Tutorial Menu
Linux Tutorial Home
Table of Contents

· Introduction to Operating Systems
· Linux Basics
· Working with the System
· Shells and Utilities
· Editing Files
· Basic Administration
· The Operating System
· The X Windowing System
· The Computer Itself
· Networking
· System Monitoring
· Solving Problems
· Security
· Installing and Upgrading
· Linux and Windows

Glossary
MoreInfo
Man Pages
Linux Topics
Test Your Knowledge

Site Menu
Site Map
FAQ
Copyright Info
Terms of Use
Privacy Info
Disclaimer
WorkBoard
Thanks
Donations
Advertising
Masthead / Impressum
Your Account

Communication
Feedback
Forums
Private Messages
Surveys

Features
HOWTOs
News Archive
Submit News
Topics
User Articles
Web Links

Google
Google


The Web
linux-tutorial.info

Who's Online
There are currently, 185 guest(s) and 0 member(s) that are online.

You are an Anonymous user. You can register for free by clicking here

  

perldata



DESCRIPTION

       Variable names

       Perl has three built-in data types: scalars, arrays of
       scalars, and associative arrays of scalars, known as
       "hashes".  A scalar is a single string (of any size, lim­
       ited only by the available memory), number, or a reference
       to something (which will be discussed in perlref).  Normal
       arrays are ordered lists of scalars indexed by number,
       starting with 0.  Hashes are unordered collections of
       scalar values indexed by their associated string key.

       Values are usually referred to by name, or through a named
       reference.  The first character of the name tells you to
       what sort of data structure it refers.  The rest of the
       name tells you the particular value to which it refers.
       Usually this name is a single identifier, that is, a
       string beginning with a letter or underscore, and contain­
       ing letters, underscores, and digits.  In some cases, it
       may be a chain of identifiers, separated by "::" (or by
       the slightly archaic "'"); all but the last are inter­
       preted as names of packages, to locate the namespace in
       which to look up the final identifier (see "Packages" in
       perlmod for details).  It's possible to substitute for a
       simple identifier, an expression that produces a reference
       to the value at runtime.   This is described in more
       detail below and in perlref.

       Perl also has its own built-in variables whose names don't
       follow these rules.  They have strange names so they don't
       accidentally collide with one of your normal variables.
       Strings that match parenthesized parts of a regular
       expression are saved under names containing only digits
       after the "$" (see perlop and perlre).  In addition, sev­
       eral special variables that provide windows into the inner
       working of Perl have names containing punctuation charac­
       ters and control characters.  These are documented in per­
       lvar.

       Scalar values are always named with '$', even when refer­
       ring to a scalar that is part of an array or a hash.  The
       '$' symbol works semantically like the English word "the"
       in that it indicates a single value is expected.

           $days               # the simple scalar value "days"
           $days[28]           # the 29th element of array @days
           $days{'Feb'}        # the 'Feb' value from hash %days
           $#days              # the last index of array @days

       Entire arrays (and slices of arrays and hashes) are
       denoted by '@', which works much like the word "these" or
       "those" does in English, in that it indicates multiple
       care about that yet (if ever :-).

       Every variable type has its own namespace, as do several
       non-variable identifiers.  This means that you can, with­
       out fear of conflict, use the same name for a scalar vari­
       able, an array, or a hash--or, for that matter, for a
       filehandle, a directory handle, a subroutine name, a for­
       mat name, or a label.  This means that $foo and @foo are
       two different variables.  It also means that $foo[1] is a
       part of @foo, not a part of $foo.  This may seem a bit
       weird, but that's okay, because it is weird.

       Because variable references always start with '$', '@', or
       '%', the "reserved" words aren't in fact reserved with
       respect to variable names.  They are reserved with respect
       to labels and filehandles, however, which don't have an
       initial special character.  You can't have a filehandle
       named "log", for instance.  Hint: you could say
       "open(LOG,'logfile')" rather than "open(log,'logfile')".
       Using uppercase filehandles also improves readability and
       protects you from conflict with future reserved words.
       Case is significant--"FOO", "Foo", and "foo" are all dif­
       ferent names.  Names that start with a letter or under­
       score may also contain digits and underscores.

       It is possible to replace such an alphanumeric name with
       an expression that returns a reference to the appropriate
       type.  For a description of this, see perlref.

       Names that start with a digit may contain only more dig­
       its.  Names that do not start with a letter, underscore,
       digit or a caret (i.e.  a control character) are limited
       to one character, e.g.,  $% or $$.  (Most of these one
       character names have a predefined significance to Perl.
       For instance, $$ is the current process id.)

       Context

       The interpretation of operations and values in Perl some­
       times depends on the requirements of the context around
       the operation or value.  There are two major contexts:
       list and scalar.  Certain operations return list values in
       contexts wanting a list, and scalar values otherwise.  If
       this is true of an operation it will be mentioned in the
       documentation for that operation.  In other words, Perl
       overloads certain operations based on whether the expected
       return value is singular or plural.  Some words in English
       work this way, like "fish" and "sheep".

       In a reciprocal fashion, an operation provides either a
       scalar or a list context to each of its arguments.  For
       example, if you say
       routine, which will then sort those lines and return them
       as a list to whatever the context of the sort was.

       Assignment is a little bit special in that it uses its
       left argument to determine the context for the right argu­
       ment.  Assignment to a scalar evaluates the right-hand
       side in scalar context, while assignment to an array or
       hash evaluates the righthand side in list context.
       Assignment to a list (or slice, which is just a list any­
       way) also evaluates the righthand side in list context.

       When you use the "use warnings" pragma or Perl's -w com­
       mand-line option, you may see warnings about useless uses
       of constants or functions in "void context".  Void context
       just means the value has been discarded, such as a state­
       ment containing only ""fred";" or "getpwuid(0);".  It
       still counts as scalar context for functions that care
       whether or not they're being called in list context.

       User-defined subroutines may choose to care whether they
       are being called in a void, scalar, or list context.  Most
       subroutines do not need to bother, though.  That's because
       both scalars and lists are automatically interpolated into
       lists.  See "wantarray" in perlfunc for how you would
       dynamically discern your function's calling context.

       Scalar values

       All data in Perl is a scalar, an array of scalars, or a
       hash of scalars.  A scalar may contain one single value in
       any of three different flavors: a number, a string, or a
       reference.  In general, conversion from one form to
       another is transparent.  Although a scalar may not
       directly hold multiple values, it may contain a reference
       to an array or hash which in turn contains multiple val­
       ues.

       Scalars aren't necessarily one thing or another.  There's
       no place to declare a scalar variable to be of type
       "string", type "number", type "reference", or anything
       else.  Because of the automatic conversion of scalars,
       operations that return scalars don't need to care (and in
       fact, cannot care) whether their caller is looking for a
       string, a number, or a reference.  Perl is a contextually
       polymorphic language whose scalars can be strings, num­
       bers, or references (which includes objects).  Although
       strings and numbers are considered pretty much the same
       thing for nearly all purposes, references are
       strongly-typed, uncastable pointers with builtin refer­
       ence-counting and destructor invocation.

       A scalar value is interpreted as TRUE in the Boolean sense
       in a place expecting a defined value, this no longer hap­
       pens except for rare cases of autovivification as
       explained in perlref.  You can use the defined() operator
       to determine whether a scalar value is defined (this has
       no meaning on arrays or hashes), and the undef() operator
       to produce an undefined value.

       To find out whether a given string is a valid non-zero
       number, it's sometimes enough to test it against both
       numeric 0 and also lexical "0" (although this will cause
       noises if warnings are on).  That's because strings that
       aren't numbers count as 0, just as they do in awk:

           if ($str == 0 && $str ne "0")  {
               warn "That doesn't look like a number";
           }

       That method may be best because otherwise you won't treat
       IEEE notations like "NaN" or "Infinity" properly.  At
       other times, you might prefer to determine whether string
       data can be used numerically by calling the POSIX::str­
       tod() function or by inspecting your string with a regular
       expression (as documented in perlre).

           warn "has nondigits"        if     /\D/;
           warn "not a natural number" unless /^\d+$/;             # rejects -3
           warn "not an integer"       unless /^-?\d+$/;           # rejects +3
           warn "not an integer"       unless /^[+-]?\d+$/;
           warn "not a decimal number" unless /^-?\d+\.?\d*$/;     # rejects .2
           warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
           warn "not a C float"
               unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;

       The length of an array is a scalar value.  You may find
       the length of array @days by evaluating $#days, as in csh.
       However, this isn't the length of the array; it's the sub­
       script of the last element, which is a different value
       since there is ordinarily a 0th element.  Assigning to
       $#days actually changes the length of the array.  Shorten­
       ing an array this way destroys intervening values.
       Lengthening an array that was previously shortened does
       not recover values that were in those elements.  (It used
       to do so in Perl 4, but we had to break this to make sure
       destructors were called when expected.)

       You can also gain some minuscule measure of efficiency by
       pre-extending an array that is going to get big.  You can
       also extend an array by assigning to an element that is
       off the end of the array.  You can truncate an array down
       to nothing by assigning the null list () to it.  The fol­
       lowing are equivalent:

       use of $[ is deprecated.)  So in general you can assume
       that

           scalar(@whatever) == $#whatever + 1;

       Some programmers choose to use an explicit conversion so
       as to leave nothing to doubt:

           $element_count = scalar(@whatever);

       If you evaluate a hash in scalar context, it returns false
       if the hash is empty.  If there are any key/value pairs,
       it returns true; more precisely, the value returned is a
       string consisting of the number of used buckets and the
       number of allocated buckets, separated by a slash.  This
       is pretty much useful only to find out whether Perl's
       internal hashing algorithm is performing poorly on your
       data set.  For example, you stick 10,000 things in a hash,
       but evaluating %HASH in scalar context reveals "1/16",
       which means only one out of sixteen buckets has been
       touched, and presumably contains all 10,000 of your items.
       This isn't supposed to happen.

       You can preallocate space for a hash by assigning to the
       keys() function.  This rounds up the allocated buckets to
       the next power of two:

           keys(%users) = 1000;                # allocate 1024 buckets

       Scalar value constructors

       Numeric literals are specified in any of the following
       floating point or integer formats:

           12345
           12345.67
           .23E-10             # a very small number
           3.14_15_92          # a very important number
           4_294_967_296       # underscore for legibility
           0xff                # hex
           0xdead_beef         # more hex
           0377                # octal
           0b011011            # binary

       You are allowed to use underscores (underbars) in numeric
       literals between digits for legibility.  You could, for
       example, group binary digits by threes (as for a Unix-
       style mode argument such as 0b110_100_100) or by fours (to
       represent nibbles, as in 0b1010_0110) or in other groups.

       String literals are usually delimited by either single or
       double quotes.  They work much like quotes in the standard
       i.e., they can end on a different line than they begin.
       This is nice, but if you forget your trailing quote, the
       error will not be reported until Perl finds another line
       containing the quote character, which may be much further
       on in the script.  Variable substitution inside strings is
       limited to scalar variables, arrays, and array or hash
       slices.  (In other words, names beginning with $ or @,
       followed by an optional bracketed expression as a sub­
       script.)  The following code segment prints out "The price
       is $100."

           $Price = '$100';    # not interpolated
           print "The price is $Price.\n";     # interpolated

       There is no double interpolation in Perl, so the $100 is
       left as is.

       As in some shells, you can enclose the variable name in
       braces to disambiguate it from following alphanumerics
       (and underscores).  You must also do this when interpolat­
       ing a variable into a string to separate the variable name
       from a following double-colon or an apostrophe, since
       these would be otherwise treated as a package separator:

           $who = "Larry";
           print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
           print "We use ${who}speak when ${who}'s here.\n";

       Without the braces, Perl would have looked for a $whos­
       peak, a $who::0, and a $who's variable.  The last two
       would be the $0 and the $s variables in the (presumably)
       non-existent package "who".

       In fact, an identifier within such curlies is forced to be
       a string, as is any simple identifier within a hash sub­
       script.  Neither need quoting.  Our earlier example,
       $days{'Feb'} can be written as $days{Feb} and the quotes
       will be assumed automatically.  But anything more compli­
       cated in the subscript will be interpreted as an expres­
       sion.

       Version Strings

       Note: Version Strings (v-strings) have been deprecated.
       They will not be available after Perl 5.8.  The marginal
       benefits of v-strings were greatly outweighed by the
       potential for Surprise and Confusion.

       A literal of the form "v1.20.300.4000" is parsed as a
       string composed of characters with the specified ordinals.
       This form, known as v-strings, provides an alternative,
       more readable way to construct strings, rather than use
       See "$^V" in perlvar.  Note that using the v-strings for
       IPv4 addresses is not portable unless you also use the
       inet_aton()/inet_ntoa() routines of the Socket package.

       Note that since Perl 5.8.1 the single-number v-strings
       (like "v65") are not v-strings before the "=>" operator
       (which is usually used to separate a hash key from a hash
       value), instead they are interpreted as literal strings
       ('v65').  They were v-strings from Perl 5.6.0 to Perl
       5.8.0, but that caused more confusion and breakage than
       good.  Multi-number v-strings like "v65.66" and 65.66.67
       continue to be v-strings always.

       Special Literals

       The special literals __FILE__, __LINE__, and __PACKAGE__
       represent the current filename, line number, and package
       name at that point in your program.  They may be used only
       as separate tokens; they will not be interpolated into
       strings.  If there is no current package (due to an empty
       "package;" directive), __PACKAGE__ is the undefined value.

       The two control characters ^D and ^Z, and the tokens
       __END__ and __DATA__ may be used to indicate the logical
       end of the script before the actual end of file.  Any fol­
       lowing text is ignored.

       Text after __DATA__ but may be read via the filehandle
       "PACKNAME::DATA", where "PACKNAME" is the package that was
       current when the __DATA__ token was encountered.  The
       filehandle is left open pointing to the contents after
       __DATA__.  It is the program's responsibility to "close
       DATA" when it is done reading from it.  For compatibility
       with older scripts written before __DATA__ was introduced,
       __END__ behaves like __DATA__ in the toplevel script (but
       not in files loaded with "require" or "do") and leaves the
       remaining contents of the file accessible via
       "main::DATA".

       See SelfLoader for more description of __DATA__, and an
       example of its use.  Note that you cannot read from the
       DATA filehandle in a BEGIN block: the BEGIN block is exe­
       cuted as soon as it is seen (during compilation), at which
       point the corresponding __DATA__ (or __END__) token has
       not yet been seen.

       Barewords

       A word that has no other interpretation in the grammar
       will be treated as if it were a quoted string.  These are
       known as "barewords".  As with filehandles and labels, a
       bareword that consists entirely of lowercase letters risks

       Arrays and slices are interpolated into double-quoted
       strings by joining the elements with the delimiter speci­
       fied in the $" variable ($LIST_SEPARATOR if "use English;"
       is specified), space by default.  The following are equiv­
       alent:

           $temp = join($", @ARGV);
           system "echo $temp";

           system "echo @ARGV";

       Within search patterns (which also undergo double-quotish
       substitution) there is an unfortunate ambiguity:  Is
       "/$foo[bar]/" to be interpreted as "/${foo}[bar]/" (where
       "[bar]" is a character class for the regular expression)
       or as "/${foo[bar]}/" (where "[bar]" is the subscript to
       array @foo)?  If @foo doesn't otherwise exist, then it's
       obviously a character class.  If @foo exists, Perl takes a
       good guess about "[bar]", and is almost always right.  If
       it does guess wrong, or if you're just plain paranoid, you
       can force the correct interpretation with curly braces as
       above.

       If you're looking for the information on how to use
       here-documents, which used to be here, that's been moved
       to "Quote and Quote-like Operators" in perlop.

       List value constructors

       List values are denoted by separating individual values by
       commas (and enclosing the list in parentheses where prece­
       dence requires it):

           (LIST)

       In a context not requiring a list value, the value of what
       appears to be a list literal is simply the value of the
       final element, as with the C comma operator.  For example,

           @foo = ('cc', '-E', $bar);

       assigns the entire list value to array @foo, but

           $foo = ('cc', '-E', $bar);

       assigns the value of variable $bar to the scalar variable
       $foo.  Note that the value of an actual array in scalar
       context is the length of the array; the following assigns
       the value 3 to $foo:

           @foo = ('cc', '-E', $bar);

       element, you might use an approach like this:

           @sauces = <<End_Lines =~ m/(\S.*\S)/g;
               normal tomato
               spicy tomato
               green chile
               pesto
               white wine
           End_Lines

       LISTs do automatic interpolation of sublists.  That is,
       when a LIST is evaluated, each element of the list is
       evaluated in list context, and the resulting list value is
       interpolated into LIST just as if each individual element
       were a member of LIST.  Thus arrays and hashes lose their
       identity in a LIST--the list

           (@foo,@bar,&SomeSub,%glarch)

       contains all the elements of @foo followed by all the ele­
       ments of @bar, followed by all the elements returned by
       the subroutine named SomeSub called in list context, fol­
       lowed by the key/value pairs of %glarch.  To make a list
       reference that does NOT interpolate, see perlref.

       The null list is represented by ().  Interpolating it in a
       list has no effect.  Thus ((),(),()) is equivalent to ().
       Similarly, interpolating an array with no elements is the
       same as if no array had been interpolated at that point.

       This interpolation combines with the facts that the open­
       ing and closing parentheses are optional (except when nec­
       essary for precedence) and lists may end with an optional
       comma to mean that multiple commas within lists are legal
       syntax. The list "1,,3" is a concatenation of two lists,
       "1," and 3, the first of which ends with that optional
       comma.  "1,,3" is "(1,),(3)" is "1,3" (And similarly for
       "1,,,3" is "(1,),(,),3" is "1,3" and so on.)  Not that
       we'd advise you to use this obfuscation.

       A list value may also be subscripted like a normal array.
       You must put the list in parentheses to avoid ambiguity.
       For example:

           # Stat returns list value.
           $time = (stat($file))[8];

           # SYNTAX ERROR HERE.
           $time = stat($file)[8];  # OOPS, FORGOT PARENTHESES

           # Find a hex digit.
           $hexdigit = ('a','b','c','d','e','f')[$digit-10];

           ($dev, $ino, undef, undef, $uid, $gid) = stat($file);

       List assignment in scalar context returns the number of
       elements produced by the expression on the right side of
       the assignment:

           $x = (($foo,$bar) = (3,2,1));       # set $x to 3, not 2
           $x = (($foo,$bar) = f());           # set $x to f()'s return count

       This is handy when you want to do a list assignment in a
       Boolean context, because most list functions return a null
       list when finished, which when assigned produces a 0,
       which is interpreted as FALSE.

       It's also the source of a useful idiom for executing a
       function or performing an operation in list context and
       then counting the number of return values, by assigning to
       an empty list and then using that assignment in scalar
       context. For example, this code:

           $count = () = $string =~ /\d+/g;

       will place into $count the number of digit groups found in
       $string.  This happens because the pattern match is in
       list context (since it is being assigned to the empty
       list), and will therefore return a list of all matching
       parts of the string. The list assignment in scalar context
       will translate that into the number of elements (here, the
       number of times the pattern matched) and assign that to
       $count. Note that simply using

           $count = $string =~ /\d+/g;

       would not have worked, since a pattern match in scalar
       context will only return true or false, rather than a
       count of matches.

       The final element of a list assignment may be an array or
       a hash:

           ($a, $b, @rest) = split;
           my($a, $b, %rest) = @_;

       You can actually put an array or hash anywhere in the
       list, but the first one in the list will soak up all the
       values, and anything after it will become undefined.  This
       may be useful in a my() or local().

       A hash can be initialized using a literal list holding
       pairs of items to be interpreted as a key and a value:

       visually distinctive synonym for a comma, but it also
       arranges for its left-hand operand to be interpreted as a
       string -- if it's a bareword that would be a legal simple
       identifier ("=>" doesn't quote compound identifiers, that
       contain double colons). This makes it nice for initializ­
       ing hashes:

           %map = (
                        red   => 0x00f,
                        blue  => 0x0f0,
                        green => 0xf00,
          );

       or for initializing hash references to be used as records:

           $rec = {
                       witch => 'Mable the Merciless',
                       cat   => 'Fluffy the Ferocious',
                       date  => '10/31/1776',
           };

       or for using call-by-named-parameter to complicated func­
       tions:

          $field = $query->radio_group(
                      name      => 'group_name',
                      values    => ['eenie','meenie','minie'],
                      default   => 'meenie',
                      linebreak => 'true',
                      labels    => \%labels
          );

       Note that just because a hash is initialized in that order
       doesn't mean that it comes out in that order.  See "sort"
       in perlfunc for examples of how to arrange for an output
       ordering.

       Subscripts

       An array is subscripted by specifying a dollary sign
       ("$"), then the name of the array (without the leading
       "@"), then the subscript inside square brackets.  For
       example:

           @myarray = (5, 50, 500, 5000);
           print "Element Number 2 is", $myarray[2], "\n";

       The array indices start with 0. A negative subscript
       retrieves its value from the end.  In our example,
       $myarray[-1] would have been 5000, and $myarray[-2] would
       have been 500.


           print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";

       Slices

       A common way to access an array or a hash is one scalar
       element at a time.  You can also subscript a list to get a
       single element from it.

           $whoami = $ENV{"USER"};             # one element from the hash
           $parent = $ISA[0];                  # one element from the array
           $dir    = (getpwnam("daemon"))[7];  # likewise, but with list

       A slice accesses several elements of a list, an array, or
       a hash simultaneously using a list of subscripts.  It's
       more convenient than writing out the individual elements
       as a list of separate scalar values.

           ($him, $her)   = @folks[0,-1];              # array slice
           @them          = @folks[0 .. 3];            # array slice
           ($who, $home)  = @ENV{"USER", "HOME"};      # hash slice
           ($uid, $dir)   = (getpwnam("daemon"))[2,7]; # list slice

       Since you can assign to a list of variables, you can also
       assign to an array or hash slice.

           @days[3..5]    = qw/Wed Thu Fri/;
           @colors{'red','blue','green'}
                          = (0xff0000, 0x0000ff, 0x00ff00);
           @folks[0, -1]  = @folks[-1, 0];

       The previous assignments are exactly equivalent to

           ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
           ($colors{'red'}, $colors{'blue'}, $colors{'green'})
                          = (0xff0000, 0x0000ff, 0x00ff00);
           ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);

       Since changing a slice changes the original array or hash
       that it's slicing, a "foreach" construct will alter
       some--or even all--of the values of the array or hash.

           foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }

           foreach (@hash{qw[key1 key2]}) {
               s/^\s+//;           # trim leading whitespace
               s/\s+$//;           # trim trailing whitespace
               s/(\w+)/\u\L$1/g;   # "titlecase" words
           }

       A slice of an empty list is still an empty list.  Thus:

               printf "%-8s %s\n", $user, $home;
           }

       As noted earlier in this document, the scalar sense of
       list assignment is the number of elements on the right-
       hand side of the assignment.  The null list contains no
       elements, so when the password file is exhausted, the
       result is 0, not 2.

       If you're confused about why you use an '@' there on a
       hash slice instead of a '%', think of it like this.  The
       type of bracket (square or curly) governs whether it's an
       array or a hash being looked at.  On the other hand, the
       leading symbol ('$' or '@') on the array or hash indicates
       whether you are getting back a singular value (a scalar)
       or a plural one (a list).

       Typeglobs and Filehandles

       Perl uses an internal type called a typeglob to hold an
       entire symbol table entry.  The type prefix of a typeglob
       is a "*", because it represents all types.  This used to
       be the preferred way to pass arrays and hashes by refer­
       ence into a function, but now that we have real refer­
       ences, this is seldom needed.

       The main use of typeglobs in modern Perl is create symbol
       table aliases.  This assignment:

           *this = *that;

       makes $this an alias for $that, @this an alias for @that,
       %this an alias for %that, &this an alias for &that, etc.
       Much safer is to use a reference.  This:

           local *Here::blue = \$There::green;

       temporarily makes $Here::blue an alias for $There::green,
       but doesn't make @Here::blue an alias for @There::green,
       or %Here::blue an alias for %There::green, etc.  See "Sym­
       bol Tables" in perlmod for more examples of this.  Strange
       though this may seem, this is the basis for the whole mod­
       ule import/export system.

       Another use for typeglobs is to pass filehandles into a
       function or to create new filehandles.  If you need to use
       a typeglob to save away a filehandle, do it this way:

           $fh = *STDOUT;

       or perhaps as a real reference, like this:

           }
           $fh = newopen('/etc/passwd');

       Now that we have the *foo{THING} notation, typeglobs
       aren't used as much for filehandle manipulations, although
       they're still needed to pass brand new file and directory
       handles into or out of functions. That's because *HAN­
       DLE{IO} only works if HANDLE has already been used as a
       handle.  In other words, *FH must be used to create new
       symbol table entries; *foo{THING} cannot.  When in doubt,
       use *FH.

       All functions that are capable of creating filehandles
       (open(), opendir(), pipe(), socketpair(), sysopen(),
       socket(), and accept()) automatically create an anonymous
       filehandle if the handle passed to them is an uninitial­
       ized scalar variable. This allows the constructs such as
       "open(my $fh, ...)" and "open(local $fh,...)" to be used
       to create filehandles that will conveniently be closed
       automatically when the scope ends, provided there are no
       other references to them. This largely eliminates the need
       for typeglobs when opening filehandles that must be passed
       around, as in the following example:

           sub myopen {
               open my $fh, "@_"
                    or die "Can't open '@_': $!";
               return $fh;
           }

           {
               my $f = myopen("</etc/motd");
               print <$f>;
               # $f implicitly closed here
           }

       Note that if an initialized scalar variable is used
       instead the result is different: "my $fh='zzz'; open($fh,
       ...)" is equivalent to "open( *{'zzz'}, ...)".  "use
       strict 'refs'" forbids such practice.

       Another way to create anonymous filehandles is with the
       Symbol module or with the IO::Handle module and its ilk.
       These modules have the advantage of not hiding different
       types of the same name during the local().  See the bottom
       of "open()" in perlfunc for an example.


SEE ALSO

       See perlvar for a description of Perl's built-in variables
       and a discussion of legal variable names.  See perlref,
       perlsub, and "Symbol Tables" in perlmod for more discus­
       sion on typeglobs and the *foo{THING} syntax.

An undefined database error occurred. SELECT distinct pages.pagepath,pages.pageid FROM pages, page2command WHERE pages.pageid = page2command.pageid AND commandid =


  
Show your Support for the Linux Tutorial

Purchase one of the products from our new online shop. For each product you purchase, the Linux Tutorial gets a portion of the proceeds to help keep us going.


Login
Nickname

Password

Security Code
Security Code
Type Security Code


Don't have an account yet? You can create one. As a registered user you have some advantages like theme manager, comments configuration and post comments with your name.

Help if you can!


Amazon Wish List

Did You Know?
The Linux Tutorial welcomes your suggestions and ideas.


Friends



Tell a Friend About Us

Bookmark and Share



Web site powered by PHP-Nuke

Is this information useful? At the very least you can help by spreading the word to your favorite newsgroups, mailing lists and forums.
All logos and trademarks in this site are property of their respective owner. The comments are property of their posters. Articles are the property of their respective owners. Unless otherwise stated in the body of the article, article content (C) 1994-2013 by James Mohr. All rights reserved. The stylized page/paper, as well as the terms "The Linux Tutorial", "The Linux Server Tutorial", "The Linux Knowledge Base and Tutorial" and "The place where you learn Linux" are service marks of James Mohr. All rights reserved.
The Linux Knowledge Base and Tutorial may contain links to sites on the Internet, which are owned and operated by third parties. The Linux Tutorial is not responsible for the content of any such third-party site. By viewing/utilizing this web site, you have agreed to our disclaimer, terms of use and privacy policy. Use of automated download software ("harvesters") such as wget, httrack, etc. causes the site to quickly exceed its bandwidth limitation and are therefore expressly prohibited. For more details on this, take a look here

PHP-Nuke Copyright © 2004 by Francisco Burzi. This is free software, and you may redistribute it under the GPL. PHP-Nuke comes with absolutely no warranty, for details, see the license.
Page Generation: 0.09 Seconds