This is Info file perl.info, produced by Makeinfo version 1.68 from the input file bigperl.texi. settitle perl  File: perl.info, Node: perlfaq7, Next: Top, Prev: Top, Up: Top Perl Language Issues ($Revision: 1.28 $, $Date: 1999/05/23 20:36:18 $) ********************************************************************** NAME ==== perlfaq7 - Perl Language Issues ($Revision: 1.28 $, $Date: 1999/05/23 20:36:18 $) DESCRIPTION =========== This section deals with general Perl language issues that don't clearly fit into any of the other sections. Can I get a BNF/yacc/RE for the Perl language? ---------------------------------------------- There is no BNF, but you can paw your way through the yacc grammar in perly.y in the source distribution if you're particularly brave. The grammar relies on very smart tokenizing code, so be prepared to venture into toke.c as well. In the words of Chaim Frenkel: "Perl's grammar can not be reduced to BNF. The work of parsing perl is distributed between yacc, the lexer, smoke and mirrors." What are all these $@%&* punctuation signs, and how do I know when to use them? ------------------------------------------------------------------------------- They are type specifiers, as detailed in *Note Perldata: perldata,: $ for scalar values (number, string or reference) @ for arrays % for hashes (associative arrays) & for subroutines (aka functions, procedures, methods) * for all types of that symbol name. In version 4 you used them like pointers, but in modern perls you can just use references. A couple of others that you're likely to encounter that aren't really type specifiers are: <> are used for inputting a record from a filehandle. \ takes a reference to something. Note that is *neither* the type specifier for files nor the name of the handle. It is the `< <' >> operator applied to the handle FILE. It reads one line (well, record - see `$', *Note Perlvar: perlvar,) from the handle FILE in scalar context, or all lines in list context. When performing open, close, or any other operation besides `< <' >> on files, or even talking about the handle, do not use the brackets. These are correct: `eof(FH)', `seek(FH, 0, 2)' and "copying from STDIN to FILE". Do I always/never have to quote my strings or use semicolons and commas? ------------------------------------------------------------------------ Normally, a bareword doesn't need to be quoted, but in most cases probably should be (and must be under `use strict'). But a hash key consisting of a simple word (that isn't the name of a defined subroutine) and the left-hand operand to the `< =' >> operator both count as though they were quoted: This is like this ------------ --------------- $foo{line} $foo{"line"} bar => stuff "bar" => stuff The final semicolon in a block is optional, as is the final comma in a list. Good style (see *Note Perlstyle: perlstyle,) says to put them in except for one-liners: if ($whoops) { exit 1 } @nums = (1, 2, 3); if ($whoops) { exit 1; } @lines = ( "There Beren came from mountains cold", "And lost he wandered under leaves", ); How do I skip some return values? --------------------------------- One way is to treat the return values as a list and index into it: $dir = (getpwnam($user))[7]; Another way is to use undef as an element on the left-hand-side: ($dev, $ino, undef, undef, $uid, $gid) = stat($file); How do I temporarily block warnings? ------------------------------------ If you are running Perl 5.6.0 or better, the `use warnings' pragma allows fine control of what warning are produced. See *Note Perllexwarn: perllexwarn, for more details. { no warnings; # temporarily turn off warnings $a = $b + $c; # I know these might be undef } If you have an older version of Perl, the $^W variable (documented in *Note Perlvar: perlvar,) controls runtime warnings for a block: { local $^W = 0; # temporarily turn off warnings $a = $b + $c; # I know these might be undef } Note that like all the punctuation variables, you cannot currently use my() on $^W, only local(). What's an extension? -------------------- A way of calling compiled C code from Perl. Reading *Note Perlxstut: perlxstut, is a good place to learn more about extensions. Why do Perl operators have different precedence than C operators? ----------------------------------------------------------------- Actually, they don't. All C operators that Perl copies have the same precedence in Perl as they do in C. The problem is with operators that C doesn't have, especially functions that give a list context to everything on their right, eg print, chmod, exec, and so on. Such functions are called "list operators" and appear as such in the precedence table in *Note Perlop: perlop,. A common mistake is to write: unlink $file || die "snafu"; This gets interpreted as: unlink ($file || die "snafu"); To avoid this problem, either put in extra parentheses or use the super low precedence or operator: (unlink $file) || die "snafu"; unlink $file or die "snafu"; The "English" operators (and, or, xor, and not) deliberately have precedence lower than that of list operators for just such situations as the one above. Another operator with surprising precedence is exponentiation. It binds more tightly even than unary minus, making `-2**2' product a negative not a positive four. It is also right-associating, meaning that `2**3**2' is two raised to the ninth power, not eight squared. Although it has the same precedence as in C, Perl's `?:' operator produces an lvalue. This assigns $x to either $a or $b, depending on the trueness of $maybe: ($maybe ? $a : $b) = $x; How do I declare/create a structure? ------------------------------------ In general, you don't "declare" a structure. Just use a (probably anonymous) hash reference. See *Note Perlref: perlref, and *Note Perldsc: perldsc, for details. Here's an example: $person = {}; # new anonymous hash $person->{AGE} = 24; # set field AGE to 24 $person->{NAME} = "Nat"; # set field NAME to "Nat" If you're looking for something a bit more rigorous, try *Note Perltoot: perltoot,. How do I create a module? ------------------------- A module is a package that lives in a file of the same name. For example, the Hello::There module would live in Hello/There.pm. For details, read *Note Perlmod: perlmod,. You'll also find *Note Exporter: (pm.info)Exporter, helpful. If you're writing a C or mixed-language module with both C and Perl, then you should study *Note Perlxstut: perlxstut,. Here's a convenient template you might wish you use when starting your own module. Make sure to change the names appropriately. package Some::Module; # assumes Some/Module.pm use strict; use warnings; BEGIN { use Exporter (); our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS); ## set the version for version checking; uncomment to use ## $VERSION = 1.00; # if using RCS/CVS, this next line may be preferred, # but beware two-digit versions. $VERSION = do{my@r=q$Revision: 1.28 $=~/\d+/g;sprintf '%d.'.'%02d'x$#r,@r}; @ISA = qw(Exporter); @EXPORT = qw(&func1 &func2 &func3); %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], # your exported package globals go here, # as well as any optionally exported functions @EXPORT_OK = qw($Var1 %Hashit); } our @EXPORT_OK; # non-exported package globals go here our @more; our $stuff; # initialize package globals, first exported ones $Var1 = ''; %Hashit = (); # then the others (which are still accessible as $Some::Module::stuff) $stuff = ''; @more = (); # all file-scoped lexicals must be created before # the functions below that use them. # file-private lexicals go here my $priv_var = ''; my %secret_hash = (); # here's a file-private function as a closure, # callable as &$priv_func; it cannot be prototyped. my $priv_func = sub { # stuff goes here. }; # make all your functions, whether exported or not; # remember to put something interesting in the {} stubs sub func1 {} # no prototype sub func2() {} # proto'd void sub func3($$) {} # proto'd to 2 scalars # this one isn't exported, but could be called! sub func4(\%) {} # proto'd to 1 hash ref END { } # module clean-up code here (global destructor) 1; # modules must return true The h2xs program will create stubs for all the important stuff for you: % h2xs -XA -n My::Module How do I create a class? ------------------------ See *Note Perltoot: perltoot, for an introduction to classes and objects, as well as *Note Perlobj: perlobj, and *Note Perlbot: perlbot,. How can I tell if a variable is tainted? ---------------------------------------- See `"Laundering and Detecting Tainted Data"', *Note Perlsec: perlsec,. Here's an example (which doesn't use any system calls, because the kill() is given no processes to signal): sub is_tainted { return ! eval { join('',@_), kill 0; 1; }; } This is not -w clean, however. There is no -w clean way to detect taintedness - take this as a hint that you should untaint all possibly-tainted data. What's a closure? ----------------- Closures are documented in *Note Perlref: perlref,. *Closure* is a computer science term with a precise but hard-to-explain meaning. Closures are implemented in Perl as anonymous subroutines with lasting references to lexical variables outside their own scopes. These lexicals magically refer to the variables that were around when the subroutine was defined (deep binding). Closures make sense in any programming language where you can have the return value of a function be itself a function, as you can in Perl. Note that some languages provide anonymous functions but are not capable of providing proper closures; the Python language, for example. For more information on closures, check out any textbook on functional programming. Scheme is a language that not only supports but encourages closures. Here's a classic function-generating function: sub add_function_generator { return sub { shift + shift }; } $add_sub = add_function_generator(); $sum = $add_sub->(4,5); # $sum is 9 now. The closure works as a *function template* with some customization slots left out to be filled later. The anonymous subroutine returned by add_function_generator() isn't technically a closure because it refers to no lexicals outside its own scope. Contrast this with the following make_adder() function, in which the returned anonymous function contains a reference to a lexical variable outside the scope of that function itself. Such a reference requires that Perl return a proper closure, thus locking in for all time the value that the lexical had when the function was created. sub make_adder { my $addpiece = shift; return sub { shift + $addpiece }; } $f1 = make_adder(20); $f2 = make_adder(555); Now `&$f1($n)' is always 20 plus whatever $n you pass in, whereas `&$f2($n)' is always 555 plus whatever $n you pass in. The $addpiece in the closure sticks around. Closures are often used for less esoteric purposes. For example, when you want to pass in a bit of code into a function: my $line; timeout( 30, sub { $line = } ); If the code to execute had been passed in as a string, `< '$line = >, there would have been no way for the hypothetical timeout() function to access the lexical variable $line back in its caller's scope. What is variable suicide and how can I prevent it? -------------------------------------------------- Variable suicide is when you (temporarily or permanently) lose the value of a variable. It is caused by scoping through my() and local() interacting with either closures or aliased foreach() iterator variables and subroutine arguments. It used to be easy to inadvertently lose a variable's value this way, but now it's much harder. Take this code: my $f = "foo"; sub T { while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } } T; print "Finally $f\n"; The $f that has "bar" added to it three times should be a new $f (`my $f' should create a new local variable each time through the loop). It isn't, however. This was a bug, now fixed in the latest releases (tested against 5.004_05, 5.005_03, and 5.005_56). How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}? --------------------------------------------------------------------------- With the exception of regexes, you need to pass references to these objects. See `"Pass by Reference"', *Note Perlsub: perlsub, for this particular question, and `"Pass by Reference"', *Note Perlref: perlref, for information on references. Passing Variables and Functions Regular variables and functions are quite easy: just pass in a reference to an existing or anonymous variable or function: func( \$some_scalar ); func( \@some_array ); func( [ 1 .. 10 ] ); func( \%some_hash ); func( { this => 10, that => 20 } ); func( \&some_func ); func( sub { $_[0] ** $_[1] } ); Passing Filehandles To pass filehandles to subroutines, use the `*FH' or `\*FH' notations. These are "typeglobs" - see `"Typeglobs and Filehandles"', *Note Perldata: perldata, and especially `"Pass by Reference"', *Note Perlsub: perlsub, for more information. Here's an excerpt: If you're passing around filehandles, you could usually just use the bare typeglob, like *STDOUT, but typeglobs references would be better because they'll still work properly under `use strict 'refs''. For example: splutter(\*STDOUT); sub splutter { my $fh = shift; print $fh "her um well a hmmm\n"; } $rec = get_rec(\*STDIN); sub get_rec { my $fh = shift; return scalar <$fh>; } If you're planning on generating new filehandles, you could do this: sub openit { my $name = shift; local *FH; return open (FH, $path) ? *FH : undef; } $fh = openit('< /etc/motd'); print <$fh>; Passing Regexes To pass regexes around, you'll need to be using a release of Perl sufficiently recent as to support the `qr//' construct, pass around strings and use an exception-trapping eval, or else be very, very clever. Here's an example of how to pass in a string to be regex compared using `qr//': sub compare($$) { my ($val1, $regex) = @_; my $retval = $val1 =~ /$regex/; return $retval; } $match = compare("old McDonald", qr/d.*D/i); Notice how `qr//' allows flags at the end. That pattern was compiled at compile time, although it was executed later. The nifty `qr//' notation wasn't introduced until the 5.005 release. Before that, you had to approach this problem much less intuitively. For example, here it is again if you don't have `qr//': sub compare($$) { my ($val1, $regex) = @_; my $retval = eval { $val1 =~ /$regex/ }; die if $@; return $retval; } $match = compare("old McDonald", q/($?i)d.*D/); Make sure you never say something like this: return eval "\$val =~ /$regex/"; # WRONG or someone can sneak shell escapes into the regex due to the double interpolation of the eval and the double-quoted string. For example: $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger'; eval "\$string =~ /$pattern_of_evil/"; Those preferring to be very, very clever might see the O'Reilly book, *Mastering Regular Expressions*, by Jeffrey Friedl. Page 273's Build_MatchMany_Function() is particularly interesting. A complete citation of this book is given in *Note Perlfaq2: perlfaq2,. Passing Methods To pass an object method into a subroutine, you can do this: call_a_lot(10, $some_obj, "methname") sub call_a_lot { my ($count, $widget, $trick) = @_; for (my $i = 0; $i < $count; $i++) { $widget->$trick(); } } Or you can use a closure to bundle up the object and its method call and arguments: my $whatnot = sub { $some_obj->obfuscate(@args) }; func($whatnot); sub func { my $code = shift; &$code(); } You could also investigate the can() method in the UNIVERSAL class (part of the standard perl distribution). How do I create a static variable? ---------------------------------- As with most things in Perl, TMTOWTDI. What is a "static variable" in other languages could be either a function-private variable (visible only within a single function, retaining its value between calls to that function), or a file-private variable (visible only to functions within the file it was declared in) in Perl. Here's code to implement a function-private variable: BEGIN { my $counter = 42; sub prev_counter { return --$counter } sub next_counter { return $counter++ } } Now prev_counter() and next_counter() share a private variable $counter that was initialized at compile time. To declare a file-private variable, you'll still use a my(), putting it at the outer scope level at the top of the file. Assume this is in file Pax.pm: package Pax; my $started = scalar(localtime(time())); sub begun { return $started } When `use Pax' or `require Pax' loads this module, the variable will be initialized. It won't get garbage-collected the way most variables going out of scope do, because the begun() function cares about it, but no one else can get it. It is not called $Pax::started because its scope is unrelated to the package. It's scoped to the file. You could conceivably have several packages in that same file all accessing the same private variable, but another file with the same package couldn't get to it. See `"Persistent Private Variables"', *Note Perlsub: perlsub, for details. What's the difference between dynamic and lexical (static) scoping? Between local() and my()? ---------------------------------------------------------------------------------------------- `local($x)' saves away the old value of the global variable $x, and assigns a new value for the duration of the subroutine, *which is visible in other functions called from that subroutine*. This is done at run-time, so is called dynamic scoping. local() always affects global variables, also called package variables or dynamic variables. `my($x)' creates a new variable that is only visible in the current subroutine. This is done at compile-time, so is called lexical or static scoping. my() always affects private variables, also called lexical variables or (improperly) static(ly scoped) variables. For instance: sub visible { print "var has value $var\n"; } sub dynamic { local $var = 'local'; # new temporary value for the still-global visible(); # variable called $var } sub lexical { my $var = 'private'; # new private variable, $var visible(); # (invisible outside of sub scope) } $var = 'global'; visible(); # prints global dynamic(); # prints local lexical(); # prints global Notice how at no point does the value "private" get printed. That's because $var only has that value within the block of the lexical() function, and it is hidden from called subroutine. In summary, local() doesn't make what you think of as private, local variables. It gives a global variable a temporary value. my() is what you're looking for if you want private variables. See `"Private Variables via my()"', *Note Perlsub: perlsub, and `"Temporary Values via local()"', *Note Perlsub: perlsub, for excruciating details. How can I access a dynamic variable while a similarly named lexical is in scope? -------------------------------------------------------------------------------- You can do this via symbolic references, provided you haven't set `use strict "refs"'. So instead of $var, use `${'var'}'. local $var = "global"; my $var = "lexical"; print "lexical is $var\n"; no strict 'refs'; print "global is ${'var'}\n"; If you know your package, you can just mention it explicitly, as in $Some_Pack::var. Note that the notation $::var is not the dynamic $var in the current package, but rather the one in the main package, as though you had written $main::var. Specifying the package directly makes you hard-code its name, but it executes faster and avoids running afoul of `use strict "refs"'. What's the difference between deep and shallow binding? ------------------------------------------------------- In deep binding, lexical variables mentioned in anonymous subroutines are the same ones that were in scope when the subroutine was created. In shallow binding, they are whichever variables with the same names happen to be in scope when the subroutine is called. Perl always uses deep binding of lexical variables (i.e., those created with my()). However, dynamic variables (aka global, local, or package variables) are effectively shallowly bound. Consider this just one more reason not to use them. See the answer to `"What's a closure?"' in this node. Why doesn't "my($foo) = ;" work right? -------------------------------------------- my() and `local()' give list context to the right hand side of =. The read operation, like so many of Perl's functions and operators, can tell which context it was called in and behaves appropriately. In general, the scalar() function can help. This function does nothing to the data itself (contrary to popular myth) but rather tells its argument to behave in whatever its scalar fashion is. If that function doesn't have a defined scalar behavior, this of course doesn't help you (such as with sort()). To enforce scalar context in this particular case, however, you need merely omit the parentheses: local($foo) = ; # WRONG local($foo) = scalar(); # ok local $foo = ; # right You should probably be using lexical variables anyway, although the issue is the same here: my($foo) = ; # WRONG my $foo = ; # right How do I redefine a builtin function, operator, or method? ---------------------------------------------------------- Why do you want to do that? :-) If you want to override a predefined function, such as open(), then you'll have to import the new definition from a different module. See `"Overriding Built-in Functions"', *Note Perlsub: perlsub,. There's also an example in `"Class::Template"', *Note Perltoot: perltoot,. If you want to overload a Perl operator, such as + or `**', then you'll want to use the `use overload' pragma, documented in *Note Overload: (pm.info)overload,. If you're talking about obscuring method calls in parent classes, see `"Overridden Methods"', *Note Perltoot: perltoot,. What's the difference between calling a function as &foo and foo()? ------------------------------------------------------------------- When you call a function as `&foo', you allow that function access to your current @_ values, and you by-pass prototypes. That means that the function doesn't get an empty @_, it gets yours! While not strictly speaking a bug (it's documented that way in *Note Perlsub: perlsub,), it would be hard to consider this a feature in most cases. When you call your function as `&foo()', then you do get a new @_, but prototyping is still circumvented. Normally, you want to call a function using `foo()'. You may only omit the parentheses if the function is already known to the compiler because it already saw the definition (use but not require), or via a forward reference or `use subs' declaration. Even in this case, you get a clean @_ without any of the old values leaking through where they don't belong. How do I create a switch or case statement? ------------------------------------------- This is explained in more depth in the *Note Perlsyn: perlsyn,. Briefly, there's no official case statement, because of the variety of tests possible in Perl (numeric comparison, string comparison, glob comparison, regex matching, overloaded comparisons, ...). Larry couldn't decide how best to do this, so he left it out, even though it's been on the wish list since perl1. The general answer is to write a construct like this: for ($variable_to_test) { if (/pat1/) { } # do something elsif (/pat2/) { } # do something else elsif (/pat3/) { } # do something else else { } # default } Here's a simple example of a switch based on pattern matching, this time lined up in a way to make it look more like a switch statement. We'll do a multi-way conditional based on the type of reference stored in $whatchamacallit: SWITCH: for (ref $whatchamacallit) { /^$/ && die "not a reference"; /SCALAR/ && do { print_scalar($$ref); last SWITCH; }; /ARRAY/ && do { print_array(@$ref); last SWITCH; }; /HASH/ && do { print_hash(%$ref); last SWITCH; }; /CODE/ && do { warn "can't print function ref"; last SWITCH; }; # DEFAULT warn "User defined type skipped"; } See `perlsyn/"Basic BLOCKs and Switch Statements"' for many other examples in this style. Sometimes you should change the positions of the constant and the variable. For example, let's say you wanted to test which of many answers you were given, but in a case-insensitive way that also allows abbreviations. You can use the following technique if the strings all start with different characters, or if you want to arrange the matches so that one takes precedence over another, as `"SEND"' has precedence over `"STOP"' here: chomp($answer = <>); if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" } elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" } elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" } elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" } elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" } A totally different approach is to create a hash of function references. my %commands = ( "happy" => \&joy, "sad", => \&sullen, "done" => sub { die "See ya!" }, "mad" => \&angry, ); print "How are you? "; chomp($string = ); if ($commands{$string}) { $commands{$string}->(); } else { print "No such command: $string\n"; } How can I catch accesses to undefined variables/functions/methods? ------------------------------------------------------------------ The AUTOLOAD method, discussed in `"Autoloading"', *Note Perlsub: perlsub, and `"AUTOLOAD: Proxy Methods"', *Note Perltoot: perltoot,, lets you capture calls to undefined functions and methods. When it comes to undefined variables that would trigger a warning under -w, you can use a handler to trap the pseudo-signal `__WARN__' like this: $SIG{__WARN__} = sub { for ( $_[0] ) { # voici un switch statement /Use of uninitialized value/ && do { # promote warning to a fatal die $_; }; # other warning cases to catch could go here; warn $_; } }; Why can't a method included in this same file be found? ------------------------------------------------------- Some possible reasons: your inheritance is getting confused, you've misspelled the method name, or the object is of the wrong type. Check out *Note Perltoot: perltoot, for details on these. You may also use `print ref($object)' to find out the class $object was blessed into. Another possible reason for problems is because you've used the indirect object syntax (eg, `find Guru "Samy"') on a class name before Perl has seen that such a package exists. It's wisest to make sure your packages are all defined before you start using them, which will be taken care of if you use the use statement instead of require. If not, make sure to use arrow notation (eg, `< Guru-'find("Samy") >>) instead. Object notation is explained in *Note Perlobj: perlobj,. Make sure to read about creating modules in *Note Perlmod: perlmod, and the perils of indirect objects in `"WARNING"', *Note Perlobj: perlobj,. How can I find out my current package? -------------------------------------- If you're just a random program, you can do this to find out what the currently compiled package is: my $packname = __PACKAGE__; But if you're a method and you want to print an error message that includes the kind of object you were called on (which is not necessarily the same as the one in which you were compiled): sub amethod { my $self = shift; my $class = ref($self) || $self; warn "called me from a $class object"; } How can I comment out a large block of perl code? ------------------------------------------------- Use embedded POD to discard it: # program is here =for nobody This paragraph is commented out # program continues =begin comment text all of this stuff here will be ignored by everyone =end comment text =cut This can't go just anywhere. You have to put a pod directive where the parser is expecting a new statement, not just in the middle of an expression or some other arbitrary yacc grammar production. How do I clear a package? ------------------------- Use this code, provided by Mark-Jason Dominus: sub scrub_package { no strict 'refs'; my $pack = shift; die "Shouldn't delete main package" if $pack eq "" || $pack eq "main"; my $stash = *{$pack . '::'}{HASH}; my $name; foreach $name (keys %$stash) { my $fullname = $pack . '::' . $name; # Get rid of everything with that name. undef $$fullname; undef @$fullname; undef %$fullname; undef &$fullname; undef *$fullname; } } Or, if you're using a recent release of Perl, you can just use the Symbol::delete_package() function instead. How can I use a variable as a variable name? -------------------------------------------- Beginners often think they want to have a variable contain the name of a variable. $fred = 23; $varname = "fred"; ++$$varname; # $fred now 24 This works *sometimes*, but it is a very bad idea for two reasons. The first reason is that they *only work on global variables*. That means above that if $fred is a lexical variable created with my(), that the code won't work at all: you'll accidentally access the global and skip right over the private lexical altogether. Global variables are bad because they can easily collide accidentally and in general make for non-scalable and confusing code. Symbolic references are forbidden under the `use strict' pragma. They are not true references and consequently are not reference counted or garbage collected. The other reason why using a variable to hold the name of another variable a bad idea is that the question often stems from a lack of understanding of Perl data structures, particularly hashes. By using symbolic references, you are just using the package's symbol-table hash (like `%main::') instead of a user-defined hash. The solution is to use your own hash or a real reference instead. $fred = 23; $varname = "fred"; $USER_VARS{$varname}++; # not $$varname++ There we're using the %USER_VARS hash instead of symbolic references. Sometimes this comes up in reading strings from the user with variable references and wanting to expand them to the values of your perl program's variables. This is also a bad idea because it conflates the program-addressable namespace and the user-addressable one. Instead of reading a string and expanding it to the actual contents of your program's own variables: $str = 'this has a $fred and $barney in it'; $str =~ s/(\$\w+)/$1/eeg; # need double eval Instead, it would be better to keep a hash around like %USER_VARS and have variable references actually refer to entries in that hash: $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all That's faster, cleaner, and safer than the previous approach. Of course, you don't need to use a dollar sign. You could use your own scheme to make it less confusing, like bracketed percent symbols, etc. $str = 'this has a %fred% and %barney% in it'; $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all Another reason that folks sometimes think they want a variable to contain the name of a variable is because they don't know how to build proper data structures using hashes. For example, let's say they wanted two hashes in their program: %fred and %barney, and to use another scalar variable to refer to those by name. $name = "fred"; $$name{WIFE} = "wilma"; # set %fred $name = "barney"; $$name{WIFE} = "betty"; # set %barney This is still a symbolic reference, and is still saddled with the problems enumerated above. It would be far better to write: $folks{"fred"}{WIFE} = "wilma"; $folks{"barney"}{WIFE} = "betty"; And just use a multilevel hash to start with. The only times that you absolutely must use symbolic references are when you really must refer to the symbol table. This may be because it's something that can't take a real reference to, such as a format name. Doing so may also be important for method calls, since these always go through the symbol table for resolution. In those cases, you would turn off `strict 'refs'' temporarily so you can play around with the symbol table. For example: @colors = qw(red blue green yellow orange purple violet); for my $name (@colors) { no strict 'refs'; # renege for the block *$name = sub { "@_" }; } All those functions (red(), blue(), green(), etc.) appear to be separate, but the real code in the closure actually was compiled only once. So, sometimes you might want to use symbolic references to directly manipulate the symbol table. This doesn't matter for formats, handles, and subroutines, because they are always global - you can't use my() on them. But for scalars, arrays, and hashes - and usually for subroutines - you probably want to use hard references only. AUTHOR AND COPYRIGHT ==================== Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. All rights reserved. When included as part of the Standard Version of Perl, or as part of its complete documentation whether printed or otherwise, this work may be distributed only under the terms of Perl's Artistic License. Any distribution of this file or derivatives thereof *outside* of that package require that special arrangements be made with copyright holder. Irrespective of its distribution, all code examples in this file are hereby placed into the public domain. You are permitted and encouraged to use this code in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit would be courteous but is not required.  File: perl.info, Node: perlfaq8, Next: Top, Prev: Top, Up: Top System Interaction ($Revision: 1.39 $, $Date: 1999/05/23 18:37:57 $) ******************************************************************** NAME ==== perlfaq8 - System Interaction ($Revision: 1.39 $, $Date: 1999/05/23 18:37:57 $) DESCRIPTION =========== This section of the Perl FAQ covers questions involving operating system interaction. This involves interprocess communication (IPC), control over the user-interface (keyboard, screen and pointing devices), and most anything else not related to data manipulation. Read the FAQs and documentation specific to the port of perl to your operating system (eg, *Note Perlvms: perlvms,, `perlplan9' in this node, ...). These should contain more detailed information on the vagaries of your perl. How do I find out which operating system I'm running under? ----------------------------------------------------------- The $^O variable ($OSNAME if you use English) contains an indication of the name of the operating system (not its release number) that your perl binary was built for. How come exec() doesn't return? ------------------------------- Because that's what it does: it replaces your currently running program with a different one. If you want to keep going (as is probably the case if you're asking this question) use system() instead. How do I do fancy stuff with the keyboard/screen/mouse? ------------------------------------------------------- How you access/control keyboards, screens, and pointing devices ("mice") is system-dependent. Try the following modules: Keyboard Term::Cap Standard perl distribution Term::ReadKey CPAN Term::ReadLine::Gnu CPAN Term::ReadLine::Perl CPAN Term::Screen CPAN Screen Term::Cap Standard perl distribution Curses CPAN Term::ANSIColor CPAN Mouse * Menu: * perl:: Perl overview (this section) * perldelta:: Perl changes since previous version * perl5005delta:: Perl changes in version 5.005 * perl5004delta:: Perl changes in version 5.004 * perlfaq:: Perl frequently asked questions * perltoc:: Perl documentation table of contents * perldata:: Perl data structures * perlsyn:: Perl syntax * perlop:: Perl operators and precedence * perlre:: Perl regular expressions * perlrun:: Perl execution and options * perlfunc:: Perl builtin functions * perlopentut:: Perl open() tutorial * perlvar:: Perl predefined variables * perlsub:: Perl subroutines * perlmod:: Perl modules: how they work * perlmodlib:: Perl modules: how to write and use * perlmodinstall:: Perl modules: how to install from CPAN * perlform:: Perl formats * perlunicode:: Perl unicode support * perllocale:: Perl locale support * perlreftut:: Perl references short introduction * perlref:: Perl references, the rest of the story * perldsc:: Perl data structures intro * perllol:: Perl data structures: arrays of arrays * perlboot:: Perl OO tutorial for beginners * perltoot:: Perl OO tutorial, part 1 * perltootc:: Perl OO tutorial, part 2 * perlobj:: Perl objects * perltie:: Perl objects hidden behind simple variables * perlbot:: Perl OO tricks and examples * perlipc:: Perl interprocess communication * perlfork:: Perl fork() information * perlthrtut:: Perl threads tutorial * perllexwarn:: Perl warnings and their control * perlfilter:: Perl source filters * perldbmfilter:: Perl DBM filters * perlcompile:: Perl compiler suite intro * perldebug:: Perl debugging * perldiag:: Perl diagnostic messages * perlnumber:: Perl number semantics * perlsec:: Perl security * perltrap:: Perl traps for the unwary * perlport:: Perl portability guide * perlstyle:: Perl style guide * perlpod:: Perl plain old documentation * perlbook:: Perl book information * perlembed:: Perl ways to embed perl in your C or C++ application * perlapio:: Perl internal IO abstraction interface * perldebguts:: Perl debugging guts and tips * perlxs:: Perl XS application programming interface * perlxstut:: Perl XS tutorial * perlguts:: Perl internal functions for those doing extensions * perlcall:: Perl calling conventions from C * perlapi:: Perl API listing (autogenerated) * perlintern:: Perl internal functions (autogenerated) * perltodo:: Perl things to do * perlhack:: Perl hackers guide * perlhist:: Perl history records * perlamiga:: Perl notes for Amiga * perlcygwin:: Perl notes for Cygwin * perldos:: Perl notes for DOS * perlhpux:: Perl notes for HP-UX * perlmachten:: Perl notes for Power MachTen * perlos2:: Perl notes for OS/2 * perlos390:: Perl notes for OS/390 * perlvms:: Perl notes for VMS * perlwin32:: Perl notes for Windows * Tk:: CPAN * Module List:(pm.info)Module List. Got your modules, right here * Function Index:: Perl functions and operators * Predefined Variable Index:: Perl predefined variables * Diagnostics Index:: Perl diagnostic messages Some of these specific cases are shown below. How do I print something out in color? -------------------------------------- In general, you don't, because you don't know whether the recipient has a color-aware display device. If you know that they have an ANSI terminal that understands color, you can use the Term::ANSIColor module from CPAN: use Term::ANSIColor; print color("red"), "Stop!\n", color("reset"); print color("green"), "Go!\n", color("reset"); Or like this: use Term::ANSIColor qw(:constants); print RED, "Stop!\n", RESET; print GREEN, "Go!\n", RESET; How do I read just one key without waiting for a return key? ------------------------------------------------------------ Controlling input buffering is a remarkably system-dependent matter. On many systems, you can just use the *stty* command as shown in `getc', *Note Perlfunc: perlfunc,, but as you see, that's already getting you into portability snags. open(TTY, "+/dev/tty 2>&1"; $key = getc(TTY); # perhaps this works # OR ELSE sysread(TTY, $key, 1); # probably this does system "stty -cbreak /dev/tty 2>&1"; The Term::ReadKey module from CPAN offers an easy-to-use interface that should be more efficient than shelling out to *stty* for each key. It even includes limited support for Windows. use Term::ReadKey; ReadMode('cbreak'); $key = ReadKey(0); ReadMode('normal'); However, that requires that you have a working C compiler and can use it to build and install a CPAN module. Here's a solution using the standard POSIX module, which is already on your systems (assuming your system supports POSIX). use HotKey; $key = readkey(); And here's the HotKey module, which hides the somewhat mystifying calls to manipulate the POSIX termios structures. # HotKey.pm package HotKey; @ISA = qw(Exporter); @EXPORT = qw(cbreak cooked readkey); use strict; use POSIX qw(:termios_h); my ($term, $oterm, $echo, $noecho, $fd_stdin); $fd_stdin = fileno(STDIN); $term = POSIX::Termios->new(); $term->getattr($fd_stdin); $oterm = $term->getlflag(); $echo = ECHO | ECHOK | ICANON; $noecho = $oterm & ~$echo; sub cbreak { $term->setlflag($noecho); # ok, so i don't want echo either $term->setcc(VTIME, 1); $term->setattr($fd_stdin, TCSANOW); } sub cooked { $term->setlflag($oterm); $term->setcc(VTIME, 0); $term->setattr($fd_stdin, TCSANOW); } sub readkey { my $key = ''; cbreak(); sysread(STDIN, $key, 1); cooked(); return $key; } END { cooked() } 1; How do I check whether input is ready on the keyboard? ------------------------------------------------------ The easiest way to do this is to read a key in nonblocking mode with the Term::ReadKey module from CPAN, passing it an argument of -1 to indicate not to block: use Term::ReadKey; ReadMode('cbreak'); if (defined ($char = ReadKey(-1)) ) { # input was waiting and it was $char } else { # no input was waiting } ReadMode('normal'); # restore normal tty settings How do I clear the screen? -------------------------- If you only have do so infrequently, use system: system("clear"); If you have to do this a lot, save the clear string so you can print it 100 times without calling a program 100 times: $clear_string = `clear`; print $clear_string; If you're planning on doing other screen manipulations, like cursor positions, etc, you might wish to use Term::Cap module: use Term::Cap; $terminal = Term::Cap->Tgetent( {OSPEED => 9600} ); $clear_string = $terminal->Tputs('cl'); How do I get the screen size? ----------------------------- If you have Term::ReadKey module installed from CPAN, you can use it to fetch the width and height in characters and in pixels: use Term::ReadKey; ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize(); This is more portable than the raw ioctl, but not as illustrative: require 'sys/ioctl.ph'; die "no TIOCGWINSZ " unless defined &TIOCGWINSZ; open(TTY, "+autoflush(1); As mentioned in the previous item, this still doesn't work when using socket I/O between Unix and Macintosh. You'll need to hardcode your line terminators, in that case. non-blocking input If you are doing a blocking read() or sysread(), you'll have to arrange for an alarm handler to provide a timeout (see `alarm', *Note Perlfunc: perlfunc,). If you have a non-blocking open, you'll likely have a non-blocking read, which means you may have to use a 4-arg select() to determine whether I/O is ready on that device (see `"select"', *Note Perlfunc: perlfunc,. While trying to read from his caller-id box, the notorious Jamie Zawinski , after much gnashing of teeth and fighting with sysread, sysopen, POSIX's tcgetattr business, and various other functions that go bump in the night, finally came up with this: sub open_modem { use IPC::Open2; my $stty = `/bin/stty -g`; open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1"); # starting cu hoses /dev/tty's stty settings, even when it has # been opened on a pipe... system("/bin/stty $stty"); $_ = ; chop; if ( !m/^Connected/ ) { print STDERR "$0: cu printed `$_' instead of `Connected'\n"; } } How do I decode encrypted password files? ----------------------------------------- You spend lots and lots of money on dedicated hardware, but this is bound to get you talked about. Seriously, you can't if they are Unix password files - the Unix password system employs one-way encryption. It's more like hashing than encryption. The best you can check is whether something else hashes to the same string. You can't turn a hash back into the original string. Programs like Crack can forcibly (and intelligently) try to guess passwords, but don't (can't) guarantee quick success. If you're worried about users selecting bad passwords, you should proactively check when they try to change their password (by modifying passwd(1), for example). How do I start a process in the background? ------------------------------------------- You could use system("cmd &") or you could use fork as documented in `"fork"', *Note Perlfunc: perlfunc,, with further examples in `"fork"', *Note Perlipc: perlipc,. Some things to be aware of, if you're on a Unix-like system: STDIN, STDOUT, and STDERR are shared Both the main process and the backgrounded one (the "child" process) share the same STDIN, STDOUT and STDERR filehandles. If both try to access them at once, strange things can happen. You may want to close or reopen these for the child. You can get around this with opening a pipe (see `"open"', *Note Perlfunc: perlfunc,) but on some systems this means that the child process cannot outlive the parent. Signals You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too. SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is sent when you write to a filehandle whose child process has closed (an untrapped SIGPIPE can cause your program to silently die). This is not an issue with `system("cmd&")'. Zombies You have to be prepared to "reap" the child process when it finishes $SIG{CHLD} = sub { wait }; See `"Signals"', *Note Perlipc: perlipc, for other examples of code to do this. Zombies are not an issue with `system("prog &")'. How do I trap control characters/signals? ----------------------------------------- You don't actually "trap" a control character. Instead, that character generates a signal which is sent to your terminal's currently foregrounded process group, which you then trap in your process. Signals are documented in `"Signals"', *Note Perlipc: perlipc, and chapter 6 of the Camel. Be warned that very few C libraries are re-entrant. Therefore, if you attempt to print() in a handler that got invoked during another stdio operation your internal structures will likely be in an inconsistent state, and your program will dump core. You can sometimes avoid this by using syswrite() instead of print(). Unless you're exceedingly careful, the only safe things to do inside a signal handler are: set a variable and exit. And in the first case, you should only set a variable in such a way that malloc() is not called (eg, by setting a variable that already has a value). For example: $Interrupted = 0; # to ensure it has a value $SIG{INT} = sub { $Interrupted++; syswrite(STDERR, "ouch\n", 5); } However, because syscalls restart by default, you'll find that if you're in a "slow" call, such as , read(), connect(), or wait(), that the only way to terminate them is by "longjumping" out; that is, by raising an exception. See the time-out handler for a blocking flock() in `"Signals"', *Note Perlipc: perlipc, or chapter 6 of the Camel. How do I modify the shadow password file on a Unix system? ---------------------------------------------------------- If perl was installed correctly, and your shadow library was written properly, the getpw*() functions described in *Note Perlfunc: perlfunc, should in theory provide (read-only) access to entries in the shadow password file. To change the file, make a new shadow password file (the format varies from system to system - see `passwd(5)' in this node for specifics) and use pwd_mkdb(8) to install it (see `pwd_mkdb(8)' in this node for more details). How do I set the time and date? ------------------------------- Assuming you're running under sufficient permissions, you should be able to set the system-wide date and time by running the date(1) program. (There is no way to set the time and date on a per-process basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT; the VMS equivalent is `set time'. However, if all you want to do is change your timezone, you can probably get away with setting an environment variable: $ENV{TZ} = "MST7MDT"; # unixish $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms system "trn comp.lang.perl.misc"; How can I sleep() or alarm() for under a second? ------------------------------------------------ If you want finer granularity than the 1 second that the sleep() function provides, the easiest way is to use the select() function as documented in `"select"', *Note Perlfunc: perlfunc,. If your system has itimers and syscall() support, you can check out the old example in http://www.perl.com/CPAN/doc/misc/ancient/tutorial/eg/itimers.pl . How can I measure time under a second? -------------------------------------- In general, you may not be able to. The Time::HiRes module (available from CPAN) provides this functionality for some systems. If your system supports both the syscall() function in Perl as well as a system call like gettimeofday(2), then you may be able to do something like this: require 'sys/syscall.ph'; $TIMEVAL_T = "LL"; $done = $start = pack($TIMEVAL_T, ()); syscall(&SYS_gettimeofday, $start, 0) != -1 or die "gettimeofday: $!"; ########################## # DO YOUR OPERATION HERE # ########################## syscall( &SYS_gettimeofday, $done, 0) != -1 or die "gettimeofday: $!"; @start = unpack($TIMEVAL_T, $start); @done = unpack($TIMEVAL_T, $done); # fix microseconds for ($done[1], $start[1]) { $_ /= 1_000_000 } $delta_time = sprintf "%.4f", ($done[0] + $done[1] ) - ($start[0] + $start[1] ); How can I do an atexit() or setjmp()/longjmp()? (Exception handling) -------------------------------------------------------------------- Release 5 of Perl added the END block, which can be used to simulate atexit(). Each package's END block is called when the program or thread ends (see *Note Perlmod: perlmod, manpage for more details). For example, you can use this to make sure your filter program managed to finish its output without filling up the disk: END { close(STDOUT) || die "stdout close failed: $!"; } The END block isn't called when untrapped signals kill the program, though, so if you use END blocks you should also use use sigtrap qw(die normal-signals); Perl's exception-handling mechanism is its eval() operator. You can use eval() as setjmp and die() as longjmp. For details of this, see the section on signals, especially the time-out handler for a blocking flock() in `"Signals"', *Note Perlipc: perlipc, and chapter 6 of the Camel. If exception handling is all you're interested in, try the exceptions.pl library (part of the standard perl distribution). If you want the atexit() syntax (and an rmexit() as well), try the AtExit module available from CPAN. Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean? ------------------------------------------------------------------------------------------------------------------------ Some Sys-V based systems, notably Solaris 2.X, redefined some of the standard socket constants. Since these were constant across all architectures, they were often hardwired into perl code. The proper way to deal with this is to "use Socket" to get the correct values. Note that even though SunOS and Solaris are binary compatible, these values are different. Go figure. How can I call my system's unique C functions from Perl? -------------------------------------------------------- In most cases, you write an external module to do it - see the answer to "Where can I learn about linking C with Perl? [h2xs, xsubpp]". However, if the function is a system call, and your system supports syscall(), you can use the syscall function (documented in *Note Perlfunc: perlfunc,). Remember to check the modules that came with your distribution, and CPAN as well - someone may already have written a module to do it. Where do I get the include files to do ioctl() or syscall()? ------------------------------------------------------------ Historically, these would be generated by the h2ph tool, part of the standard perl distribution. This program converts cpp(1) directives in C header files to files containing subroutine definitions, like &SYS_getitimer, which you can use as arguments to your functions. It doesn't work perfectly, but it usually gets most of the job done. Simple files like `errno.h', `syscall.h', and `socket.h' were fine, but the hard ones like `ioctl.h' nearly always need to hand-edited. Here's how to install the *.ph files: 1. become super-user 2. cd /usr/include 3. h2ph *.h */*.h If your system supports dynamic loading, for reasons of portability and sanity you probably ought to use h2xs (also part of the standard perl distribution). This tool converts C header files to Perl extensions. See *Note Perlxstut: perlxstut, for how to get started with h2xs. If your system doesn't support dynamic loading, you still probably ought to use h2xs. See *Note Perlxstut: perlxstut, and *Note ExtUtils/MakeMaker: (pm.info)ExtUtils/MakeMaker, for more information (in brief, just use *make perl* instead of a plain make to rebuild perl with a new static extension). Why do setuid perl scripts complain about kernel problems? ---------------------------------------------------------- Some operating systems have bugs in the kernel that make setuid scripts inherently insecure. Perl gives you a number of options (described in *Note Perlsec: perlsec,) to work around such systems. How can I open a pipe both to and from a command? ------------------------------------------------- The IPC::Open2 module (part of the standard perl distribution) is an easy-to-use approach that internally uses pipe(), fork(), and exec() to do the job. Make sure you read the deadlock warnings in its documentation, though (see *Note IPC/Open2: (pm.info)IPC/Open2,). See `"Bidirectional Communication with Another Process"', *Note Perlipc: perlipc, and `"Bidirectional Communication with Yourself"', *Note Perlipc: perlipc, You may also use the IPC::Open3 module (part of the standard perl distribution), but be warned that it has a different order of arguments from IPC::Open2 (see *Note IPC/Open3: (pm.info)IPC/Open3,). Why can't I get the output of a command with system()? ------------------------------------------------------ You're confusing the purpose of system() and backticks ("). system() runs a command and returns exit status information (as a 16 bit value: the low 7 bits are the signal the process died from, if any, and the high 8 bits are the actual exit value). Backticks (") run a command and return what it sent to STDOUT. $exit_status = system("mail-users"); $output_string = `ls`; How can I capture STDERR from an external command? -------------------------------------------------- There are three basic ways of running external commands: system $cmd; # using system() $output = `$cmd`; # using backticks (``) open (PIPE, "cmd |"); # using open() With system(), both STDOUT and STDERR will go the same place as the script's versions of these, unless the command redirects them. Backticks and open() read *only* the STDOUT of your command. With any of these, you can change file descriptors before the call: open(STDOUT, ">logfile"); system("ls"); or you can use Bourne shell file-descriptor redirection: $output = `$cmd 2>some_file`; open (PIPE, "cmd 2>some_file |"); You can also use file-descriptor redirection to make STDERR a duplicate of STDOUT: $output = `$cmd 2>&1`; open (PIPE, "cmd 2>&1 |"); Note that you *cannot* simply open STDERR to be a dup of STDOUT in your Perl program and avoid calling the shell to do the redirection. This doesn't work: open(STDERR, ">&STDOUT"); $alloutput = `cmd args`; # stderr still escapes This fails because the open() makes STDERR go to where STDOUT was going at the time of the open(). The backticks then make STDOUT go to a string, but don't change STDERR (which still goes to the old STDOUT). Note that you must use Bourne shell (sh(1)) redirection syntax in backticks, not csh(1)! Details on why Perl's system() and backtick and pipe opens all use the Bourne shell are in http://www.perl.com/CPAN/doc/FMTEYEWTK/versus/csh.whynot . To capture a command's STDERR and STDOUT together: $output = `cmd 2>&1`; # either with backticks $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe while () { } # plus a read To capture a command's STDOUT but discard its STDERR: $output = `cmd 2>/dev/null`; # either with backticks $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe while () { } # plus a read To capture a command's STDERR but discard its STDOUT: $output = `cmd 2>&1 1>/dev/null`; # either with backticks $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe while () { } # plus a read To exchange a command's STDOUT and STDERR in order to capture the STDERR but leave its STDOUT to come out our old STDERR: $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe while () { } # plus a read To read both a command's STDOUT and its STDERR separately, it's easiest and safest to redirect them separately to files, and then read from those files when the program is done: system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr"); Ordering is important in all these examples. That's because the shell processes file descriptor redirections in strictly left to right order. system("prog args 1>tmpfile 2>&1"); system("prog args 2>&1 1>tmpfile"); The first command sends both standard out and standard error to the temporary file. The second command sends only the old standard output there, and the old standard error shows up on the old standard out. Why doesn't open() return an error when a pipe open fails? ---------------------------------------------------------- Because the pipe open takes place in two steps: first Perl calls fork() to start a new process, then this new process calls exec() to run the program you really wanted to open. The first step reports success or failure to your process, so open() can only tell you whether the fork() succeeded or not. To find out if the exec() step succeeded, you have to catch SIGCHLD and wait() to get the exit status. You should also catch SIGPIPE if you're writing to the child-you may not have found out the exec() failed by the time you write. This is documented in *Note Perlipc: perlipc,. In some cases, even this won't work. If the second argument to a piped open() contains shell metacharacters, perl fork()s, then exec()s a shell to decode the metacharacters and eventually run the desired program. Now when you call wait(), you only learn whether or not the *shell* could be successfully started. Best to avoid shell metacharacters. On systems that follow the spawn() paradigm, open() *might* do what you expect-unless perl uses a shell to start your command. In this case the fork()/exec() description still applies. What's wrong with using backticks in a void context? ---------------------------------------------------- Strictly speaking, nothing. Stylistically speaking, it's not a good way to write maintainable code because backticks have a (potentially humongous) return value, and you're ignoring it. It's may also not be very efficient, because you have to read in all the lines of output, allocate memory for them, and then throw it away. Too often people are lulled to writing: `cp file file.bak`; And now they think "Hey, I'll just always use backticks to run programs." Bad idea: backticks are for capturing a program's output; the system() function is for running programs. Consider this line: `cat /etc/termcap`; You haven't assigned the output anywhere, so it just wastes memory (for a little while). Plus you forgot to check $? to see whether the program even ran correctly. Even if you wrote print `cat /etc/termcap`; In most cases, this could and probably should be written as system("cat /etc/termcap") == 0 or die "cat program failed!"; Which will get the output quickly (as it is generated, instead of only at the end) and also check the return value. system() also provides direct control over whether shell wildcard processing may take place, whereas backticks do not. How can I call backticks without shell processing? -------------------------------------------------- This is a bit tricky. Instead of writing @ok = `grep @opts '$search_string' @filenames`; You have to do this: my @ok = (); if (open(GREP, "-|")) { while () { chomp; push(@ok, $_); } close GREP; } else { exec 'grep', @opts, $search_string, @filenames; } Just as with system(), no shell escapes happen when you exec() a list. Further examples of this can be found in `"Safe Pipe Opens"', *Note Perlipc: perlipc,. Note that if you're stuck on Microsoft, no solution to this vexing issue is even possible. Even if Perl were to emulate fork(), you'd still be hosed, because Microsoft gives no argc/argv-style API. Their API always reparses from a single string, which is fundamentally wrong, but you're not likely to get the Gods of Redmond to acknowledge this and fix it for you. Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)? ----------------------------------------------------------------------------------- Because some stdio's set error and eof flags that need clearing. The POSIX module defines clearerr() that you can use. That is the technically correct way to do it. Here are some less reliable workarounds: 1. Try keeping around the seekpointer and go there, like this: $where = tell(LOG); seek(LOG, $where, 0); 2. If that doesn't work, try seeking to a different part of the file and then back. 3. If that doesn't work, try seeking to a different part of the file, reading something, and then seeking back. 4. If that doesn't work, give up on your stdio package and use sysread. How can I convert my shell script to perl? ------------------------------------------ Learn Perl and rewrite it. Seriously, there's no simple converter. Things that are awkward to do in the shell are easy to do in Perl, and this very awkwardness is what would make a shell->perl converter nigh-on impossible to write. By rewriting it, you'll think about what you're really trying to do, and hopefully will escape the shell's pipeline datastream paradigm, which while convenient for some matters, causes many inefficiencies. Can I use perl to run a telnet or ftp session? ---------------------------------------------- Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from CPAN). http://www.perl.com/CPAN/scripts/netstuff/telnet.emul.shar will also help for emulating the telnet protocol, but Net::Telnet is quite probably easier to use.. If all you want to do is pretend to be telnet but don't need the initial telnet handshaking, then the standard dual-process approach will suffice: use IO::Socket; # new in 5.004 $handle = IO::Socket::INET->new('www.perl.com:80') || die "can't connect to port 80 on www.perl.com: $!"; $handle->autoflush(1); if (fork()) { # XXX: undef means failure select($handle); print while ; # everything from stdin to socket } else { print while <$handle>; # everything from socket to stdout } close $handle; exit; How can I write expect in Perl? ------------------------------- Once upon a time, there was a library called chat2.pl (part of the standard perl distribution), which never really got finished. If you find it somewhere, *don't use it*. These days, your best bet is to look at the Expect module available from CPAN, which also requires two other modules from CPAN, IO::Pty and IO::Stty. Is there a way to hide perl's command line from programs such as "ps"? ---------------------------------------------------------------------- First of all note that if you're doing this for security reasons (to avoid people seeing passwords, for example) then you should rewrite your program so that critical information is never given as an argument. Hiding the arguments won't make your program completely secure. To actually alter the visible command line, you can assign to the variable $0 as documented in *Note Perlvar: perlvar,. This won't work on all operating systems, though. Daemon programs like sendmail place their state there, as in: $0 = "orcus [accepting connections]"; I {changed directory, modified my environment} in a perl script. How come the change disappeared when I exited the script? How do I get my changes to be visible? ------------------------------------------------------------------------------------------------------------------------------------------------------------------- Unix In the strictest sense, it can't be done - the script executes as a different process from the shell it was started from. Changes to a process are not reflected in its parent, only in its own children created after the change. There is shell magic that may allow you to fake it by eval()ing the script's output in your shell; check out the comp.unix.questions FAQ for details. How do I close a process's filehandle without waiting for it to complete? ------------------------------------------------------------------------- Assuming your system supports such things, just send an appropriate signal to the process (see `"kill"', *Note Perlfunc: perlfunc,. It's common to first send a TERM signal, wait a little bit, and then send a KILL signal to finish it off. How do I fork a daemon process? ------------------------------- If by daemon process you mean one that's detached (disassociated from its tty), then the following process is reported to work on most Unixish systems. Non-Unix users should check their Your_OS::Process module for other solutions. * Open /dev/tty and use the TIOCNOTTY ioctl on it. See `tty(4)' in this node for details. Or better yet, you can just use the POSIX::setsid() function, so you don't have to worry about process groups. * Change directory to / * Reopen STDIN, STDOUT, and STDERR so they're not connected to the old tty. * Background yourself like this: fork && exit; The Proc::Daemon module, available from CPAN, provides a function to perform these actions for you. How do I make my program run with sh and csh? --------------------------------------------- See the `eg/nih' script (part of the perl source distribution). How do I find out if I'm running interactively or not? ------------------------------------------------------ Good question. Sometimes `-t STDIN' and `-t STDOUT' can give clues, sometimes not. if (-t STDIN && -t STDOUT) { print "Now what? "; } On POSIX systems, you can test whether your own process group matches the current process group of your controlling terminal as follows: use POSIX qw/getpgrp tcgetpgrp/; open(TTY, "/dev/tty") or die $!; $tpgrp = tcgetpgrp(fileno(*TTY)); $pgrp = getpgrp(); if ($tpgrp == $pgrp) { print "foreground\n"; } else { print "background\n"; } How do I timeout a slow event? ------------------------------ Use the alarm() function, probably in conjunction with a signal handler, as documented in `"Signals"', *Note Perlipc: perlipc, and chapter 6 of the Camel. You may instead use the more flexible Sys::AlarmCall module available from CPAN. How do I set CPU limits? ------------------------ Use the BSD::Resource module from CPAN. How do I avoid zombies on a Unix system? ---------------------------------------- Use the reaper code from `"Signals"', *Note Perlipc: perlipc, to call wait() when a SIGCHLD is received, or else use the double-fork technique described in `fork', *Note Perlfunc: perlfunc,. How do I use an SQL database? ----------------------------- There are a number of excellent interfaces to SQL databases. See the DBD::* modules available from http://www.perl.com/CPAN/modules/DBD . A lot of information on this can be found at http://www.symbolstone.org/technology/perl/DBI/ How do I make a system() exit on control-C? ------------------------------------------- You can't. You need to imitate the system() call (see *Note Perlipc: perlipc, for sample code) and then have a signal handler for the INT signal that passes the signal on to the subprocess. Or you can check for it: $rc = system($cmd); if ($rc & 127) { die "signal death" } How do I open a file without blocking? -------------------------------------- If you're lucky enough to be using a system that supports non-blocking reads (most Unixish systems do), you need only to use the O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with sysopen(): use Fcntl; sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644) or die "can't open /tmp/somefile: $!": How do I install a module from CPAN? ------------------------------------ The easiest way is to have a module also named CPAN do it for you. This module comes with perl version 5.004 and later. To manually install the CPAN module, or any well-behaved CPAN module for that matter, follow these steps: 1. Unpack the source into a temporary area. 2. perl Makefile.PL 3. make 4. make test 5. make install If your version of perl is compiled without dynamic loading, then you just need to replace step 3 (make) with *make perl* and you will get a new `perl' binary with your extension linked in. See *Note ExtUtils/MakeMaker: (pm.info)ExtUtils/MakeMaker, for more details on building extensions. See also the next question. What's the difference between require and use? ---------------------------------------------- Perl offers several different ways to include code from one file into another. Here are the deltas between the various inclusion constructs: 1) do $file is like eval `cat $file`, except the former: 1.1: searches @INC and updates %INC. 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code. 2) require $file is like do $file, except the former: 2.1: checks for redundant loading, skipping already loaded files. 2.2: raises an exception on failure to find, compile, or execute $file. 3) require Module is like require "Module.pm", except the former: 3.1: translates each "::" into your system's directory separator. 3.2: primes the parser to disambiguate class Module as an indirect object. 4) use Module is like require Module, except the former: 4.1: loads the module at compile time, not run-time. 4.2: imports symbols and semantics from that package to the current one. In general, you usually want use and a proper Perl module. How do I keep my own module/library directory? ---------------------------------------------- When you build modules, use the PREFIX option when generating Makefiles: perl Makefile.PL PREFIX=/u/mydir/perl then either set the PERL5LIB environment variable before you run scripts that use the modules/libraries (see *Note Perlrun: perlrun,) or say use lib '/u/mydir/perl'; This is almost the same as: BEGIN { unshift(@INC, '/u/mydir/perl'); } except that the lib module checks for machine-dependent subdirectories. See Perl's *Note Lib: (pm.info)lib, for more information. How do I add the directory my program lives in to the module/library search path? --------------------------------------------------------------------------------- use FindBin; use lib "$FindBin::Bin"; use your_own_modules; How do I add a directory to my include path at runtime? ------------------------------------------------------- Here are the suggested ways of modifying your include path: the PERLLIB environment variable the PERL5LIB environment variable the perl -Idir command line flag the use lib pragma, as in use lib "$ENV{HOME}/myown_perllib"; The latter is particularly useful because it knows about machine dependent architectures. The lib.pm pragmatic module was first included with the 5.002 release of Perl. What is socket.ph and where do I get it? ---------------------------------------- It's a perl4-style file defining values for system networking constants. Sometimes it is built using h2ph when Perl is installed, but other times it is not. Modern programs `use Socket;' instead. AUTHOR AND COPYRIGHT ==================== Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. All rights reserved. When included as part of the Standard Version of Perl, or as part of its complete documentation whether printed or otherwise, this work may be distributed only under the terms of Perl's Artistic License. Any distribution of this file or derivatives thereof *outside* of that package require that special arrangements be made with copyright holder. Irrespective of its distribution, all code examples in this file are hereby placed into the public domain. You are permitted and encouraged to use this code in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit would be courteous but is not required.