PERL 5 Manual

Name

perl - Practical Extraction and Report Language (version 5)

Synopsis

For ease of access, the Perl manual has been split up into a number of sections.
You can also search the Perl manual.

perl Perl overview (this section)
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
perlvar Perl predefined variables
perlsub Perl subroutines
perlmod Perl modules
perlref Perl references and nested data structures
perlobj Perl objects
perlbot Perl OO tricks and examples
perldebug Perl debugging
perldiag Perl diagnostic messages
perlform Perl formats
perlipc Perl interprocess communication
perlsec Perl security
perltrap Perl traps for the unwary
perlstyle Perl style guide
perlapi Perl application programming interface
perlguts Perl internal functions for those doing extensions
perlcall Perl calling conventions from C
perlovl Perl overloading semantics
perlbook Perl book information

(If you're intending to read these straight through for the first time, the suggested order will tend to reduce the number of forward references.)

If something strange has gone wrong with your program and you're not sure where you should look for help, try the -w switch first. It will often point out exactly where the trouble is.

Description

Perl is an interpreted language optimized for scanning arbitrary text files, extracting information from those text files, and printing reports based on that information. It's also a good language for many system management tasks. The language is intended to be practical (easy to use, efficient, complete) rather than beautiful (tiny, elegant, minimal). It combines (in the author's opinion, anyway) some of the best features of C, sed, awk, and sh, so people familiar with those languages should have little difficulty with it. (Language historians will also note some vestiges of csh, Pascal, and even BASIC-PLUS.) Expression syntax corresponds quite closely to C expression syntax. Unlike most Unix utilities, Perl does not arbitrarily limit the size of your data--if you've got the memory, Perl can slurp in your whole file as a single string. Recursion is of unlimited depth. And the hash tables used by associative arrays grow as necessary to prevent degraded performance. Perl uses sophisticated pattern matching techniques to scan large amounts of data very quickly. Although optimized for scanning text, Perl can also deal with binary data, and can make dbm files look like associative arrays (where dbm is available). Setuid Perl scripts are safer than C programs through a dataflow tracing mechanism which prevents many stupid security holes. If you have a problem that would ordinarily use sed or awk or sh, but it exceeds their capabilities or must run a little faster, and you don't want to write the silly thing in C, then Perl may be for you. There are also translators to turn your sed and awk scripts into Perl scripts.

But wait, there's more...

Perl version 5 is nearly a complete rewrite, and provides the following additional benefits:

* Many usability enhancements: It is now possible to write much more readable Perl code (even within regular expressions). Formerly cryptic variable names can be replaced by mnemonic identifiers. Error messages are more informative, and the optional warnings will catch many of the mistakes a novice might make. This cannot be stressed enough. Whenever you get mysterious behavior, try the -w switch!!! Whenever you don't get mysterious behavior, try using -w anyway.
* Simplified grammar: The new yacc grammar is one half the size of the old one. Many of the arbitrary grammar rules have been regularized. The number of reserved words has been cut by 2/3. Despite this, nearly all old Perl scripts will continue to work unchanged.
* Lexical scoping: Perl variables may now be declared within a lexical scope, like "auto" variables in C. Not only is this more efficient, but it contributes to better privacy for "programming in the large".
* Arbitrarily nested data structures: Any scalar value, including any array element, may now contain a reference to any other variable or subroutine. You can easily create anonymous variables and subroutines. Perl manages your reference counts for you.
* Modularity and reusability: The Perl library is now defined in terms of modules which can be easily shared among various packages. A package may choose to import all or a portion of a module's published interface. Pragmas (that is, compiler directives) are defined and used by the same mechanism.
* Object-oriented programming: A package can function as a class. Dynamic multiple inheritance and virtual methods are supported in a straightforward manner and with very little new syntax. Filehandles may now be treated as objects.
* Embeddible and Extensible: Perl may now be embedded easily in your C or C++ application, and can either call or be called by your routines through a documented interface. The XS preprocessor is provided to make it easy to glue your C or C++ routines into Perl. Dynamic loading of modules is supported.
* POSIX compliant: A major new module is the POSIX module, which provides access to all available POSIX routines and definitions, via object classes where appropriate.
* Package constructors and destructors: The new BEGIN and END blocks provide means to capture control as a package is being compiled, and after the program exits. As a degenerate case they work just like awk's BEGIN and END when you use the -p or -n switches.
* Multiple simultaneous DBM implementations: A Perl program may now access DBM, NDBM, SDBM, GDBM, and Berkeley DB files from the same script simultaneously. In fact, the old dbmopen interface has been generalized to allow any variable to be tied to an object class which defines its access methods.
* Subroutine definitions may now be autoloaded: In fact, the AUTOLOAD mechanism also allows you to define any arbitrary semantics for undefined subroutine calls. It's not just for autoloading.
* Regular expression enhancements: You can now specify non-greedy quantifiers. You can now do grouping without creating a backreference. You can now write regular expressions with embedded whitespace and comments for readability. A consistent extensibility mechanism has been added that is upwardly compatible with all old regular expressions.

Ok, that's definitely enough hype.

Environment

HOME: Used if chdir has no argument.
LOGDIR: Used if chdir has no argument and HOME is not set.
PATH: Used in executing subprocesses, and in finding the script if -S is used.
PERL5LIB: A colon-separated list of directories in which to look for Perl library files before looking in the standard library and the current directory. If PERL5LIB is not defined, PERLLIB is used.
PERL5DB: The command used to get the debugger code. If unset, uses
PERLLIB: A colon-separated list of directories in which to look for Perl library files before looking in the standard library and the current directory. If PERL5LIB is defined, PERLLIB is not used.

Apart from these, Perl uses no other environment variables, except to make them available to the script being executed, and to child processes. However, scripts running setuid would do well to execute the following lines before doing anything else, just to keep people honest:

        $ENV{'PATH'} = '/bin:/usr/bin';    # or whatever you need
        $ENV{'SHELL'} = '/bin/sh' if defined $ENV{'SHELL'};
        $ENV{'IFS'} = ''          if defined $ENV{'IFS'};

Author

Larry Wall <F

Files

     "/tmp/perl-e$$"        temporary file for -e commands
     "@INC"                        locations of perl 5 libraries

Diagnostics

The -w switch produces some lovely diagnostics.

See the perldiag manpage for explanations of all Perl's diagnostics.

Compilation errors will tell you the line number of the error, with an indication of the next token or token type that was to be examined. (In the case of a script passed to Perl via -e switches, each -e is counted as one line.)

Setuid scripts have additional constraints that can produce error messages such as "Insecure dependency". See the perlsec manpage .

Did we mention that you should definitely consider using the -w switch?

Bugs

The -w switch is not mandatory.

Perl is at the mercy of your machine's definitions of various operations such as type casting, atof() and sprintf().

If your stdio requires an seek or eof between reads and writes on a particular stream, so does Perl. (This doesn't apply to sysread() and syswrite().)

While none of the built-in data types have any arbitrary size limits (apart from memory size), there are still a few arbitrary limits: a given identifier may not be longer than 255 characters, and no component of your PATH may be longer than 255 if you use -S . A regular expression may not compile to more than 32767 bytes internally.

Perl actually stands for Pathologically Eclectic Rubbish Lister, but don't tell anyone I said that.

Notes

The Perl motto is "There's more than one way to do it." Divining how many more is left as an exercise to the reader.

The three principle virtues of a programmer are Laziness, Impatience, and Hubris. See the Camel Book for why.