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Manual Reference Pages  -  SHELL::BASE (3)

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Shell::Base - A generic class to build line-oriented command interpreters.



  package My::Shell;

  use Shell::Base;
  use base qw(Shell::Base);

  sub do_greeting {
      return "Hello!"


Shell::Base is a base class designed for building command line programs. It defines a number of useful defaults, simplifies adding commands and help, and integrates well with Term::ReadLine.

After writing several REP (Read-Eval-Print) loops in Perl, I found myself wishing for something a little more convenient than starting with:

  while(1) {
      my $line = <STDIN>;
      last unless defined $line;

      chomp $line;
      if ($line =~ /^...


Shell::Base provides simple access to many of the things I always write into my REP’s, as well as support for many thing that I always intend to, but never find time for:
readline support Shell::Base provides simple access to the readline library via Term::ReadLine, including built-in tab-completion and easy integration with the history file features.

If a subclass does want or need Term::ReadLine support, then it can be replaced in subclasses by overriding a few methods. See Using Shell::Base Without readline, below.

Trivial to add commands Adding commands to your shell is as simple as creating methods: the command foo is dispatched to do_foo. In addition, there are hooks for unknown commands and for when the user just hits <Return>, both of which a subclass can override.
Integrated help system Shell::Base makes it simple to integrate online help within alongside your command methods. Help for a command foo can be retrieved with help foo, with the addition of one method. In addition, a general help command lists all possible help commands; this list is generated at run time, so there’s no possibility of forgetting to add help methods to the list of available topics.
Pager integration Output can be sent through the user’s default pager (as defined by $ENV{’PAGER’}, with a reasonable default) or dumped directly to STDOUT.
Customizable output stream(s) Printing is handled through a print() method, which can be overridden in a subclass to send output anywhere.
Pre- and post-processing methods Input received from readline() can be processed before it is parsed, and output from command methods can be post-processed before it is sent to print().
Automatic support for RC files A simple RC-file parser is built in, which handles name = value type configuration files. This parser handles comments, whitespace, multiline definitions, boolean and (name, value) option types, and multiple files (e.g., /etc/foorc, $HOME/.foorc).
Shell::Base was originally based, conceptually, on Python’s cmd.Cmd class, though it has expanded far beyond what Cmd offers.


There are two basic types of methods: methods that control how a Shell::Base-derived object behaves, and methods that add command to the shell.

All aspects of a Shell::Base-derived object are available via accessors, from the Term::ReadLine instance to data members, to make life easier for subclass implementors.

NB: The following list isn’t really in any order!
new The constructor is called new, and should be inherited from Shell::Base (and not overridden). new should be called with a reference to a hash of name => value parameters:

  my %options = (HISTFILE => glob("~/.myshell_history"),
                 OPTION_1 => $one,
                 OPTION_2 => $two);

  my $shell = My::Shell->new(\%options);

new calls a number of initializing methods, each of which will be called with a reference to the passed in hash of parameters as the only argument:
init_rl(\%args) init_rl initializes the Term::ReadLine instance. If a subclass does not intend to use Term::ReadLine, this method can be overridden. (There are other methods that need to be overridden to eliminate readline completely; see Using Shell::Base Without readline for more details.)

The completion method, complete, is set here, though the list of possible completions is generated in the init_completions method.

If a HISTFILE parameter is passed to init_rl, then the internal Term::ReadLine instance will attempt to use that file for history functions. See History Functions in Term::ReadLine::Gnu for more details.

init_rcfiles(\%args) init_rcfiles treats each element in the RCFILES array (passed into the contructor) as a configuration file, and attempts to read and parse it. See RC Files, below.
init_help(\%args) init_help generates the list of available help topics, which is all methods that match the pattern ^help_, by default. Once this list is generated, it is stored using the helps method (see helps).
init_completions(\%args) init_completions creates the list of methods that are tab-completable, and sets them using the completions method. By default, it finds all methods that begin with ^do_ in the current class and superclass(es).

The default completion method, complete, chooses completions from this list based on the line and word being completed. See complete.

init(\%args) A general purpose init method, designed to be overridden by subclasses. The default init method in Shell::Base does nothing.

In general, subclass-specific initializations should go in this method.

A subclass’s init method should be carful about deleting from the hash that they get as a parameter — items removed from the hash are really gone. At the same time, items can be added to the hash, and will persist. The original parameters can be retrieved at run time using the args method.

Similarly, configuration data parsed from RCFILES can be retrieved using the config method.

run The main loop of the program is a method called run — all other methods are called in preparation for the call to run, or are called from within run. run takes no parameters, and does not return.

  $shell = My::Shell->new();

At the top of the loop, run prints the value of $self->intro, if it is defined:

  my $intro = $self->intro();
      if defined $intro;

run does several things for each iteration of the REP loop that are worth noting:
o Reads a line of input using $self->readline(), passing the value of $self->prompt():

  $line = $self->readline($self->prompt);

o Passes that line through $self->precmd(), for possible manipulation:

  $line = $self->precmd($line);

o Parses the line:

  ($cmd, $env, @args) = $self->parseline($line);

See parseline for details about parseline, and what $cmd, $env, and @args are.

o Update environment variables with entries from %$env, for the command $cmd only.
o Checks the contents of $cmd; there are a few special cases:
o If $cmd matches $Shell::Base::RE_QUIT, the method quit is invoked:

  $output = $self->quit();

$RE_QUIT is ^(?i)\s*(quit|exit|logout) by default

o Otherwise, if $cmd matches $Shell::Base::RE_HELP, the method help is invoked, with @args as parameters:

  $output = $self->help(@args);

$RE_HELP is ^(?i)\s*(help|\?) by default.

o Otherwise, if $cmd matches $Shell::Base::RE_SHEBANG, the method do_shell is invoked, with @args as parameters:

  $output = $self->do_shell(@args);

$RE_SHEBANG is ^\s*!\s*$ by default.

o Otherwise, the command do_$cmd is invoked, with @args as parameters:

  my $method = "do_$cmd";
  $output = $self->$method(@args);

o $output is passed to $self->postcmd() for postprocessing:

  $output = $self->postcmd($output);

o Finally, if $output is not undef, it is passed to $self->print(), with a newline appended:

      if defined $output;

When the main loop ends, usually through the exit or quit commands, or when the user issues CTRL-D, run calls the quit method.

args([$what]) The original hash of arguments passed into the constructor is stored in the instance, and can be retrieved using the args method, which is an accessor only (though the hash returned by args is live, and changes will propogate).

If args is passed a value, then the value associated with that key will be returned. An example:

  my $shell = My::Shell->new(FOO => "foo", BAR => "bar");

  my $foo = $shell->args("FOO");  # $foo contains "foo"
  my $bar = $shell->args("BAR");  # $bar contains "bar"
  my $baz = $shell->args("BAZ");  # $baz is undefined
  my $args = $shell->args();      # $args is a ref to the whole hash

As a convenience, if a specified argument is not found, it is uppercased, and then tried again, so:

  my $foo = $shell->args("FOO");


  my $foo = $shell->args("foo");

are identical if there is a FOO arg and no foo arg.

config([$what]) Configuration data gleaned from RCFILES can be retrieved using the config method. config behaves similarly to the args method.
helps When called without arguments, helps returns a list of all the available help_foo methods, as a list.

When called with arguments, helps uses these arguments to set the current list of help methods.

This is the method called by init_help to fill in the list of available help methods, and help when it needs to figure out the available help topics.

completions Similar to helps, except that completions returns or sets the list of completions possible when the user hits <tab>.
print The print method, well, prints its data. print is a method so that subclasses can override it; here is a small example class, Tied::Shell, that wraps around a Tie::File instance, in which all data is printed to the Tie::File instance, as well as to the normal place. This makes it ideal for (e.g.) logging sessions:

  package Tied::Shell;

  use Shell::Base;
  use Tie::File;

  use strict;
  use base qw(Shell::Base);

  sub init {
      my ($self, $args) = @_;
      my @file;

      tie @file, Tie::File, $args->{ FILENAME };

      $self->{ TIEFILE } = \@file;

  # Append to self, then call SUPER::print
  sub print {
      my ($self, @lines) = @_;
      push @{ $self->{ TIEFILE } }, @lines;

      return $self->SUPER::print(@lines);

  sub quit {
      my $self = shift;
      untie @{ $self->{ TIEFILE } };

(See Tie::File for the appropriate details.)

readline The readline method is a wrapper for $self->term->readline; it is called at the top of the REP loop within run to get the next line of input. readline is it’s own method so that subclasses which do not use Term::ReadLine can override it specifically. A very basic, non-readline readline could look like:

  sub readline {
      my ($self, $prompt) = @_;
      my $line;

      print $prompt;
      chomp($line = <STDIN>);

      return $line;

As implied by the example, readline will be passed the prompt to be displayed, which should be a string (it will be treated like one).

A good example of when this might be overridden would be on systems that prefer to use editline instead of GNU readline, using the Term::EditLine module (e.g., NetBSD):

  # Initialize Term::EditLine
  sub init_rl {
      my ($self, $args) = @_;

      require Term::EditLine;
      $self->{ TERM } = Term::EditLine->new(ref $self);

      return $self;

  # Return the Term::EditLine instance
  sub term {
      my $self = shift;
      return $self->{ TERM };

  # Get a line of input
  sub readline {
      my ($self, $prompt) = @_;
      my $line;
      my $term = $self->term;

      $line = $term->gets();

      return $line;

default When an unknown command is received, the default method is invoked, with ($cmd, @args) as the arguments. The default default method simply returns an error string, but this can of course be overridden in a subclass:

  sub default {
      my ($self, @cmd) = @_;
      my $output = `@cmd`;
      chomp $output;  # everything is printed with an extra "\n"
      return $output;

precmd precmd is called after a line of input is read, but before it is parsed. precmd will be called with $line as the sole argument, and it is expected to return a string suitable for splitting with parseline. Any amount of massaging can be done to $line, of course.

The default precmd method does nothing:

  sub precmd {
      my ($self, $line) = @_;
      return $line;

This would be a good place to handle things tilde-expansion:

  sub precmd {
      my ($self, $line) = @_;
      $line =~ s{~([\w\d_-]*)}
                { $1 ? (getpwnam($1))[7] : $ENV{HOME} }e;
      return $line;

postcmd postcmd is called immediately before any output is printed. postcmd will be passed a scalar containing the output of whatever command run invoked. postcmd is expected to return a string suitable for printing; if the return of postcmd is undef, then nothing will be printed.

The default postcmd method does nothing:

  sub postcmd {
      my ($self, $output) = @_;
      return $output;

You can do fun output filtering here:

  use Text::Bastardize;
  my $bastard = Text::Bastardize->new;
  sub postcmd {
      my ($self, $output) = @_;


      return $bastard->k3wlt0k()

Or translation:

  use Text::Iconv;
  my $converter;
  sub postcmd {
      my ($self, $output) = @_;

      unless (defined $converter) {
          # Read these values from the config files
          my $from_lang = $self->config("from_lang");
          my $to_lang = $self->config("to_lang");

          $converter = Text::Iconv->new($from_lang, $to_lang);

          # Return undef on error, dont croak

      # Fall back to unconverted output, not croak
      return $completer->convert($output) || $output;

Or put the tildes back in:

  sub postcmd {
      my ($self, $line) = @_;
      $line =~ s{(/home/([^/ ]+))}
                { -d $1 ? "~$2" : $1 }ge;
      return $line;

pager The pager method attempts to determine what the user’s preferred pager is, and return it. This can be used within an overridden print method, for example, to send everything through a pager:

  sub print {
      my ($self, @stuff) = @_;
      my $pager = $self->pager;

      open my $P, "|$pager" or carp "Cant open $pager: $!";
      CORE::print $P @stuff;
      close $P;

Note the explicit use of CORE::print, to prevent infinite recursion.

parseline A line is divided into ($command, %env, @args) using $self->parseline(). A command foo is dispatched to a method do_foo, with @args passed as an array, and with %ENV updated to include %env.

If there is no do_foo method for a command foo, then the method default will be called. Subclasses can override the default method.

%ENV is localized and updated with the contents of %env for the current command. %env is populated in a similar fashion to how /bin/sh does; the command:

    FOO=bar baz

Invokes the do_baz method with $ENV{’FOO’} = bar.

Shell::Base doesn’t (currently) do anything interesting with pipelines; the command:

  foo | bar

will be parsed by parseline() as:

  ("foo", {}, "|", "bar")

rather than as two separate connected commands. Support for pipelines in on the TODO list.

prompt Gets or sets the current prompt. The default prompt is:

  sprintf "(%s) \$ ", __PACKAGE__;

The prompt method can be overridden, of course, possibly using something like String::Format:

  use Cwd;
  use File::Basename qw(basename);
  use Net::Domain qw(hostfqdn);
  use String::Format qw(stringf);
  use Sys::Hostname qw(hostname);

  sub prompt {
      my $self = shift;
      my $fmt = $self->{ PROMPT_FMT };
      return stringf $fmt => {
          $ => $$,
          w => cwd,
          W => basename(cwd),
          0 => $self->progname,
          ! => $self->prompt_no,
          u => scalar getpwuid($<),
          g => scalar getgrgid($(),
          c => ref($self),
          h => hostname,
          H => hostfqdn,

Then $self->{ PROMPT_FMT } can be set to, for example, %u@%h %w %%, which might yield a prompt like:

  darren@tumbleweed /tmp/Shell-Base %

(See String::Format for the appropriate details.)

The value passed to prompt can be a code ref; if so, it is invoked with $self and any additional arguments passed to prompt as the arguments:

    $self->prompt(\&func, @stuff);

Will call:

    &$func($self, @stuff);

and use the return value as the prompt string.

intro / outro Text that is displayed when control enters run (intro) and quit (outro). If the method returns a non-undef result, it will be passed to $self->print().
quit The quit method currently handles closing the history file; if it is overridden, $self->SUPER::quit() should be called, so that the history file will be written out.

The results of $self->outro() will be passed to $self->print() as well.

    Methods That Add Commands

Any command that run() doesn’t recognize will be treated as a command; a method named do_$command will be invoked, in an eval block. Remember that a line is parsed into ($command, %env, @args); do_$command will be invoked with @args as @_, and %ENV updated to include the contents of %env. The effect is similar to:

  my ($command, $env, @args) = $self->parseline($line);
  my $method = "do_$command";
  local %ENV = (%ENV, %$env);

  my $output = $self->$method(@args);

$output will be passed to $self->print() if it is defined.

Here is method that implements the env command:

  sub do_env {
      my ($self, @args) = @_;
      my @output;

      for (keys %ENV) {
          push @output, "$_=$ENV{$_}";

      return join "\n", @output;

And here is an rm command:

  sub do_rm {
      my ($self, @files) = @_;
      my ($file, @errors);

      for $file (@files) {
          unlink $file
              or push @errors, $file;

      if (@errors) {
          return "Couldnt delete " . join ", ", @errors;



    Quick Imports

If Shell::Base, or any Shell::Base subclass that does not does implement an import method, is invoked as:

  use My::Shell qw(shell);

a function named shell is installed in the calling package. This shell function is very simple, and turns this:


into this:


This is most useful for one-liners:

  $ perl -MMy::Shell=shell -e shell

    RC Files

The rcfile parser is simple, and parses (name, value) tuples from config files, according to these rules:
Definitions Most definitions are in name = value format:

  foo = bar
  baz = quux

Boolean defitions in the form


are allowed, and define wiffle as 1. Any definition without an = is considered a boolean definition. Boolean definitions in the form nowiffle define wiffle as 0:


Comments Everything after a # is considered a comment, and is stripped from the line immediately
Whitespace Whitespace is (mostly) ignored. The following are equivalent:

  foo    =    bar

Whitespace after the beginning of the value is not ignored:

  foo =    bar baz  quux

foo contains bar baz quux.

Line continuations Lines ending with \ are continued on the next line:

  form_letter = Dear %s,\
  How are you today? \
  Love, \

    Using Shell::Base Without readline

The appropriate methods to override in this case are:
init_rl The readline initialization method.
term Returns the Term::ReadLine instance; primarily used by the other methods listed in this section.
readline Returns the next line of input. Will be passed 1 argument, the prompt to display. See readline for an example of overriding readline.
print Called with the data to be printed. By default, this method prints to $self->term->OUT, but subclasses that aren’t using Term::ReadLine will want to provide a useful alternative. One possibily might be:

  sub print {
      my ($self, @print_me) = @_;

Another good example was given above, in pager:

  sub print {
      my ($self, @stuff) = @_;
      my $pager = $self->pager;

      open my $P, "|$pager" or carp "Cant open $pager: $!";
      CORE::print $P @stuff;
      close $P;


Some parts of this API will likely change in the future. In an upcoming version, do_$foo methods will mostly likely be expected to return a ($status, $output) pair rather than simply $output. Any API changes that are likely to break existing applications will be noted.


abbreviations Add abbreviation support, by default via Text::Abbrev, but overriddable, so that a shell can have (for example), \x type commands, or /x type commands. This can currently be done by overriding the precmd() method or parseline() methods; for example, this parseline() method strips a leading /, for IRC-like commands (/foo, /bar)

  sub parseline {
      my ($self, $line) = @_;
      my ($cmd, $env, @args) = $self->SUPER::parseline($line);

      $cmd =~ s:^/::;
      return ($cmd, $env, @args);

Another way to implement abbreviations would be to override the complete method.

command pipelines I have some ideas about how to implement pipelines, but, since I have yet to look at the code in any existing shells, I might be completely insane and totally on the wrong track. I therefore reserve the right to not implement this feature now, until I’ve looked at how some proper shells implement pipelines.


darren chamberlain <>


This documentation describes Shell::Base, $Revision: 1.5 $.


Copyright (C) 2003 darren chamberlain. All Rights Reserved.

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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