PERLDL(1)

📦

version 2.100.0 · 5 versions

💖 perldl — the manual, but make it cute 💖

okay bestie, gather round, we're reading the perldl(1) page from User Contributed Perl Documentation and I'm gonna hold your hand through every single line of it. no skimming, no skipping, we're doing this properly. 🫶

🌸 NAME 🌸

perldl — a simple shell for PDL. that's it. that's the whole name section. iconic. ✨

🦋 SYNOPSIS 🦋

Use PDL interactively:

bash$ perldl
pdl> $x = sequence(10) # or any other perl or PDL command

bash$ pdl
pdl> print "Hello, world!\n";

pdl> with_time { print +($A->matmult($B))->info, "\n" } for 1..5;
pdl> gv $A, 'output.png' # uses GraphViz2 to show connected transforms etc
pdl> 1+1 # no output
pdl> do_print 1 # or put this in your .perldlrc - as of 2.099
pdl> 1+1; # no output, like MATLAB
pdl> 1+1
2

Run a script:

bash$ cat > pdlscript
#!/usr/bin/pdl
print "Hello, world!\n";
...

👑 DESCRIPTION 👑

so the program perldl is a simple shell (written in perl, obvs) for using PDL interactively. it's a command-line interface that supports immediate interpretation of perl commands and expressions. perl expressions — including PDL constructs — can be typed straight at the keyboard and they get compiled and executed immediately. the syntax is not exactly identical to Perl, because under most circumstances ending a line causes immediate execution of the command you've entered so far. no trailing ; required, we're free 💅

the synonym pdl is a compiled executable that's useful as a script interpreter using UNIX shebang (#!) syntax. super handy for generating and re-executing command-journal files from perldl. 🎀

the perldl shell runs an initial startup file (~/.perldlrc) that you can use to pre-load perl modules or configure the global perl environment. it's got a path mechanism for autoloading perl subroutines, a command-history mechanism, and several other useful features like command preprocessing, shortcuts for commonly used commands such as print, and the ability to execute arbitrary code whenever a prompt is printed. main character energy fr fr. ✨

depending on your configuration settings, perldl can be set to honor or ignore the ^D (end-of-file) character when it's sent from a terminal, or to attempt to do the Right Thing when a block construct spanning multiple lines shows up. Right Thing behavior, we love to see it.

perldl and pdl support several command-line options, which are discussed near the end of this document. (stay tuned bestie, we're getting there 🌈)

📖 Reference manual & online help 📖

the PDL reference manual and online help are available from within perldl, using the help and apropos commands (which may also be abbreviated ? and ??.) the help command all by itself prints a summary of help syntax, and help <module-name> will print POD documentation from the module you mention. (POD is the Perl format for embedding documentation in your perl code; see perlpod for details.) 🧚

if you include POD documentation in your autoload subroutines (see path mechanism below), then both help and apropos will find it and be able to format and display it on demand. obsessed.

🕰️ History mechanism 🕰️

if you have the perl modules ReadLines and ReadKeys installed, then perldl supports a history and line-editing mechanism using editing keys similar to emacs. the last 500 commands are always stored in the file .perldl_hist in your home directory between sessions. set $PERLDL::HISTFILESIZE to change the number of lines saved. the command l [number] shows you the last number commands you typed, where number defaults to 20. 💖

e.g.:

bash$ perldl
ReadLines enabled
pdl> $x = rfits "foo.fits"
BITPIX =  -32  size = 88504 pixels
Reading  354016 bytes
BSCALE =  &&  BZERO =

pdl> imag log($x+400)
Displaying 299 x 296 image from 4.6939525604248 to 9.67116928100586 ...

⚡ Command execution ⚡

if you enter a simple command at the perldl command line, it is immediately executed in a Perl eval(). the environment is almost identical to that within a perl script, with some important exceptions:

$_ is not preserved across lines

$_ is used to hold the command line for initial processing, so at the beginning of processing of each command line, $_ contains the command itself. use variables other than $_ to store values across lines. no cap.

Scope is not preserved across lines

each command line is executed in a separate eval block within perl, so scoping commands such as my and local may not perform exactly as expected — in particular, if you declare a variable with my, it is local to the particular command line on which you typed the my command, which means that it will evaporate before the next prompt is printed. (you can use my variables in a multi-line block or to isolate values within a single command line, of course 💅)

Execution is immediate

under most circumstances, as soon as you end a line of input the line is parsed and executed. this breaks Perl's normal dependence on semicolons as command delimiters. for example, the two-line expression

print "Hello ",
   "world";

prints the phrase Hello world in Perl, but (under most circumstances) Hello in perldl. 😭

Multi-line execution

in multiline mode (which is enabled by default, see Shell variables, below), perldl searches for searches for block-like constructs with curly braces, parentheses, quotes, and related delimiters. if you leave such a construct open, perldl accepts more lines of input until you close the construct or explicitly end the multi-line expression with ^D. following the example above, the phrase

{ print "Hello ",
     "world"; }

will print Hello world from either Perl or (in multi-line mode) perldl. slay.

⚠️ Warning: the multi-line parsing uses Damian Conway's Text::Balanced module, which contains some flaws — so it can be fooled by quote-like operators such as q/.../, included POD documentation, multi-line << quotes, and some particularly bizarre-but-valid m/.../ matches and s/.../.../ substitutions. in such cases, use ^D to close out the multi-line construct and force compilation-and-execution. 💀

if you want to preserve this behavior in a script (for example to replay a command journal file; see below on how to create one), you can use pdl instead of perl as the interpreter in the script's initial shebang line. 🎀

🚪 Terminating `perldl` 🚪

a perldl session can be terminated with any of the commands quit, exit or the shorthands x or q. if EOF handling is switched on (the default) you can also type ^D at the command prompt. bye bestie 👋

if the command input is NOT a terminal (for example if you're running from a command journal file), then EOF will always terminate perldl.

🛑 Terminating commands (Ctrl-C handling) 🛑

commands executed within perldl can be terminated prematurely using Ctrl-C (or whichever key sequence sends an INT signal to the process on your terminal). provided your PDL code does not ignore sigints this should throw you back at the perldl command prompt:

pdl> $result = start_lengthy_computation()
 <Ctrl-C>
Ctrl-C detected

pdl>

as of 2.077, this requires Sys::SigAction to be installed (without that, and before 2.077, for Perl >5.8 it didn't actually interrupt things). noted. 📝

✂️ Shortcuts and aliases ✂️

the shell aliases p to be a convenient short form of print, e.g.

pdl> p ones 5,3

[
 [1 1 1 1 1]
 [1 1 1 1 1]
 [1 1 1 1 1]
]

q and x are short-hand for quit.
l lists the history buffer

pdl> l # list last 20 commands

pdl> l 40 # list last 40 commands

? is an alias for help

pdl> ? vars    # show information about ndarrays in current namespace

?? is an alias for apropos

pdl> ?? PDL::Doc

help, apropos, usage and sig: all words after these commands are used verbatim and not evaluated by perl. so you can write, e.g.,

pdl> help help

instead of

pdl> help 'help'

with_time runs the following code-block, and tells you how long it took, in milliseconds. requires Time::HiRes.

pdl> with_time { print +($A->matmult($B))->info, "\n" } for 1..5;

gv uses pdumpgraph in PDL::Core etc to visualise the given ndarray, with all connected transformations, etc, into the given file. requires Graph and GraphViz2.

pdl> gv $A, 'output.png'

🎛️ Command-line options 🎛️

perldl and pdl support several command-line options to adjust the behavior of the session. most of them are equivalent to commands that can be entered at the pdl> prompt. they are:

-glut — load OpenGL when starting the shell (the perl OpenGL module, which is available from CPAN, must be installed). this enables readline event loop processing. Don't use with -tk.
-tk — load Tk when starting the shell (the perl Tk module, which is available from CPAN, must be installed). this enables readline event loop processing. Don't use with -glut.
-f file — loads the file before processing any user input. any errors during the execution of the file are fatal. 😭
-w — runs with warning messages (i.e. the normal perl -w warnings) turned-on.
-M module — loads the module before processing any user input. compare corresponding perl switch.
-m module — unloads the module before processing any user input.
-I directory — adds directory to the include path. (i.e. the @INC array) compare corresponding perl switch.
-V — prints a summary of PDL config. this information should be included with any PDL bug report. compare corresponding perl switch.

🏠 The startup file `~/.perldlrc` 🏠

if the file ~/.perldlrc is found it is sourced at start-up to load default modules, set shell variables, etc. if it is NOT found the distribution file PDL/default.perldlrc is read instead. this loads various modules considered useful by default, and which ensure compatibility with v1.11. if you don't like this and want a more streamlined set of your own favourite modules, simple create your own ~/.perldlrc. you may wish to start from the existing PDL/default.perldlrc as a template since it will not be sourced once you replace it with your own version. 🎀

to set even more local defaults the file local.perldlrc (in the current directory) is sourced if found. this lets you load modules and define subroutines for the project in the current directory. 💅

the name is chosen specifically because it was found hidden files were NOT wanted in these circumstances. periodt.

the startup file should normally include use PDL::AutoLoader;, as many of the nicer interactive features won't work without it. 👑

🐚 Shell variables 🐚

Shell variables: (Note: if you don't like the defaults change them in ~/.perldlrc)

$PERLDL::ESCAPE - default value #
Any line starting with this character is treated as a shell escape. The default value is chosen because it escapes the code from the standard perl interpreter. 🐚
$PERLDL::HISTFILESIZE — default value 500. This is how many lines of perldl shell command history we keep, bestie. Your receipts, preserved. 📜
$PERLDL::PAGER — default value "more". It's the external program that filters the output of commands. Using "more" prints output one screenful at a time. On Unix, setting page(1) and $PERLDL::PAGER to "tee -a outfile" will keep a record of the output generated by subsequent perldl commands (without paging). ✨
$PERLDL::PROMPT — default value 'pdl> '. Enough said 💅 But it can also be set to a subroutine reference, e.g. $PERLDL::PROMPT = sub {join(':',(gmtime)[2,1,0]).'> '} puts the current time into the prompt. Iconic behavior honestly.
$PERLDL::MULTI — default value 1. If this is set to a true value, then perldl will parse multi-line perl blocks: your input will not be executed until you finish a line with no outstanding group operators (such as quotes, blocks, parenthesis, or brackets) still active. Continuation lines have a different prompt that shows you what delimiters are still active. So you always know what's still open. 🫶

Note that this is not (yet!) a complete perl parser. In particular, Text::Balanced appears to be able to ignore quoting operators like "q/ ... /" within a line, but not to be able to extend them across lines. Likewise, there is no support for the '<<' operator.

Multiline conventional strings and {}, [], and () groupings are well supported.

$PERLDL::NO_EOF — default value 0 / 1 on MSWin32. Protects against accidental use of "^D" from the terminal. If this is set to a true value, then you can't accidentally exit perldl by typing "^D". If you set it to a value larger than 1 (and PERLDL::MULTI is set), then you can't use "^D" to exit multiline commands either. If you're piping commands in from a file or pipe, this variable has no effect. Safety rails, periodt. 🛡️
$HOME — the user's home directory. 🏠
$PERLDL::TERM — this is the Term::ReadLine object associated with the perldl shell. It can be used by routines called from perldl if your command is interactive.
$PDL::toolongtoprint — the maximal size pdls to print (defaults to 10,000 elements). This is not just a "perldl" variable but it is something that is usually needed in an interactive debugging session. Lowkey a lifesaver when your array is the size of a small galaxy. 🌌

✨ Executing scripts from the "perldl" prompt ✨

A useful idiom for developing perldl scripts or editing functions on-line is

pdl> # emacs script &
  -- add perldl code to script and save the file

pdl> do 'script'

-- substitute your favourite window-based editor for 'emacs' (you may also need to change the '&' on non-Unix systems). Pick your fighter, no judgment. 🎀

Running do 'script' again updates any variables and function definitions from the current version of 'script'. Live edits, instantly reflected. It's giving hot reload. 🔥

🚀 Executing perldl scripts from the command line 🚀

PDL scripts are just perl scripts that happen to use PDL (and possibly PDL::NiceSlice). But for the truly lazy, perldl can be invoked as a script interpreter. Because perldl is itself an interpreted perl script, most unices won't allow you to say #!/usr/bin/perldl at the top of your script.

Instead, say #!/usr/bin/pdl and your script will be executed exactly as if you typed it, line-by-line, into the perldl shell. Chef's kiss. 👑

🧪 Command preprocessing 🧪

NOTE: This feature is used by default by PDL::NiceSlice. See below for more about slicing at the "perldl" prompt.

In some cases, it is convenient to process commands before they are sent to perl for execution. For example, this is the case where the shell is being presented to people unfamiliar with perl but who wish to take advantage of commands added locally (eg by automatically quoting arguments to certain commands).

NOTE: The preprocessing interface has changed from earlier versions! The old way using $PERLDL::PREPROCESS will still work but is strongly deprecated and might go away in the future. So maybe don't, bestie. 💀

You can enable preprocessing by registering a filter with the preproc_add function. preproc_add takes one argument which is the filter to be installed. A filter is a Perl code reference (usually set in a local configuration file) that will be called, with the current command string as argument, just prior to the string being executed by the shell. The modified string should be returned. Note that you can make perldl completely unusable if you fail to return the modified string; quitting is then your only option. 😭 No cap — that's a real trap.

Filters can be removed from the preprocessing pipeline by calling preproc_del with the filter to be removed as argument. To find out if a filter is currently installed in the preprocessing pipeline use preproc_registered:

pdl> preproc_add $myfilter unless preproc_registered $myfilter;

Previous versions of perldl used the variable $PERLDL::PREPROCESS. This will still work but should be avoided. Please change your scripts to use the preproc_add etc functions.

The following code would check for a call to function 'mysub' and bracket arguments with qw.

$filter = preproc_add sub {
   my $str = shift;
   $str =~ s/^\s+//;  # Strip leading space
   if ($str =~ /^mysub/) {
      my ($command, $arguments) = split(/\s+/,$str, 2);
      $str = "$command qw( $arguments )"
      if (defined $arguments && $arguments !~ /^qw/);
   };
   # Return the input string, modified as required
   return $str;
 };

This would convert:

pdl> mysub arg1 arg2

pdl> mysub qw( arg1 arg2 )

which Perl will understand as a list. Obviously, a little more effort is required to check for cases where the caller has supplied a normal list (and so does not require automatic quoting) or variable interpolation is required. So yeah, read the room before you auto-quote. 🙈

You can remove this preprocessor using the preproc_del function which takes one argument (the filter to be removed, it must be the same coderef that was returned from a previous preproc_add call):

pdl> preproc_del $filter;

An example of actual usage can be found in the perldl script. Look at the function trans to see how the niceslicing preprocessor is enabled/disabled. Go peek, it's cute. 🦋

🌸 "perldl" and PDL::NiceSlice 🌸

PDL::NiceSlice introduces a more convenient slicing syntax for ndarrays. In the current version of perldl niceslicing is enabled by default (if the required CPAN modules are installed on your machine).

At startup perldl will let you know if niceslicing is enabled. The startup message will contain info to this end, something like this:

perlDL shell v1.XX
 PDL comes with ABSOLUTELY NO WARRANTY. For details, see the file
 'COPYING' in the PDL distribution. This is free software and you
 are welcome to redistribute it under certain conditions, see
 the same file for details.
ReadLines, NiceSlice  enabled
Reading /home/csoelle/.perldlrc...
Type 'demo' for online demos
Loaded PDL v2.XX

When you get such a message that indicates "NiceSlice" is enabled you can use the enhanced slicing syntax:

pdl> $x = sequence 10;
pdl> p $x(3:8:2)

For details consult PDL::NiceSlice.

PDL::NiceSlice installs a filter in the preprocessing pipeline (see above) to enable the enhanced slicing syntax. You can use a few commands in the perldl shell to switch this preprocessing on or off and also explicitly check the substitutions that the NiceSlice filter makes.

You can switch the PDL::NiceSlice filter on and off by typing

pdl> trans # switch niceslicing on

and

pdl> notrans # switch niceslicing off

respectively. The filter is on by default. 💖

To see how your commands are translated switch reporting on:

pdl> report 1;
pdl> p $x(3:8:2)
processed p $x->slice([3,8,2])
[3 5 7]

Similarly, switch reporting off as needed

pdl> report 0;
pdl>  p $x(3:8:2)
[3 5 7]

Reporting is off by default.

🎀 Automatically execute your own hooks 🎀

The variable @PERLDL::AUTO is a simple list of perl code strings and/or code reference. It is used to define code to be executed automatically every time the user enters a new line.

A simple example would be to print the time of each command:

pdl> push @PERLDL::AUTO,'print scalar(gmtime),"\n"'

pdl> print zeroes(3,3)
Sun May  3 04:49:05 1998

[
 [0 0 0]
 [0 0 0]
 [0 0 0]
]

pdl> print "Boo"
Sun May  3 04:49:18 1998
Boo
pdl>

Or to make sure any changes in the file 'local.perldlrc' are always picked up :-

pdl> push @PERLDL::AUTO,"do 'local.perldlrc'"

This code can of course be put in 'local.perldlrc', but be careful :-) [Hint: add "unless ($started++)" to above to ensure it only gets done once!] Because yes, infinite loops are not the vibe. 🌀

Another example application is as a hook for Autoloaders (e.g. PDL::AutoLoader) to add code too which allows them to automatically re-scan their files for changes. This is extremely convenient at the interactive command line. Since this hook is only in the shell it imposes no inefficiency on PDL scripts.

Finally note this is a very powerful facility - which means it should be used with caution! 👑

And that's a wrap on perldl(1) bestie — this page ships with perl v5.42.0 and is dated 2025-07-25. Go forth and slice some ndarrays, you absolute main character. 🫶💜

📦 where this page comes from

fedora 43: perl-PDL 2.100.0-3.fc43 · GPL-1.0-or-later OR Artistic-1.0-Perl
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-13: a language model's rewrite of the original, under its terms