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Shell pipes without a shell but Raku.


use v6.d;
use Shell::Piping;

my int $exitcode = 0;
my &RED = $*OUT.t ?? { „\e[31m$_\e[0m“ } !! { $_ };

sub MAIN(Str $where = ‚/tmp/.‘) {
    my @result;
    my @err;

    px«find $where» |» { /a/ ?? $_ !! Nil } |» px<sort -r> |» @result :stderr(@err) :done({$exitcode ⚛= 1 if .exitcodes});

    .say for @result.head(10);

    if $exitcode {
        $*ERR.put: @err».&RED.join(„\n“);

    exit $exitcode;


This module provides the operator (aliased to |>>) to implement shell-like piping using Proc::Async objects, Code objects, Channel, Supply and custom objects. A quote construct like operator px is provided to create Proc::Async instances.

px<>, px«», px{}

These operators take a single argument without a space between px and the argument. It will then split the argument on white spaces. The first element is considered a command and the remaining elements arguments to that command. If a command does not contain a directory separator, %*ENV<PATH> will be searched for that command and the first hit used to create a Proc::Async. If a directory separator is used the first argument is assumed to be a IO::Path. In both cases the resulting file is checked for existence and filesystem access rights to execute it. The exceptions X::Shell::CommandNotFound and X::Shell::CommandNoAccess will be thrown when those tests fail. Please note that the file might be deleted between this check and the actual execution of the command. The semantics of the provided argument follow general Raku subscript rules. As such px<foo bar> and px«foo $bar» will generate an argument list automatically. While the code inside px{foo, bar} has to return that list by your effort.

my $proc = px<foo $not-interpolated>; # no interpolation, $PATH is queried
my $var = "42";
$proc = px«/usr/bin/meaning $var»; # interpolation and $PATH is not queried
$proc = px{ 'C:/WINDOWS/SYSTEM32/VIOLATE-PRIVACY'.subst('/', '\') ~ '.exe', secrets.txt };
await $proc.start;

It is not the resposibility of px to actually do anything with the resulting Proc::Async instance.

The adverb :timeout(Numerical) takes seconds with fractions and kills the spawned process when at least this time has passed. If the adverb is used Proc::Async::Timeout will be returned instead of Proc::Async. When the timeout is hit X::Proc::Async::Timeout will be thrown.

my @a;

loop {
    (px<curl https://www.raku.org>:timeout(60)) |» @a;
    CATCH {
        when X::Proc::Async::Timeout {
            put „{.command} timed out. Trying again and again and again.“;

multi infix:<|»> and multi infix:«|>>»

The MMD candidates of this operator take two arguments and return a Shell::Pipe object. This object implements .sink and .start, whereby the first will call the latter. When .sink is called all members of a pipe will be wired up, started in the right order and awaited. In sink context the whole pipe will block until the last Proc::Async returned from its .start method.

Members of a pipe can be Proc::Async, Code objects, Channel, Supply, IO::Handle, IO::Path and Array-like objects. The latter are identified by a subset.

subset Arrayish of Any where { !.isa('Code') && .^can(‚push‘) && .^can(‚list‘) }

Proc::Async has its STDOUT fed line-by-line to the next element in the pipe. If it is a RHS argument to its STDIN is written to with the output of the LHS. STDERR is left untouched unless the adverbs :quiet or :stderr are used.

px<find /tmp> |» px<sort> :quiet; # equivalent to `find /tmp 2>/dev/null | sort 2>/dev/null`;

To capture STDOUT and output to the terminal at the same time, set $*echo to on.

Code objects can be used at any place in a pipe. The semantics however vary. At the beginning of a pipe the object has to return an Iterable or implement .list. It will be called once and iterated over its return value. As such we support gather/take, sequence operators and many buildins. Each value returned from an iteration will be added a newline, encoded as utf8 and fed to the next member of the pipe. If a code object is in the middle of a pipe it will be called each time a line of text is produced to its left and its return value fed to the right. If Nil is returned this value will be skipped. At the end of the pipe the code object is called with each line produced by its left neighbour.

my @a;
{ 2,4,8 … 2**32 } |» px<sha256sum> |» @a;
px<find /tmp> |» { /a/ ?? .lc !! Nil } |» px<sort>;
px<find /tmp> |» { .say } :quiet;

Channel and Supplier/Supply can be used at the start and end of a pipe. If they are closed, the entire pipe will have STDIN/STDOUT closed. This allows a pipe to be controlled from the outside. Any case to complex for a Code object should therefore be handled with a Channel.

my $c = Channel.new;
my $sort = px<sort>;
start {
    await $sort.ready; # this line is optional
    for 1..∞ {
        $c.send: $^a;

Promise.in(60).then: { $c.close }; # a timeout
$c |» $sort |» px<uniq> |» { .say };

IO::Path objects are opened for reading at the begin of a pipe and for writing at the end. IO::Handle objects are expected to be open already and must be open for writing at the end. File handles will not be closed by the pipe.

px<find /tmp> |» px<sort> |» { .uc  } |» ‚/tmp/sorted.txt‘.IO :quiet;

Array-like objects can be used at both ends of a pipe. If used as a first element its .list method will be called and iterated. At the end of a pipe the .push method is called. That means lines from a LHS are always added to this object.

class Custom {
    has @.buffer;
    method push -> \v { @.buffer.push: v; @.buffer.shift if +@.buffer > 100; self }
    method list { @.buffer.list }

my $c = Custom.new;
px<find /usr -iname *.txt> |» $c;
$c |» px<sort> |» { .say };

Slurpy list deconstruction

List deconstruction is supported on the right hand side of , whereby a Whatever is required as a first or last element of the declarator list. Used as a first element Whatever indicated that elements at the beginning of the output are to be skipped. With Whatever as the last element, only the first n-elements are to be kept. This is useful if you are only interested in the first or last few elements of the output of a script.

px<find /usr -iname *.txt> |» my (Whatever, $second-last, $last-line);
px<find /usr -iname *.txt> |» out ($first-line, $, $third-line, Whatever);
px<find /usr -iname *.txt> |» my $first-line-only;

That declarator lists don't accept * is a limitation of Raku (for now).

A single scalar that is not Arrayish will have the first ouput line assigned. If there are more lines we warn.

To capture STDOUT and output to the terminal at the same time, set $*echo to on.


:done(&c(Shell::Pipe $pipe))

Will be called after the last command of a pipe has exited and before X::Shell::NonZeroExitcode will be thrown. The argument $pipe can be used for error handling via .exitcodes and introspection via .pipees.


This adverb redirects all STDERR into objects similar to what ‚|»‘ accepts. Error text is processed line by line and forwarded as a pair of (Int $index, Str $text). Whereby $index is the position of the pipee producing the text starting with 0.

px<find /usr> |» px<sort> |» @a :stderr(@err) :done({.exitcodes});
for @err.grep({.head == 0}) {
    say ‚find warned about: ‘, .Str;

To log to a file :stderr() takes an IO::Handle that is open for writing or a IO::Path that will be opened for writing. To close the handle call .stderr.close in the :done() callback.

Multiple targets for the STDERR stream can be provides with a Junction. For now only & junctions are supported. All targets will receive the same lines of text. Whereby no particular order should be assumed.

px<find /usr> |» px<sort> |» @a :stderr('logfile.txt'.IO & @err & Capture);

The value Capture can have an Int mixed in to limit capturing to the last n lines.

my $n = 10; # at most $n lines of STDERR are captured
px<find /usr> |» px<sort> |» @a :stderr(Capture but $n);


The adverb :quiet will gobble up all STDERR streams and discard them. This can be made the default by setting $*quiet to the exported symbol on.

my $*quiet = on;
my @a;
px<find -iname *.raku> |» @a;

Error handling

When any Proc::Async in a pipe finished with a non-zero exitcode the pipe returns a Failure of X::Shell::NonZeroExitcode. Calling .exitcode on the pipe will mark this Failure as handled. The callback in :done() is called before the Failure can throw. Handling exitcodes by hand has to go there. Individual exitcodes of pipe commands are stored in an Array with an index that corresponds to the commands position in the pipe. If STDERR output is captured with :stderr(Capture). The text per command is available, again as a list of ($idx, $text). This can be made the default by setting $*capture-stderr to the exported symbol on.

sub error-handler($pipe) {
    my @a = $pipe.exitcodes;
    for @a {
px«find /usr» |» px«sort» :done(&error-handler) :stderr(Capture);

The class Shell::Pipe::Exitcode supports smartmatching against Int, Str and Regex. This can be used for handling exceptions.

px«find /usr» |» px«sort» :stderr(Capture);

    when X::Shell::NonZeroExitcode { 
        for .pipe.exitcodes {
            when ‚find‘ & 1 & /‘(<![‘]>+)‘: Permission denied/ {
                say „did not look in $0“;



    when X::Shell::CommandNotFound {
        say .cmd ~ ‚was not found‘;
    when X::Shell::CommandNoAccess {
        say .cmd ~ ‚was unaccessable‘;
    when X::Shell::NonZeroExitcode {
        for .pipe.exitcodes {
            say .command, .exitcode, .pipe.stderr ~~ Capture ?? .STDERR !! ‚‘;
            when ‚find‘ & 1 & /‘(<![‘]>+)‘: Permission denied/ {
                say „did not look in $0“;
    when X::Shell::NoExitcodeYet {
        say .^name, „\n“, .message;

Refining Exceptions

The exceptions X::Shell::CommandNotFound and X::Shell::CommandNoAccess are refinable. This means the error message can be tweaked with the class method .refine. This method takes two Callables. When .message is called with the exception instance and expected to return Bool. On True the 2nd callback is called with the exception instance and supposed to return a text. This text will be used instead of the default text and returned from .message. Replacing this message will act on the class and even on created but yet to be thrown exceptions.

    (my &b = {.cmd eq ‚raku‘}),
    { ‚Please install Rakudo with `apt install rakudo`.‘ }
X::Shell::CommandNotFound.refine(&b, :revert);

The method .revert also takes one Callable and the adverb :revert to remove one refinement or all refinements with :revert-all.


Will be thrown by px«» or when the pipe is started if the file used as a command is not found. The meaning of "not found" depends on the OS. If the command was searched for in %*ENV<PATH>, that path will be shown in the exception message. This exception also checks for dangling symlinks and provides an alternate error message for this case.


Will be thrown by px«» or when the pipe is started if the file used as a command exists but can not be executed. Filesystem access rights depend on the OS.


This will be thrown after the last pipee exits and holds a Shell::Pipe in .pipe. If :stderr(Capture) is used the exception message contains all error text grouped by the shell command names. When an Int is mixed in, only that many lines will be captured.

The command line will be clipped at 180 characters. This limit can be changed by setting $*max-exitcode-command to any Int or Inf.


Will be thrown if .exitcodes is accessed before the pipe finished. Please note that filling the underlying Array is not atomic. When or after .done is called using .exitcodes is fine.

Colour control

Exceptions will print their error messges in red to STDERR if send to a terminal. This can be controlled by %*ENV<SHELLPIPINGNOCOLOR> and $*colored-exceptions. The environment variable can be set to any value. The dynamic variable to the exported symbols on and off whereby on is the default when the variable is not declared by any caller.

use Shell::Piping;
use Shell::Piping::Whereceptions;

sub s(IO(Str) $f where &it-is-a-file) {

my $*colored-exceptions = off;

my @a;
px<find /tmp/> |» @a;


Are subs to be used in where clauses to test for conditions that would throw later on. Whereceptions will output to STDERR in red unless %*ENV<SHELLPIPINGNOCOLOR> is set to any value. When sensible there will be checks for dangling symlinks and an alternate error message will be returned by the exceptions. All exceptions are subclasses of X::IO::Whereception.


sub works-with-files(IO::Path(Str) $file where &it-is-a-file) {
    say ‚answer‘ for $file.lines.grep(42);

sub works-with-directories(IO::Path(Str) $dir where &it-is-a-directory) {
    for $dir {
        .&works-with-files when .IO.f;
        .IO.dir()».&?BLOCK when .IO.d;


sub will-shell-out(IO::Path(Str) $file where &it-is-executable) {
    px<find -iname '42'> |» px«$file» |» (my @stdout);

sub it-is-a-file(IO() $f)

Will call .e and .f and throw X::IO::FileNotFound.

sub it-is-a-directory(IO() $d)

Will call .d and throw X::IO::DirectoryNotFound.

sub it-is-executable(IO() $exec)

Will call .x and throw X::IO::FileNotExecutable.


All files (unless noted otherwise) can be used, modified and redistributed under the terms of the Artistic License Version 2. Examples (in the documentation, in tests or distributed as separate files) can be considered public domain.

ⓒ2020 Wenzel P. P. Peppmeyer