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Single-file packages

For small or script-like applications, DUB supports a special mode where the whole package is contained in a single .d file. The package recipe description can be embedded into code comments within the file:

#!/usr/bin/env dub
/+ dub.sdl:
    name "hello"
void main() {
    import std.stdio : writeln;
    writeln("Hello, World!");

Console example

$ chmod +x hello.d
$ ./hello.d
Hello, World!
# these are the same and will NOT show dub output: $ ./hello.d $ dub ./hello.d # these are the same, but WILL show dub output: $ dub --single ./hello.d $ dub run --single ./hello.d

This application can be executed by running dub run --single hello.d, or just compiled with dub build --single hello.d.

In addition to the normal method of passing commandline arguments, you can use the shorthand dub hello.d <arguments to hello>. This shorthand and the optional shebang allow you to run applications via ./hello <arguments> from your shell if you set the executable bit of the file.

Startup delay

Directly calling ./hello.d or using dub hello.d does not cache the built executable and will rebuild on every invocation. This may be fine for smaller scripts, but will add noticable start-up delay, especially when put behind something like keyboard shortcuts.

For repeatedly used CLI tools, prefer using dub build --single hello.d, which generates hello / hello.exe, which can be run immediately without compilation startup delay.

This may be fixed in the future, utilizing the temporary directory of the OS. See DUB#2672

Single-file packages cannot be used to create library packages.

Differences between dub hello.d and dub --single hello.d

When the --single flag is explicitly set, DUB will use the regular CLI parsing, so you need to specify custom arguments afterwards using -- such as dub --single hello.d -- my args here. This however also means that you can use all the dub commands and arguments to change how the build is done.

Without that flag, DUB has a special CLI parsing mode which will just pass-through all the arguments and not attempt to parse any of them itself. This means you don't have to pass a --, but also that you can't pass any DUB arguments to modify compilation or execution.

DUB uses the special parsing mode if any of those is the case:

  • the first argument is -
    • in which case the source code is read from stdin, temporarily saved and executed
  • the first argument ends with .d
  • the first argument is not a built-in command name, does not start with - and exists on file
  • the first argument is not a built-in command name, does not start with - and exists on file with a .d suffix
    • to support dub hello when hello.d is in the current folder - note this has some issues, see DUB#2681

Internal transformation in case of special parsing mode:

dub -
dub run -q --temp-build --single /tmp/app_[random UUID].d --
dub foo.d bar baz
dub run -q --temp-build --single foo.d -- bar baz

Last update: September 16, 2023
Created: September 1, 2023