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FHS Bundles.md

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Installed Bundles on UNIX-like Systems

This document covers {CF,NS,}Bundle behavior on Linux, with an eye to keeping things working for non-Linux OSes that have similar POSIX-y setups (in particular, the BSD family.) It covers two different styles of bundle, which are suitable both for things that are installed systemwide (e.g. as part of a platform-supplied package manager), and for freestanding bundles that are embedded (e.g. resource bundles) or executed (e.g. app bundles) but not installed systemwide.

The aim of this proposal is to provide idiomatic ways for POSIX-y systems to use bundle API, including resource access, in a way equivalent to what Darwin OSes do for bundles installed in /System or /Library.

Installed bundles

An installed bundle is intended to be installed in a directory hierarchy that models the use of --prefix=… in autoconf-like configuration tools. This is suitable for the kind of bundles that we would install in /System/Library or in /Applications, systemwide, on Darwin OSes: system frameworks, apps installed for all users, and so on.

This setup complies with the Filesystem Hierarchy Standard (FHS) 3.0, used by most Linux distributions. It also fits with the intent of the /usr and /usr/local directories of many BSD systems (e.g. from FreeBSD's hier(7).)

Definition

An installed bundle exists if there is:

  • A directory with the .resources extension;
  • contained in a directory named share.

The base name of this bundle is the name of this directory, removing the extension. (E.g., for /usr/share/MyFramework.resources, the base name is MyFramework.)

Bundle Paths

The bundle's .resources directory contains resources much like an iOS-style flat bundle. Unlike on iOS, however, executables are not contained directly inside this directory. Instead, given that the bundle exists in share/MyFramework.resources, then an installed bundle will source from the following additional paths within the prefix the bundle is installed in (the parent directory of the share directory):

  • The main executable will be searched in the bin, sbin, or lib… directories.
    • Executable bundles will search bin, then sbin, for an executable with the same base name as the resources folder, plus any platform prefix or suffix for executables. e.g.: bin/MyApp.
    • Framework bundles will search the appropriate lib… directories for a library of the same name, adding appropriate platform prefixes or suffixes for shared library file (falling back to just lib otherwise). e.g.: lib/libMyFramework.so.
  • Auxiliary executables and libraries will be in, or in subdirectories of, libexec/MyFramework.executables;
  • For framework bundles, include/MyFramework will contain headers, though this directory does affect the return values of the runtime API.

Both paths are optional. If they don't exist, the API will behave much like on Darwin platforms.

For installed bundles, the bundle's main executable is not searched within its .resources path. Instead, we expect that it be installed in the bin or sbin directory, if executable, or the appropriate lib… directory, if a shared library file. These files should be named the same as the main executable name (from Info.plist, or the framework's base name as a default value) plus any appropriate executable or library suffixes or prefixes for the platform. For example:

share/MyApp.resources
bin/MyApp

or:

share/MyFramework.resources
lib64/libMyFramework.so

The bundle path for an installed bundle is the same as its resources path. That's the path that needs to be passed into CFBundleCreate() and equivalents for the bundle to be successfully created; passing associated directory names will return NULL. ⊗

As an example, the following values are produced by invoking these functions on the bundle created with CFBundleCreate(NULL, …(…"/usr/local/share/MyFramework.resources")):

Function Path returned
CFBundleCopyBundleURL /usr/local/share/MyFramework.resources
CFBundleCopyExecutableURL /usr/local/lib/libMyFramework.so
CFBundleCopyResourcesDirectoryURL /usr/local/share/MyFramework.resources
CFBundleCopySharedSupportURL /usr/local/share/MyFramework.resources/SharedSupport
CFBundleCopyPrivateFrameworksURL /usr/local/libexec/MyFramework.executables/Frameworks
CFBundleCopySharedFrameworksURL /usr/local/libexec/MyFramework.executables/SharedFrameworks
CFBundleCopyBuiltInPlugInsURL /usr/local/libexec/MyFramework.executables/PlugIns
CFBundleCopyAuxiliaryExecutableURL(…, CFSTR("myexec")) /usr/local/libexec/MyFramework.executables/myexec
CFBundleCopyResourceURL(…, CFSTR("Welcome"), CFSTR("txt") …) /usr/local/share/MyFramework.resources/en.lproj/Welcome.txt

The structure inside any of these paths is the same as that of an iOS bundle containing the appropriate subset of files for its location; for example, the resources folder contains localization .lproj subdirectories, the Frameworks directory inside the ….executables contains frameworks, and so on.

Freestanding bundles

We will also support freestanding bundles on platforms that also support installed bundles. These bundles are structured much like their Windows counterparts, where there is a binary and a .resources directory on the side, like so:

./MyFramework.resources
./libMyFramework.so

Bundle Paths

The bundle path for an installed bundle is the same as its resources path. That's the path that needs to be passed into CFBundleCreate() and equivalents for the bundle to be successfully created; passing associated paths, including the path to the executable, will return NULL. ⊗

The .resources directory functions exactly like an iOS bundle, returning the same paths. By way of example, for a freestanding bundle created with CFBundleCreate(NULL, …(…"/opt/myapp/MyFramework.resources")):

Function Path returned
CFBundleCopyBundleURL /opt/myapp/MyFramework.resources
CFBundleCopyExecutableURL /opt/myapp/libMyFramework.so
CFBundleCopyResourcesDirectoryURL /opt/myapp/MyFramework.resources
CFBundleCopySharedSupportURL /opt/myapp/MyFramework.resources/SharedSupport
CFBundleCopyPrivateFrameworksURL /opt/myapp/MyFramework.resources/Frameworks
CFBundleCopySharedFrameworksURL /opt/myapp/MyFramework.resources/SharedFrameworks
CFBundleCopyBuiltInPlugInsURL /opt/myapp/MyFramework.resources/PlugIns
CFBundleCopyAuxiliaryExecutableURL(…, CFSTR("myexec")) /opt/myapp/MyFramework.resources/myexec
CFBundleCopyResourceURL(…, CFSTR("Welcome"), CFSTR("txt") …) /opt/myapp/MyFramework.resources/en.lproj/Welcome.txt

Embedded Frameworks

The two formats interact in the case of embedded bundles. Since the inside of any bundle is not compliant with the LHS, bundles inside other bundles must be freestanding frameworks. This includes frameworks in the private or shared frameworks paths, built-in plug-ins, and so on.

Alternatives considered

XDG

There are two filesystem specs that are relevant to Linux distribution: the Filesystem Hierarchy Specification (FHS), maintained by the Linux Foundation; and the XDG Base Directory Specification, maintained by freedesktop.org.

While both define where files can be installed on a Linux system, they differ substantially:

  • The FHS defines the directory structure for /, the root directory. In particular, it dictates where software and libraries are installed and the structure of /usr and /usr/local, which is where autoconf-style installations end up by default, and whose structure is mimicked when using ./configure --prefix=… to isolate app and library environments to a new prefix.

  • The XDG spec defines the directory structure for user data, such as preferences, within a single user's home directory. Compatible desktop systems will set environment variables to guide an application to write and read per-user information in paths the current user can read and write to. No part of this spec defines where code should be located, though we will have to heed it for such things as NSSearchPathsFor… and current-user defaults reading and writing.

In comparing the two, I produced a bundle structure suitable for use with the FHS so that it can be incorporated mostly as-is into the lifecycle of autoconf or CMake-driven development on Linux (i.e., making make install mostly just work).

Applications, both UI and server, are usually installed systemwide on Linux (in /usr or /usr/local), or in appropriate prefixes (generally under /opt) for specialized needs, and there isn't a concept of a standalone bundle that may end up in the home directory as it may happen with Mac apps, so the XDG spec becomes a little less relevant.

Use the executable's path as the bundle location

We tried this as our first attempt to avoid using the containing directory path for installed bundles, since all bundles would then have the same location. We moved away from this to the use of the .resources directory because it is likely that code will hardcode the assumption of a bundle location being a directory.

Footnotes

⊖: Should we mandate that an executable that could be in an installed bundle (because the binary is in a bin or lib… directory) must be in an installed bundle, e.g. that we won't fall back to searching for a freestanding bundle?

⊗: This is consistent with Darwin OSes' behavior of returning NULL for any path beside the bundle path, even if that path is the auxiliary or main executable path for some bundle.