This document describes how the AppBundle runtime operates, including how it reads its own information, extracts static tools, determines environment variables, and handles runtime flags.

Execution Flow

When an AppBundle is executed, the runtime performs the following steps:

1. Read Runtime Information:

   • The runtime reads the .pbundle_runtime_info section from the ELF file, which contains CBOR-encoded metadata.
   • This section includes:
     • AppBundleID: A unique identifier for the AppBundle. (e.g: “com.brave.Browser-xplshn-2025-05-19”. You’re not forced to follow this format, but if you do, you can create a dbin repository that countains your AppBundle by using our appstream-helper tool. $NAME-$MAINTAINER-$DATE or preferably: $APPSTREAM_ID-$MAINTAINER-$DATE, so that you don’t have to include an AppStream file within the AppDir for appstream-helper to get metadata from it)
     • PelfVersion: The version of the pelf tool used to create the AppBundle.
     • HostInfo: System information from uname -mrsp(v) of the build machine.
     • FilesystemType: Either “dwarfs” or “squashfs”.
     • Hash: A hash of the filesystem image for integrity verification.
     • DisableRandomWorkDir: A boolean indicating whether to use a fixed working directory.
     • MountOrExtract: A uint8 value (0–3) specifying the run behavior (see below).
   • The runtime uses this information to configure its behavior and locate the filesystem image.

2. Extract Static Tools:

   • The runtime accesses the static tools required for mounting or extracting the filesystem (e.g., dwarfs, dwarfsextract, squashfuse, unsquashfs).
   • The handling of static tools depends on the build mode:
     • noEmbed Edition: The tools are embedded in the .pbundle_static_tools ELF section as a ZSTD-compressed tar archive. The runtime determines the filesystem mounting and extraction commands at runtime, extracts the needed files from this archive to a temporary directory (cfg.staticToolsDir), and uses them to either mount or extract the filesystem.
     • Embed Edition: The tools are embedded directly in the binary using Go’s embed package, without compression. The runtime accesses these tools directly from the embedded filesystem, without needing to extract a compressed archive.

3. Exported Env Variables:

   • The runtime sets up several environment variables to facilitate execution:
     • HOME: If a portable home directory (.AppBundleID.home) exists in the same directory as the AppBundle, it is used as $HOME.
     • XDG_DATA_HOME: If a portable share directory (.AppBundleID.share) exists, it is used as $XDG_DATA_HOME.
     • XDG_CONFIG_HOME: If a portable config directory (.AppBundleID.config) exists, it is used as $XDG_CONFIG_HOME.
     • APPDIR: Set to the mount or extraction directory
     • SELF: The absolute path to the AppBundle executable.
     • ARGV0: The basename of $SELF
     • PATH: Augmented to include the AppBundle’s bin directory and the directory containing the static tools.

4. Mount or Extract Filesystem:

   • The runtime decides whether to mount or extract the filesystem image based on the MountOrExtract value:
     • 0: Mounts the filesystem using FUSE (e.g., dwarfs or squashfuse) and fails if FUSE is unavailable.
     • 1: Extracts the filesystem to a temporary directory (usually in tmpfs) and runs from there.
     • 2: Attempts to mount with FUSE; falls back to extraction if FUSE is unavailable.
     • 3: Similar to 2, but only falls back to extraction if the AppBundle is smaller than 350MB.

5. Execute the Application:

   • The runtime executes the AppRun script within the AppDir.
   • If a specific command is provided via --pbundle_link, the runtime executes that command within the AppBundle’s environment, instead of executing the AppRun.

Runtime Flags

The AppBundle runtime supports several command-line flags to modify its behavior:

• --pbundle_help: Displays help information, including the PelfVersion, HostInfo, and internal configuration variables (e.g., cfg.exeName, cfg.mountDir).
• --pbundle_list: Lists the contents of the AppBundle’s filesystem, including static tools.
• --pbundle_link <binary>: Executes a specified command within the AppBundle’s environment, leveraging its PATH and other variables.
• --pbundle_pngIcon: Outputs the base64-encoded .DirIcon (PNG) if it exists; otherwise, exits with error code 1.
• --pbundle_svgIcon: Outputs the base64-encoded .DirIcon.svg if it exists; otherwise, exits with error code 1.
• --pbundle_appstream: Outputs the base64-encoded first .xml file (AppStream metadata) found in the AppDir.
• --pbundle_desktop: Outputs the base64-encoded first .desktop file found in the AppDir.
• --pbundle_portableHome: Creates a portable home directory (.AppBundleID.home) in the same directory as the AppBundle.
• --pbundle_portableConfig: Creates a portable config directory (.AppBundleID.config) in the same directory as the AppBundle.
• --pbundle_cleanup: Unmounts and removes the AppBundle’s working directory and mount point, affecting only instances of the same AppBundle.
• --pbundle_mount: Mounts the filesystem to a specified or default directory and keeps the mount active.
• --pbundle_extract [globs]: Extracts the filesystem to a directory (default: <rExeName>_<filesystemType> or squashfs-root for AppImage compatibility). Supports selective extraction with glob patterns.
• --pbundle_extract_and_run: Extracts the filesystem and immediately executes the entrypoint.
• --pbundle_offset: Outputs the offset of the filesystem image within the AppBundle.
• AppImage Compatibility Flags:
  • --appimage-extract: Same as --pbundle_extract, but uses squashfs-root as the output directory.
  • --appimage-extract-and-run: Same as --pbundle_extract_and_run.
  • --appimage-mount: Same as --pbundle_mount.
  • --appimage-offset: Same as --pbundle_offset.

Notes

• The choice between noEmbed and embed modes affects how static tools are stored and accessed. The noEmbed mode uses a compressed archive for flexibility, while the embed mode simplifies access by avoiding compression.
• The AppRun script (e.g., AppRun.rootfs-based, AppRun.sharun, or AppRun.sharun.ovfsProto) determines sandboxing and execution behavior, such as using bwrap or unionfs-fuse.
• The runtime ensures cleanup of temporary directories unless --pbundle_cleanup is explicitly called or noCleanup is set.
• The noEmbed build tag for the appbundle-runtime allows you to build a single appbundle-runtime binary, that determines which filesystem to use at runtime, after having read its .pbundle_runtime_info and decompressed the .tar.zst data within the .pbundle_static_tools ELF section
  • If you’re writting a new runtime, I recommend you implement appbundle-runtime.go, cli.go and noEmbed.go. This edition of the runtime is the most portable and flexible. It is simplifies a lot the build process.