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The root of your project must contain a configuration file named .buckconfig. Before executing, Buck2 reads this file to incorporate any customizations it specifies.

Performance impact of Buck2 configuration changes

Because configuration settings are sometimes included in the cache keys that Buck2 uses in its caching system, changes to Buck's configuration can invalidate previously-built artifacts in Buck's caches. If this occurs, Buck2 rebuilds those artifacts, which can impact your build time.

The .buckconfig file uses the INI file format

The .buckconfig file uses the INI file format. That is, it is divided into sections where each section contains a collection of key names and key values. The .buckconfig implementation supports some modifications to the INI file format; these are discussed below.

Other INI file parsers

As mentioned previously, we have extended the INI file parser that Buck2 uses to parse configuration files. As a result, INI file parsers provided by other languages or libraries are often not able to parse Buck's configuration files successfully.

Dot character not supported in section names

We do not support the use of the dot character (.) in section names within Buck2 configuration files. For example, the following is not supported—although Buck2 does not issue a warning or error.


Note that sometimes you might need to define your own custom sections, such as for platform flavors for C++ or Python. These scenarios are examples of when you should be careful not to introduce the dot character in section names. This constraint is because Buck2 uses the dot character to delimit section names and key names in other contexts such as the --config command-line parameter.

Character encoding

To ensure that any character can be encoded in a .buckconfig key value, you can use escape sequences to encode characters that would otherwise be problematic. The following escape sequences are supported.

\"double quote
\rcarriage return
\x##Unicode character with code point ## (in hex)
\u####Unicode character with code point #### (in hex)
\U########Unicode character with code point ######## (in hex)

Key values as lists

Although the standard INI format supports only key values that represent a single item, Buck2 supports key values that represent a list of items. The syntax is to separate the items in the list using the space (0x20) character. For example, a key value for the list of command-line flags to be passed to a compiler could be represented as a list of the flags separated by spaces:

flags = -foo -bar -baz -qux

When a key value is parsed as a list instead of a single item, the separator character is interpreted as a separator only when it occurs outside of double quotes. For example, if flags is a key value interpreted as a list of items separated by spaces, then

flags = -foo "-bar \u0429"

results in the two strings: foo and -bar Щ; the space character between -bar and \u0429 is not interpreted as a separator.

Transclusion of values from one key to another

Values from other keys can be transcluded into the current key using the following syntax inside the current key value.

$(config <section>.<field>)

For example, to use the [go].vendor_path in a custom setting:

[custom_section]custom_value = $(config go.vendor_path)


In addition to the semicolon (;), you can use the pound sign (#), as a comment character in .buckconfig.


The root of your project may contain a second configuration file named .buckconfig.local. Its format is the same as that of .buckconfig, but settings in .buckconfig.local override those in .buckconfig. In practice, .buckconfig is a version-controlled file that contains settings that are applicable to all team members, whereas .buckconfig.local is excluded from version control to allow users to define personal settings, such as personal aliases.

Other initialization files

In addition to the .buckconfig and .buckconfig.local files in the project root, Buck2 reads configuration settings from the following additional locations, some of which are actually directories:

  1. Directory .buckconfig.d located in the project root directory.
  2. File .buckconfig and directory .buckconfig.d located in the current user's home directory which, on Unix-like systems, is available from the HOME environment variable or through the ~ symbol.
  3. File buckconfig and directory buckconfig.d located in system directory /etc/.

Buck2 treats any file—irrespective of name—in a .buckconfig.d(buckconfig.d) directory (excluding files found in subdirectories) as a Buck2 configuration file, provided that it adheres to .buckconfig syntax. Note that a .buckconfig.d directory is distinct from the similarly-named .buckd directory which is used by the Buck2 Daemon (buckd) . For a description of how Buck2 resolves collisions between settings in these configuration files, see the section Precedence of Buck2 configuration specifications below.

Command-line control of configuration

In addition to the above configuration files, Buck2 supports specifying additional configuration files from the Buck2 command line using the --config-file parameter. You can also specify configuration settings individually on the Buck2 command line using the --config (-c) parameter. Furthermore, you can aggregate these settings into flag files using the --flagfile parameter. A flag file provides similar functionality to a configuration file but uses a different syntax. Flag files are sometimes called mode files or at (@) files.

Precedence of Buck2 configuration specifications

The following list shows the order of precedence for how Buck2 interprets its configuration specifications. Settings specified using a method closer to the top of the list have higher precedence and will override those lower on the list. For example, the .buckconfig file in the repo overrides a .buckconfig file in the user's HOME directory.

  1. Configuration specified on the command line using --config (-c), --config-file and --flagfile. Configuration specified later on the command line overrides configuration specified earlier.
  2. .buckconfig.local in the repo.
  3. .buckconfig in the repo.
  4. Files in a .buckconfig.d folder of the repo.
  5. .buckconfig.local in user's HOME directory.
  6. Files in a .buckconfig.d folder in user's HOME directory.
  7. The global file /etc/buckconfig
  8. Files in the global directory /etc/buckconfig.d

Files in a .buckconfig.d (buckconfig.d) directory have precedence according to the lexicographical order of their file names. Files later in the lexicographical order have precedence over files earlier in that order.

Configuration files can include other files

Any of the configuration files that we've discussed so far can also include by reference other files that contain configuration information. These included files can contain complete .buckconfig sections or they can contain a group of key name/value pairs that constitute part of a section. In this second use case, you'll need to ensure that the included file is referenced beneath the appropriate section in the including file. Because of this additional complexity, we recommend that you include only files that contain complete sections. Note: Inclusion of files is a Buck-specific extension to the INI file parser that Buck2 uses. Therefore, if you use this feature, your Buck2 configuration files will probably not be parsable by other more-generic INI file parsers. The syntax to include a file is


where path-to-included-file is either a relative path from the including file (recommended) or an absolute path from the root of the file system. You can also specify that the file should be included only if it exists by prefixing with a question mark (?).


If you use this prefix, it is not an error condition if the file does not exist; Buck2 just silently continues to process the rest of the configuration file. In the following example, the .buckconfig file includes the file cxx-other-platform.include which exists in the subdirectory cxx-other-platform. The .buckconfig file will also include the file future-platform from the directory future-platform.include if that file exists.

# .buckconfig
cxxppflags="-D MYMACRO=\"Buck\""


# cxx-other-platform.include
cxxppflags="-D MYMACRO=\"Watchman\""


Below is an incomplete list of supported buckconfigs.


This section contains definitions of build target aliases.

[alias]app     = //apps/myapp:app
apptest = //apps/myapp:test

These aliases can then be used from the command line:

$ buck2 build app
$ buck2 test apptest


Lists the cells that constitute the Buck2 project. Buck2 builds that are part of this project—that is, which use this .buckconfig—can access the cells specified in this section.

buck = .
bazel_skylib = ./third-party/skylark/bazel-skylib

The string on the left-hand side of the equals sign is the alias for the cell. The string on the right-hand side of the equals sign is the path to the cell from the directory that contains this .buckconfig file. It is not necessary to include the current cell in this section, but we consider it a best practice to do so:

buck = .

You can view the contents of this section using the buck2 audit cell command.

[repositories] is additionally supported as a deprecated alternative name for this section.