3 Syntax and Operators

BitBake files have their own syntax. The syntax has similarities to several other languages but also has some unique features. This section describes the available syntax and operators as well as provides examples.

3.1 Basic Syntax

This section provides some basic syntax examples.

3.1.1 Basic Variable Setting

The following example sets VARIABLE to “value”. This assignment occurs immediately as the statement is parsed. It is a “hard” assignment.

VARIABLE = "value"

As expected, if you include leading or trailing spaces as part of an assignment, the spaces are retained:

VARIABLE = " value"
VARIABLE = "value "

Setting VARIABLE to “” sets it to an empty string, while setting the variable to “ “ sets it to a blank space (i.e. these are not the same values).


You can use single quotes instead of double quotes when setting a variable’s value. Doing so allows you to use values that contain the double quote character:

VARIABLE = 'I have a " in my value'


Unlike in Bourne shells, single quotes work identically to double quotes in all other ways. They do not suppress variable expansions.

3.1.2 Modifying Existing Variables

Sometimes you need to modify existing variables. Following are some cases where you might find you want to modify an existing variable:

  • Customize a recipe that uses the variable.

  • Change a variable’s default value used in a *.bbclass file.

  • Change the variable in a *.bbappend file to override the variable in the original recipe.

  • Change the variable in a configuration file so that the value overrides an existing configuration.

Changing a variable value can sometimes depend on how the value was originally assigned and also on the desired intent of the change. In particular, when you append a value to a variable that has a default value, the resulting value might not be what you expect. In this case, the value you provide might replace the value rather than append to the default value.

If after you have changed a variable’s value and something unexplained occurs, you can use BitBake to check the actual value of the suspect variable. You can make these checks for both configuration and recipe level changes:

  • For configuration changes, use the following:

    $ bitbake -e

    This command displays variable values after the configuration files (i.e. local.conf, bblayers.conf, bitbake.conf and so forth) have been parsed.


    Variables that are exported to the environment are preceded by the string “export” in the command’s output.

  • To find changes to a given variable in a specific recipe, use the following:

    $ bitbake recipename -e | grep VARIABLENAME=\"

    This command checks to see if the variable actually makes it into a specific recipe.

3.1.3 Line Joining

Outside of functions, BitBake joins any line ending in a backslash character (”\”) with the following line before parsing statements. The most common use for the “\” character is to split variable assignments over multiple lines, as in the following example:

FOO = "bar \
       baz \

Both the “\” character and the newline character that follow it are removed when joining lines. Thus, no newline characters end up in the value of FOO.

Consider this additional example where the two assignments both assign “barbaz” to FOO:

FOO = "barbaz"
FOO = "bar\


BitBake does not interpret escape sequences like “\n” in variable values. For these to have an effect, the value must be passed to some utility that interprets escape sequences, such as printf or echo -n.

3.1.4 Variable Expansion

Variables can reference the contents of other variables using a syntax that is similar to variable expansion in Bourne shells. The following assignments result in A containing “aval” and B evaluating to “preavalpost”.

A = "aval"
B = "pre${A}post"


Unlike in Bourne shells, the curly braces are mandatory: Only ${FOO} and not $FOO is recognized as an expansion of FOO.

The “=” operator does not immediately expand variable references in the right-hand side. Instead, expansion is deferred until the variable assigned to is actually used. The result depends on the current values of the referenced variables. The following example should clarify this behavior:

A = "${B} baz"
B = "${C} bar"
C = "foo"
*At this point, ${A} equals "foo bar baz"*
C = "qux"
*At this point, ${A} equals "qux bar baz"*
B = "norf"
*At this point, ${A} equals "norf baz"*

Contrast this behavior with the Immediate variable expansion (:=) operator.

If the variable expansion syntax is used on a variable that does not exist, the string is kept as is. For example, given the following assignment, BAR expands to the literal string “${FOO}” as long as FOO does not exist.

BAR = "${FOO}"

3.1.5 Setting a default value (?=)

You can use the “?=” operator to achieve a “softer” assignment for a variable. This type of assignment allows you to define a variable if it is undefined when the statement is parsed, but to leave the value alone if the variable has a value. Here is an example:

A ?= "aval"

If A is set at the time this statement is parsed, the variable retains its value. However, if A is not set, the variable is set to “aval”.


This assignment is immediate. Consequently, if multiple “?=” assignments to a single variable exist, the first of those ends up getting used.

3.1.6 Setting a weak default value (??=)

The weak default value of a variable is the value which that variable will expand to if no value has been assigned to it via any of the other assignment operators. The “??=” operator takes effect immediately, replacing any previously defined weak default value. Here is an example:

W ??= "x"
A := "${W}" # Immediate variable expansion
W ??= "y"
B := "${W}" # Immediate variable expansion
W ??= "z"
C = "${W}"
W ?= "i"

After parsing we will have:

A = "x"
B = "y"
C = "i"
W = "i"

Appending and prepending non-override style will not substitute the weak default value, which means that after parsing:

W ??= "x"
W += "y"

we will have:

W = " y"

On the other hand, override-style appends/prepends/removes are applied after any active weak default value has been substituted:

W ??= "x"
W:append = "y"

After parsing we will have:

W = "xy"

3.1.7 Immediate variable expansion (:=)

The “:=” operator results in a variable’s contents being expanded immediately, rather than when the variable is actually used:

T = "123"
A := "test ${T}"
T = "456"
B := "${T} ${C}"
C = "cval"
C := "${C}append"

In this example, A contains “test 123”, even though the final value of T is “456”. The variable B will end up containing “456 cvalappend”. This is because references to undefined variables are preserved as is during (immediate)expansion. This is in contrast to GNU Make, where undefined variables expand to nothing. The variable C contains “cvalappend” since ${C} immediately expands to “cval”.

3.1.8 Appending (+=) and prepending (=+) With Spaces

Appending and prepending values is common and can be accomplished using the “+=” and “=+” operators. These operators insert a space between the current value and prepended or appended value.

These operators take immediate effect during parsing. Here are some examples:

B = "bval"
B += "additionaldata"
C = "cval"
C =+ "test"

The variable B contains “bval additionaldata” and C contains “test cval”.

3.1.9 Appending (.=) and Prepending (=.) Without Spaces

If you want to append or prepend values without an inserted space, use the “.=” and “=.” operators.

These operators take immediate effect during parsing. Here are some examples:

B = "bval"
B .= "additionaldata"
C = "cval"
C =. "test"

The variable B contains “bvaladditionaldata” and C contains “testcval”.

3.1.10 Appending and Prepending (Override Style Syntax)

You can also append and prepend a variable’s value using an override style syntax. When you use this syntax, no spaces are inserted.

These operators differ from the “:=”, “.=”, “=.”, “+=”, and “=+” operators in that their effects are applied at variable expansion time rather than being immediately applied. Here are some examples:

B = "bval"
B:append = " additional data"
C = "cval"
C:prepend = "additional data "
D = "dval"
D:append = "additional data"

The variable B becomes “bval additional data” and C becomes “additional data cval”. The variable D becomes “dvaladditional data”.


You must control all spacing when you use the override syntax.


The overrides are applied in this order, “:append”, “:prepend”, “:remove”.

It is also possible to append and prepend to shell functions and BitBake-style Python functions. See the “Shell Functions” and “BitBake-Style Python Functions” sections for examples.

3.1.11 Removal (Override Style Syntax)

You can remove values from lists using the removal override style syntax. Specifying a value for removal causes all occurrences of that value to be removed from the variable. Unlike “:append” and “:prepend”, there is no need to add a leading or trailing space to the value.

When you use this syntax, BitBake expects one or more strings. Surrounding spaces and spacing are preserved. Here is an example:

FOO = "123 456 789 123456 123 456 123 456"
FOO:remove = "123"
FOO:remove = "456"
FOO2 = " abc def ghi abcdef abc def abc def def"
FOO2:remove = "\
    def \
    abc \
    ghi \

The variable FOO becomes “ 789 123456 “ and FOO2 becomes “ abcdef “.

Like “:append” and “:prepend”, “:remove” is applied at variable expansion time.


The overrides are applied in this order, “:append”, “:prepend”, “:remove”. This implies it is not possible to re-append previously removed strings. However, one can undo a “:remove” by using an intermediate variable whose content is passed to the “:remove” so that modifying the intermediate variable equals to keeping the string in:

FOOREMOVE = "123 456 789"
FOO:remove = "${FOOREMOVE}"
FOOREMOVE = "123 789"

This expands to FOO:remove = "123 789".


Override application order may not match variable parse history, i.e. the output of bitbake -e may contain “:remove” before “:append”, but the result will be removed string, because “:remove” is handled last.

3.1.12 Override Style Operation Advantages

An advantage of the override style operations “:append”, “:prepend”, and “:remove” as compared to the “+=” and “=+” operators is that the override style operators provide guaranteed operations. For example, consider a class foo.bbclass that needs to add the value “val” to the variable FOO, and a recipe that uses foo.bbclass as follows:

inherit foo
FOO = "initial"

If foo.bbclass uses the “+=” operator, as follows, then the final value of FOO will be “initial”, which is not what is desired:

FOO += "val"

If, on the other hand, foo.bbclass uses the “:append” operator, then the final value of FOO will be “initial val”, as intended:

FOO:append = " val"


It is never necessary to use “+=” together with “:append”. The following sequence of assignments appends “barbaz” to FOO:

FOO:append = "bar"
FOO:append = "baz"

The only effect of changing the second assignment in the previous example to use “+=” would be to add a space before “baz” in the appended value (due to how the “+=” operator works).

Another advantage of the override style operations is that you can combine them with other overrides as described in the “Conditional Syntax (Overrides)” section.

3.1.13 Variable Flag Syntax

Variable flags are BitBake’s implementation of variable properties or attributes. It is a way of tagging extra information onto a variable. You can find more out about variable flags in general in the “Variable Flags” section.

You can define, append, and prepend values to variable flags. All the standard syntax operations previously mentioned work for variable flags except for override style syntax (i.e. “:prepend”, “:append”, and “:remove”).

Here are some examples showing how to set variable flags:

FOO[a] = "abc"
FOO[b] = "123"
FOO[a] += "456"

The variable FOO has two flags: [a] and [b]. The flags are immediately set to “abc” and “123”, respectively. The [a] flag becomes “abc 456”.

No need exists to pre-define variable flags. You can simply start using them. One extremely common application is to attach some brief documentation to a BitBake variable as follows:

CACHE[doc] = "The directory holding the cache of the metadata."


Variable flag names starting with an underscore (_) character are allowed but are ignored by d.getVarFlags("VAR") in Python code. Such flag names are used internally by BitBake.

3.1.14 Inline Python Variable Expansion

You can use inline Python variable expansion to set variables. Here is an example:

DATE = "${@time.strftime('%Y%m%d',time.gmtime())}"

This example results in the DATE variable being set to the current date.

Probably the most common use of this feature is to extract the value of variables from BitBake’s internal data dictionary, d. The following lines select the values of a package name and its version number, respectively:

PN = "${@bb.parse.vars_from_file(d.getVar('FILE', False),d)[0] or 'defaultpkgname'}"
PV = "${@bb.parse.vars_from_file(d.getVar('FILE', False),d)[1] or '1.0'}"


Inline Python expressions work just like variable expansions insofar as the “=” and “:=” operators are concerned. Given the following assignment, foo() is called each time FOO is expanded:

FOO = "${@foo()}"

Contrast this with the following immediate assignment, where foo() is only called once, while the assignment is parsed:

FOO := "${@foo()}"

For a different way to set variables with Python code during parsing, see the “Anonymous Python Functions” section.

3.1.15 Unsetting variables

It is possible to completely remove a variable or a variable flag from BitBake’s internal data dictionary by using the “unset” keyword. Here is an example:

unset DATE
unset do_fetch[noexec]

These two statements remove the DATE and the do_fetch[noexec] flag.

3.1.16 Providing Pathnames

When specifying pathnames for use with BitBake, do not use the tilde (“~”) character as a shortcut for your home directory. Doing so might cause BitBake to not recognize the path since BitBake does not expand this character in the same way a shell would.

Instead, provide a fuller path as the following example illustrates:

    /home/scott-lenovo/LayerA \

3.2 Exporting Variables to the Environment

You can export variables to the environment of running tasks by using the export keyword. For example, in the following example, the do_foo task prints “value from the environment” when run:

ENV_VARIABLE = "value from the environment"

do_foo() {
    bbplain "$ENV_VARIABLE"


BitBake does not expand $ENV_VARIABLE in this case because it lacks the obligatory {} . Rather, $ENV_VARIABLE is expanded by the shell.

It does not matter whether export ENV_VARIABLE appears before or after assignments to ENV_VARIABLE.

It is also possible to combine export with setting a value for the variable. Here is an example:

export ENV_VARIABLE = "variable-value"

In the output of bitbake -e, variables that are exported to the environment are preceded by “export”.

Among the variables commonly exported to the environment are CC and CFLAGS, which are picked up by many build systems.

3.3 Conditional Syntax (Overrides)

BitBake uses OVERRIDES to control what variables are overridden after BitBake parses recipes and configuration files. This section describes how you can use OVERRIDES as conditional metadata, talks about key expansion in relationship to OVERRIDES, and provides some examples to help with understanding.

3.3.1 Conditional Metadata

You can use OVERRIDES to conditionally select a specific version of a variable and to conditionally append or prepend the value of a variable.


Overrides can only use lower-case characters, digits and dashes. In particular, colons are not permitted in override names as they are used to separate overrides from each other and from the variable name.

  • Selecting a Variable: The OVERRIDES variable is a colon-character-separated list that contains items for which you want to satisfy conditions. Thus, if you have a variable that is conditional on “arm”, and “arm” is in OVERRIDES, then the “arm”-specific version of the variable is used rather than the non-conditional version. Here is an example:

    OVERRIDES = "architecture:os:machine"
    TEST = "default"
    TEST:os = "osspecific"
    TEST:nooverride = "othercondvalue"

    In this example, the OVERRIDES variable lists three overrides: “architecture”, “os”, and “machine”. The variable TEST by itself has a default value of “default”. You select the os-specific version of the TEST variable by appending the “os” override to the variable (i.e. TEST:os).

    To better understand this, consider a practical example that assumes an OpenEmbedded metadata-based Linux kernel recipe file. The following lines from the recipe file first set the kernel branch variable KBRANCH to a default value, then conditionally override that value based on the architecture of the build:

    KBRANCH = "standard/base"
    KBRANCH:qemuarm = "standard/arm-versatile-926ejs"
    KBRANCH:qemumips = "standard/mti-malta32"
    KBRANCH:qemuppc = "standard/qemuppc"
    KBRANCH:qemux86 = "standard/common-pc/base"
    KBRANCH:qemux86-64 = "standard/common-pc-64/base"
    KBRANCH:qemumips64 = "standard/mti-malta64"
  • Appending and Prepending: BitBake also supports append and prepend operations to variable values based on whether a specific item is listed in OVERRIDES. Here is an example:

    DEPENDS = "glibc ncurses"
    OVERRIDES = "machine:local"
    DEPENDS:append:machine = "libmad"

    In this example, DEPENDS becomes “glibc ncurses libmad”.

    Again, using an OpenEmbedded metadata-based kernel recipe file as an example, the following lines will conditionally append to the KERNEL_FEATURES variable based on the architecture:

    KERNEL_FEATURES:append:qemux86=" cfg/sound.scc cfg/paravirt_kvm.scc"
    KERNEL_FEATURES:append:qemux86-64=" cfg/sound.scc cfg/paravirt_kvm.scc"
  • Setting a Variable for a Single Task: BitBake supports setting a variable just for the duration of a single task. Here is an example:

    FOO:task-configure = "val 1"
    FOO:task-compile = "val 2"

    In the previous example, FOO has the value “val 1” while the do_configure task is executed, and the value “val 2” while the do_compile task is executed.

    Internally, this is implemented by prepending the task (e.g. “task-compile:”) to the value of OVERRIDES for the local datastore of the do_compile task.

    You can also use this syntax with other combinations (e.g. “:prepend”) as shown in the following example:

    EXTRA_OEMAKE:prepend:task-compile = "${PARALLEL_MAKE} "


Before BitBake 1.52 (Honister 3.4), the syntax for OVERRIDES used _ instead of :, so you will still find a lot of documentation using _append, _prepend, and _remove, for example.

For details, see the Overrides Syntax Changes section in the Yocto Project manual migration notes.

3.3.2 Key Expansion

Key expansion happens when the BitBake datastore is finalized. To better understand this, consider the following example:

A${B} = "X"
B = "2"
A2 = "Y"

In this case, after all the parsing is complete, BitBake expands ${B} into “2”. This expansion causes A2, which was set to “Y” before the expansion, to become “X”.

3.3.3 Examples

Despite the previous explanations that show the different forms of variable definitions, it can be hard to work out exactly what happens when variable operators, conditional overrides, and unconditional overrides are combined. This section presents some common scenarios along with explanations for variable interactions that typically confuse users.

There is often confusion concerning the order in which overrides and various “append” operators take effect. Recall that an append or prepend operation using “:append” and “:prepend” does not result in an immediate assignment as would “+=”, “.=”, “=+”, or “=.”. Consider the following example:

A = "Z"
A:foo:append = "X"

For this case, A is unconditionally set to “Z” and “X” is unconditionally and immediately appended to the variable A:foo. Because overrides have not been applied yet, A:foo is set to “X” due to the append and A simply equals “Z”.

Applying overrides, however, changes things. Since “foo” is listed in OVERRIDES, the conditional variable A is replaced with the “foo” version, which is equal to “X”. So effectively, A:foo replaces A.

This next example changes the order of the override and the append:

A = "Z"
A:append:foo = "X"

For this case, before overrides are handled, A is set to “Z” and A:append:foo is set to “X”. Once the override for “foo” is applied, however, A gets appended with “X”. Consequently, A becomes “ZX”. Notice that spaces are not appended.

This next example has the order of the appends and overrides reversed back as in the first example:

A = "Y"
A:foo:append = "Z"
A:foo:append = "X"

For this case, before any overrides are resolved, A is set to “Y” using an immediate assignment. After this immediate assignment, A:foo is set to “Z”, and then further appended with “X” leaving the variable set to “ZX”. Finally, applying the override for “foo” results in the conditional variable A becoming “ZX” (i.e. A is replaced with A:foo).

This final example mixes in some varying operators:

A = "1"
A:append = "2"
A:append = "3"
A += "4"
A .= "5"

For this case, the type of append operators are affecting the order of assignments as BitBake passes through the code multiple times. Initially, A is set to “1 45” because of the three statements that use immediate operators. After these assignments are made, BitBake applies the “:append” operations. Those operations result in A becoming “1 4523”.

3.4 Sharing Functionality

BitBake allows for metadata sharing through include files (.inc) and class files (.bbclass). For example, suppose you have a piece of common functionality such as a task definition that you want to share between more than one recipe. In this case, creating a .bbclass file that contains the common functionality and then using the inherit directive in your recipes to inherit the class would be a common way to share the task.

This section presents the mechanisms BitBake provides to allow you to share functionality between recipes. Specifically, the mechanisms include include, inherit, INHERIT, and require directives.

3.4.1 Locating Include and Class Files

BitBake uses the BBPATH variable to locate needed include and class files. Additionally, BitBake searches the current directory for include and require directives.


The BBPATH variable is analogous to the environment variable PATH .

In order for include and class files to be found by BitBake, they need to be located in a “classes” subdirectory that can be found in BBPATH.

3.4.2 inherit Directive

When writing a recipe or class file, you can use the inherit directive to inherit the functionality of a class (.bbclass). BitBake only supports this directive when used within recipe and class files (i.e. .bb and .bbclass).

The inherit directive is a rudimentary means of specifying functionality contained in class files that your recipes require. For example, you can easily abstract out the tasks involved in building a package that uses Autoconf and Automake and put those tasks into a class file and then have your recipe inherit that class file.

As an example, your recipes could use the following directive to inherit an autotools.bbclass file. The class file would contain common functionality for using Autotools that could be shared across recipes:

inherit autotools

In this case, BitBake would search for the directory classes/autotools.bbclass in BBPATH.


You can override any values and functions of the inherited class within your recipe by doing so after the “inherit” statement.

If you want to use the directive to inherit multiple classes, separate them with spaces. The following example shows how to inherit both the buildhistory and rm_work classes:

inherit buildhistory rm_work

An advantage with the inherit directive as compared to both the include and require directives is that you can inherit class files conditionally. You can accomplish this by using a variable expression after the inherit statement. Here is an example:

inherit ${VARNAME}

If VARNAME is going to be set, it needs to be set before the inherit statement is parsed. One way to achieve a conditional inherit in this case is to use overrides:

VARIABLE:someoverride = "myclass"

Another method is by using anonymous Python. Here is an example:

python () {
    if condition == value:
        d.setVar('VARIABLE', 'myclass')
        d.setVar('VARIABLE', '')

Alternatively, you could use an in-line Python expression in the following form:

inherit ${@'classname' if condition else ''}
inherit ${@functionname(params)}

In all cases, if the expression evaluates to an empty string, the statement does not trigger a syntax error because it becomes a no-op.

3.4.3 include Directive

BitBake understands the include directive. This directive causes BitBake to parse whatever file you specify, and to insert that file at that location. The directive is much like its equivalent in Make except that if the path specified on the include line is a relative path, BitBake locates the first file it can find within BBPATH.

The include directive is a more generic method of including functionality as compared to the inherit directive, which is restricted to class (i.e. .bbclass) files. The include directive is applicable for any other kind of shared or encapsulated functionality or configuration that does not suit a .bbclass file.

As an example, suppose you needed a recipe to include some self-test definitions:

include test_defs.inc


The include directive does not produce an error when the file cannot be found. Consequently, it is recommended that if the file you are including is expected to exist, you should use require instead of include . Doing so makes sure that an error is produced if the file cannot be found.

3.4.4 require Directive

BitBake understands the require directive. This directive behaves just like the include directive with the exception that BitBake raises a parsing error if the file to be included cannot be found. Thus, any file you require is inserted into the file that is being parsed at the location of the directive.

The require directive, like the include directive previously described, is a more generic method of including functionality as compared to the inherit directive, which is restricted to class (i.e. .bbclass) files. The require directive is applicable for any other kind of shared or encapsulated functionality or configuration that does not suit a .bbclass file.

Similar to how BitBake handles include, if the path specified on the require line is a relative path, BitBake locates the first file it can find within BBPATH.

As an example, suppose you have two versions of a recipe (e.g. foo_1.2.2.bb and foo_2.0.0.bb) where each version contains some identical functionality that could be shared. You could create an include file named foo.inc that contains the common definitions needed to build “foo”. You need to be sure foo.inc is located in the same directory as your two recipe files as well. Once these conditions are set up, you can share the functionality using a require directive from within each recipe:

require foo.inc

3.4.5 INHERIT Configuration Directive

When creating a configuration file (.conf), you can use the INHERIT configuration directive to inherit a class. BitBake only supports this directive when used within a configuration file.

As an example, suppose you needed to inherit a class file called abc.bbclass from a configuration file as follows:

INHERIT += "abc"

This configuration directive causes the named class to be inherited at the point of the directive during parsing. As with the inherit directive, the .bbclass file must be located in a “classes” subdirectory in one of the directories specified in BBPATH.


Because .conf files are parsed first during BitBake’s execution, using INHERIT to inherit a class effectively inherits the class globally (i.e. for all recipes).

If you want to use the directive to inherit multiple classes, you can provide them on the same line in the local.conf file. Use spaces to separate the classes. The following example shows how to inherit both the autotools and pkgconfig classes:

INHERIT += "autotools pkgconfig"

3.5 Functions

As with most languages, functions are the building blocks that are used to build up operations into tasks. BitBake supports these types of functions:

  • Shell Functions: Functions written in shell script and executed either directly as functions, tasks, or both. They can also be called by other shell functions.

  • BitBake-Style Python Functions: Functions written in Python and executed by BitBake or other Python functions using bb.build.exec_func().

  • Python Functions: Functions written in Python and executed by Python.

  • Anonymous Python Functions: Python functions executed automatically during parsing.

Regardless of the type of function, you can only define them in class (.bbclass) and recipe (.bb or .inc) files.

3.5.1 Shell Functions

Functions written in shell script are executed either directly as functions, tasks, or both. They can also be called by other shell functions. Here is an example shell function definition:

some_function () {
    echo "Hello World"

When you create these types of functions in your recipe or class files, you need to follow the shell programming rules. The scripts are executed by /bin/sh, which may not be a bash shell but might be something such as dash. You should not use Bash-specific script (bashisms).

Overrides and override-style operators like :append and :prepend can also be applied to shell functions. Most commonly, this application would be used in a .bbappend file to modify functions in the main recipe. It can also be used to modify functions inherited from classes.

As an example, consider the following:

do_foo() {
    bbplain first

fn:prepend() {
    bbplain second

fn() {
    bbplain third

do_foo:append() {
    bbplain fourth

Running do_foo prints the following:

recipename do_foo: first
recipename do_foo: second
recipename do_foo: third
recipename do_foo: fourth


Overrides and override-style operators can be applied to any shell function, not just tasks.

You can use the bitbake -e recipename command to view the final assembled function after all overrides have been applied.

3.5.2 BitBake-Style Python Functions

These functions are written in Python and executed by BitBake or other Python functions using bb.build.exec_func().

An example BitBake function is:

python some_python_function () {
    d.setVar("TEXT", "Hello World")
    print d.getVar("TEXT")

Because the Python “bb” and “os” modules are already imported, you do not need to import these modules. Also in these types of functions, the datastore (“d”) is a global variable and is always automatically available.


Variable expressions (e.g. ${X} ) are no longer expanded within Python functions. This behavior is intentional in order to allow you to freely set variable values to expandable expressions without having them expanded prematurely. If you do wish to expand a variable within a Python function, use d.getVar("X") . Or, for more complicated expressions, use d.expand().

Similar to shell functions, you can also apply overrides and override-style operators to BitBake-style Python functions.

As an example, consider the following:

python do_foo:prepend() {

python do_foo() {

python do_foo:append() {

Running do_foo prints the following:

recipename do_foo: first
recipename do_foo: second
recipename do_foo: third

You can use the bitbake -e recipename command to view the final assembled function after all overrides have been applied.

3.5.3 Python Functions

These functions are written in Python and are executed by other Python code. Examples of Python functions are utility functions that you intend to call from in-line Python or from within other Python functions. Here is an example:

def get_depends(d):
    if d.getVar('SOMECONDITION'):
        return "dependencywithcond"
        return "dependency"

DEPENDS = "${@get_depends(d)}"

This would result in DEPENDS containing dependencywithcond.

Here are some things to know about Python functions:

  • Python functions can take parameters.

  • The BitBake datastore is not automatically available. Consequently, you must pass it in as a parameter to the function.

  • The “bb” and “os” Python modules are automatically available. You do not need to import them.

3.5.4 BitBake-Style Python Functions Versus Python Functions

Following are some important differences between BitBake-style Python functions and regular Python functions defined with “def”:

  • Only BitBake-style Python functions can be tasks.

  • Overrides and override-style operators can only be applied to BitBake-style Python functions.

  • Only regular Python functions can take arguments and return values.

  • Variable flags such as [dirs], [cleandirs], and [lockfiles] can be used on BitBake-style Python functions, but not on regular Python functions.

  • BitBake-style Python functions generate a separate ${T}/run.function-name.pid script that is executed to run the function, and also generate a log file in ${T}/log.function-name.pid if they are executed as tasks.

    Regular Python functions execute “inline” and do not generate any files in ${T}.

  • Regular Python functions are called with the usual Python syntax. BitBake-style Python functions are usually tasks and are called directly by BitBake, but can also be called manually from Python code by using the bb.build.exec_func() function. Here is an example:

    bb.build.exec_func("my_bitbake_style_function", d)


    bb.build.exec_func() can also be used to run shell functions from Python code. If you want to run a shell function before a Python function within the same task, then you can use a parent helper Python function that starts by running the shell function with bb.build.exec_func() and then runs the Python code.

    To detect errors from functions executed with bb.build.exec_func(), you can catch the bb.build.FuncFailed exception.


    Functions in metadata (recipes and classes) should not themselves raise bb.build.FuncFailed. Rather, bb.build.FuncFailed should be viewed as a general indicator that the called function failed by raising an exception. For example, an exception raised by bb.fatal() will be caught inside bb.build.exec_func(), and a bb.build.FuncFailed will be raised in response.

Due to their simplicity, you should prefer regular Python functions over BitBake-style Python functions unless you need a feature specific to BitBake-style Python functions. Regular Python functions in metadata are a more recent invention than BitBake-style Python functions, and older code tends to use bb.build.exec_func() more often.

3.5.5 Anonymous Python Functions

Sometimes it is useful to set variables or perform other operations programmatically during parsing. To do this, you can define special Python functions, called anonymous Python functions, that run at the end of parsing. For example, the following conditionally sets a variable based on the value of another variable:

python () {
    if d.getVar('SOMEVAR') == 'value':
        d.setVar('ANOTHERVAR', 'value2')

An equivalent way to mark a function as an anonymous function is to give it the name “__anonymous”, rather than no name.

Anonymous Python functions always run at the end of parsing, regardless of where they are defined. If a recipe contains many anonymous functions, they run in the same order as they are defined within the recipe. As an example, consider the following snippet:

python () {
    d.setVar('FOO', 'foo 2')

FOO = "foo 1"

python () {
    d.appendVar('BAR',' bar 2')

BAR = "bar 1"

The previous example is conceptually equivalent to the following snippet:

FOO = "foo 1"
BAR = "bar 1"
FOO = "foo 2"
BAR += "bar 2"

FOO ends up with the value “foo 2”, and BAR with the value “bar 1 bar 2”. Just as in the second snippet, the values set for the variables within the anonymous functions become available to tasks, which always run after parsing.

Overrides and override-style operators such as “:append” are applied before anonymous functions run. In the following example, FOO ends up with the value “foo from anonymous”:

FOO = "foo"
FOO:append = " from outside"

python () {
    d.setVar("FOO", "foo from anonymous")

For methods you can use with anonymous Python functions, see the “Functions You Can Call From Within Python” section. For a different method to run Python code during parsing, see the “Inline Python Variable Expansion” section.

3.5.6 Flexible Inheritance for Class Functions

Through coding techniques and the use of EXPORT_FUNCTIONS, BitBake supports exporting a function from a class such that the class function appears as the default implementation of the function, but can still be called if a recipe inheriting the class needs to define its own version of the function.

To understand the benefits of this feature, consider the basic scenario where a class defines a task function and your recipe inherits the class. In this basic scenario, your recipe inherits the task function as defined in the class. If desired, your recipe can add to the start and end of the function by using the “:prepend” or “:append” operations respectively, or it can redefine the function completely. However, if it redefines the function, there is no means for it to call the class version of the function. EXPORT_FUNCTIONS provides a mechanism that enables the recipe’s version of the function to call the original version of the function.

To make use of this technique, you need the following things in place:

  • The class needs to define the function as follows:


    For example, if you have a class file bar.bbclass and a function named do_foo, the class must define the function as follows:

  • The class needs to contain the EXPORT_FUNCTIONS statement as follows:

    EXPORT_FUNCTIONS functionname

    For example, continuing with the same example, the statement in the bar.bbclass would be as follows:

  • You need to call the function appropriately from within your recipe. Continuing with the same example, if your recipe needs to call the class version of the function, it should call bar_do_foo. Assuming do_foo was a shell function and EXPORT_FUNCTIONS was used as above, the recipe’s function could conditionally call the class version of the function as follows:

    do_foo() {
        if [ somecondition ] ; then
            # Do something else

    To call your modified version of the function as defined in your recipe, call it as do_foo.

With these conditions met, your single recipe can freely choose between the original function as defined in the class file and the modified function in your recipe. If you do not set up these conditions, you are limited to using one function or the other.

3.6 Tasks

Tasks are BitBake execution units that make up the steps that BitBake can run for a given recipe. Tasks are only supported in recipes and classes (i.e. in .bb files and files included or inherited from .bb files). By convention, tasks have names that start with “do_”.

3.6.1 Promoting a Function to a Task

Tasks are either shell functions or BitBake-style Python functions that have been promoted to tasks by using the addtask command. The addtask command can also optionally describe dependencies between the task and other tasks. Here is an example that shows how to define a task and declare some dependencies:

python do_printdate () {
    import time
    print time.strftime('%Y%m%d', time.gmtime())
addtask printdate after do_fetch before do_build

The first argument to addtask is the name of the function to promote to a task. If the name does not start with “do_”, “do_” is implicitly added, which enforces the convention that all task names start with “do_”.

In the previous example, the do_printdate task becomes a dependency of the do_build task, which is the default task (i.e. the task run by the bitbake command unless another task is specified explicitly). Additionally, the do_printdate task becomes dependent upon the do_fetch task. Running the do_build task results in the do_printdate task running first.


If you try out the previous example, you might see that the do_printdate task is only run the first time you build the recipe with the bitbake command. This is because BitBake considers the task “up-to-date” after that initial run. If you want to force the task to always be rerun for experimentation purposes, you can make BitBake always consider the task “out-of-date” by using the [nostamp] variable flag, as follows:

do_printdate[nostamp] = "1"

You can also explicitly run the task and provide the -f option as follows:

$ bitbake recipe -c printdate -f

When manually selecting a task to run with the bitbake recipe -c task command, you can omit the “do_” prefix as part of the task name.

You might wonder about the practical effects of using addtask without specifying any dependencies as is done in the following example:

addtask printdate

In this example, assuming dependencies have not been added through some other means, the only way to run the task is by explicitly selecting it with bitbake recipe -c printdate. You can use the do_listtasks task to list all tasks defined in a recipe as shown in the following example:

$ bitbake recipe -c listtasks

For more information on task dependencies, see the “Dependencies” section.

See the “Variable Flags” section for information on variable flags you can use with tasks.


While it’s infrequent, it’s possible to define multiple tasks as dependencies when calling addtask. For example, here’s a snippet from the OpenEmbedded class file package_tar.bbclass:

addtask package_write_tar before do_build after do_packagedata do_package

Note how the package_write_tar task has to wait until both of do_packagedata and do_package complete.

3.6.2 Deleting a Task

As well as being able to add tasks, you can delete them. Simply use the deltask command to delete a task. For example, to delete the example task used in the previous sections, you would use:

deltask printdate

If you delete a task using the deltask command and the task has dependencies, the dependencies are not reconnected. For example, suppose you have three tasks named do_a, do_b, and do_c. Furthermore, do_c is dependent on do_b, which in turn is dependent on do_a. Given this scenario, if you use deltask to delete do_b, the implicit dependency relationship between do_c and do_a through do_b no longer exists, and do_c dependencies are not updated to include do_a. Thus, do_c is free to run before do_a.

If you want dependencies such as these to remain intact, use the [noexec] varflag to disable the task instead of using the deltask command to delete it:

do_b[noexec] = "1"

3.6.3 Passing Information Into the Build Task Environment

When running a task, BitBake tightly controls the shell execution environment of the build tasks to make sure unwanted contamination from the build machine cannot influence the build.


By default, BitBake cleans the environment to include only those things exported or listed in its passthrough list to ensure that the build environment is reproducible and consistent. You can prevent this “cleaning” by setting the BB_PRESERVE_ENV variable.

Consequently, if you do want something to get passed into the build task environment, you must take these two steps:

  1. Tell BitBake to load what you want from the environment into the datastore. You can do so through the BB_ENV_PASSTHROUGH and BB_ENV_PASSTHROUGH_ADDITIONS variables. For example, assume you want to prevent the build system from accessing your $HOME/.ccache directory. The following command adds the the environment variable CCACHE_DIR to BitBake’s passthrough list to allow that variable into the datastore:

  2. Tell BitBake to export what you have loaded into the datastore to the task environment of every running task. Loading something from the environment into the datastore (previous step) only makes it available in the datastore. To export it to the task environment of every running task, use a command similar to the following in your local configuration file local.conf or your distribution configuration file:

    export CCACHE_DIR


    A side effect of the previous steps is that BitBake records the variable as a dependency of the build process in things like the setscene checksums. If doing so results in unnecessary rebuilds of tasks, you can also flag the variable so that the setscene code ignores the dependency when it creates checksums.

Sometimes, it is useful to be able to obtain information from the original execution environment. BitBake saves a copy of the original environment into a special variable named BB_ORIGENV.

The BB_ORIGENV variable returns a datastore object that can be queried using the standard datastore operators such as getVar(, False). The datastore object is useful, for example, to find the original DISPLAY variable. Here is an example:

origenv = d.getVar("BB_ORIGENV", False)
bar = origenv.getVar("BAR", False)

The previous example returns BAR from the original execution environment.

3.7 Variable Flags

Variable flags (varflags) help control a task’s functionality and dependencies. BitBake reads and writes varflags to the datastore using the following command forms:

variable = d.getVarFlags("variable")
self.d.setVarFlags("FOO", {"func": True})

When working with varflags, the same syntax, with the exception of overrides, applies. In other words, you can set, append, and prepend varflags just like variables. See the “Variable Flag Syntax” section for details.

BitBake has a defined set of varflags available for recipes and classes. Tasks support a number of these flags which control various functionality of the task:

  • [cleandirs]: Empty directories that should be created before the task runs. Directories that already exist are removed and recreated to empty them.

  • [depends]: Controls inter-task dependencies. See the DEPENDS variable and the “Inter-Task Dependencies” section for more information.

  • [deptask]: Controls task build-time dependencies. See the DEPENDS variable and the “Build Dependencies” section for more information.

  • [dirs]: Directories that should be created before the task runs. Directories that already exist are left as is. The last directory listed is used as the current working directory for the task.

  • [file-checksums]: Controls the file dependencies for a task. The baseline file list is the set of files associated with SRC_URI. May be used to set additional dependencies on files not associated with SRC_URI.

    The value set to the list is a file-boolean pair where the first value is the file name and the second is whether or not it physically exists on the filesystem.

    do_configure[file-checksums] += "${MY_DIRPATH}/my-file.txt:True"

    It is important to record any paths which the task looked at and which didn’t exist. This means that if these do exist at a later time, the task can be rerun with the new additional files. The “exists” True or False value after the path allows this to be handled.

  • [lockfiles]: Specifies one or more lockfiles to lock while the task executes. Only one task may hold a lockfile, and any task that attempts to lock an already locked file will block until the lock is released. You can use this variable flag to accomplish mutual exclusion.

  • [network]: When set to “1”, allows a task to access the network. By default, only the do_fetch task is granted network access. Recipes shouldn’t access the network outside of do_fetch as it usually undermines fetcher source mirroring, image and licence manifests, software auditing and supply chain security.

  • [noexec]: When set to “1”, marks the task as being empty, with no execution required. You can use the [noexec] flag to set up tasks as dependency placeholders, or to disable tasks defined elsewhere that are not needed in a particular recipe.

  • [nostamp]: When set to “1”, tells BitBake to not generate a stamp file for a task, which implies the task should always be executed.


    Any task that depends (possibly indirectly) on a [nostamp] task will always be executed as well. This can cause unnecessary rebuilding if you are not careful.

  • [number_threads]: Limits tasks to a specific number of simultaneous threads during execution. This varflag is useful when your build host has a large number of cores but certain tasks need to be rate-limited due to various kinds of resource constraints (e.g. to avoid network throttling). number_threads works similarly to the BB_NUMBER_THREADS variable but is task-specific.

    Set the value globally. For example, the following makes sure the do_fetch task uses no more than two simultaneous execution threads: do_fetch[number_threads] = “2”


    • Setting the varflag in individual recipes rather than globally can result in unpredictable behavior.

    • Setting the varflag to a value greater than the value used in the BB_NUMBER_THREADS variable causes number_threads to have no effect.

  • [postfuncs]: List of functions to call after the completion of the task.

  • [prefuncs]: List of functions to call before the task executes.

  • [rdepends]: Controls inter-task runtime dependencies. See the RDEPENDS variable, the RRECOMMENDS variable, and the “Inter-Task Dependencies” section for more information.

  • [rdeptask]: Controls task runtime dependencies. See the RDEPENDS variable, the RRECOMMENDS variable, and the “Runtime Dependencies” section for more information.

  • [recideptask]: When set in conjunction with recrdeptask, specifies a task that should be inspected for additional dependencies.

  • [recrdeptask]: Controls task recursive runtime dependencies. See the RDEPENDS variable, the RRECOMMENDS variable, and the “Recursive Dependencies” section for more information.

  • [stamp-extra-info]: Extra stamp information to append to the task’s stamp. As an example, OpenEmbedded uses this flag to allow machine-specific tasks.

  • [umask]: The umask to run the task under.

Several varflags are useful for controlling how signatures are calculated for variables. For more information on this process, see the “Checksums (Signatures)” section.

  • [vardeps]: Specifies a space-separated list of additional variables to add to a variable’s dependencies for the purposes of calculating its signature. Adding variables to this list is useful, for example, when a function refers to a variable in a manner that does not allow BitBake to automatically determine that the variable is referred to.

  • [vardepsexclude]: Specifies a space-separated list of variables that should be excluded from a variable’s dependencies for the purposes of calculating its signature.

  • [vardepvalue]: If set, instructs BitBake to ignore the actual value of the variable and instead use the specified value when calculating the variable’s signature.

  • [vardepvalueexclude]: Specifies a pipe-separated list of strings to exclude from the variable’s value when calculating the variable’s signature.

3.8 Events

BitBake allows installation of event handlers within recipe and class files. Events are triggered at certain points during operation, such as the beginning of operation against a given recipe (i.e. *.bb), the start of a given task, a task failure, a task success, and so forth. The intent is to make it easy to do things like email notification on build failures.

Following is an example event handler that prints the name of the event and the content of the FILE variable:

addhandler myclass_eventhandler
python myclass_eventhandler() {
    from bb.event import getName
    print("The name of the Event is %s" % getName(e))
    print("The file we run for is %s" % d.getVar('FILE'))
myclass_eventhandler[eventmask] = "bb.event.BuildStarted

In the previous example, an eventmask has been set so that the handler only sees the “BuildStarted” and “BuildCompleted” events. This event handler gets called every time an event matching the eventmask is triggered. A global variable “e” is defined, which represents the current event. With the getName(e) method, you can get the name of the triggered event. The global datastore is available as “d”. In legacy code, you might see “e.data” used to get the datastore. However, realize that “e.data” is deprecated and you should use “d” going forward.

The context of the datastore is appropriate to the event in question. For example, “BuildStarted” and “BuildCompleted” events run before any tasks are executed so would be in the global configuration datastore namespace. No recipe-specific metadata exists in that namespace. The “BuildStarted” and “BuildCompleted” events also run in the main cooker/server process rather than any worker context. Thus, any changes made to the datastore would be seen by other cooker/server events within the current build but not seen outside of that build or in any worker context. Task events run in the actual tasks in question consequently have recipe-specific and task-specific contents. These events run in the worker context and are discarded at the end of task execution.

During a standard build, the following common events might occur. The following events are the most common kinds of events that most metadata might have an interest in viewing:

  • bb.event.ConfigParsed(): Fired when the base configuration; which consists of bitbake.conf, base.bbclass and any global INHERIT statements; has been parsed. You can see multiple such events when each of the workers parse the base configuration or if the server changes configuration and reparses. Any given datastore only has one such event executed against it, however. If BB_INVALIDCONF is set in the datastore by the event handler, the configuration is reparsed and a new event triggered, allowing the metadata to update configuration.

  • bb.event.HeartbeatEvent(): Fires at regular time intervals of one second. You can configure the interval time using the BB_HEARTBEAT_EVENT variable. The event’s “time” attribute is the time.time() value when the event is triggered. This event is useful for activities such as system state monitoring.

  • bb.event.ParseStarted(): Fired when BitBake is about to start parsing recipes. This event’s “total” attribute represents the number of recipes BitBake plans to parse.

  • bb.event.ParseProgress(): Fired as parsing progresses. This event’s “current” attribute is the number of recipes parsed as well as the “total” attribute.

  • bb.event.ParseCompleted(): Fired when parsing is complete. This event’s “cached”, “parsed”, “skipped”, “virtuals”, “masked”, and “errors” attributes provide statistics for the parsing results.

  • bb.event.BuildStarted(): Fired when a new build starts. BitBake fires multiple “BuildStarted” events (one per configuration) when multiple configuration (multiconfig) is enabled.

  • bb.build.TaskStarted(): Fired when a task starts. This event’s “taskfile” attribute points to the recipe from which the task originates. The “taskname” attribute, which is the task’s name, includes the do_ prefix, and the “logfile” attribute point to where the task’s output is stored. Finally, the “time” attribute is the task’s execution start time.

  • bb.build.TaskInvalid(): Fired if BitBake tries to execute a task that does not exist.

  • bb.build.TaskFailedSilent(): Fired for setscene tasks that fail and should not be presented to the user verbosely.

  • bb.build.TaskFailed(): Fired for normal tasks that fail.

  • bb.build.TaskSucceeded(): Fired when a task successfully completes.

  • bb.event.BuildCompleted(): Fired when a build finishes.

  • bb.cooker.CookerExit(): Fired when the BitBake server/cooker shuts down. This event is usually only seen by the UIs as a sign they should also shutdown.

This next list of example events occur based on specific requests to the server. These events are often used to communicate larger pieces of information from the BitBake server to other parts of BitBake such as user interfaces:

  • bb.event.TreeDataPreparationStarted()

  • bb.event.TreeDataPreparationProgress()

  • bb.event.TreeDataPreparationCompleted()

  • bb.event.DepTreeGenerated()

  • bb.event.CoreBaseFilesFound()

  • bb.event.ConfigFilePathFound()

  • bb.event.FilesMatchingFound()

  • bb.event.ConfigFilesFound()

  • bb.event.TargetsTreeGenerated()

3.9 Variants — Class Extension Mechanism

BitBake supports multiple incarnations of a recipe file via the BBCLASSEXTEND variable.

The BBCLASSEXTEND variable is a space separated list of classes used to “extend” the recipe for each variant. Here is an example that results in a second incarnation of the current recipe being available. This second incarnation will have the “native” class inherited.



The mechanism for this class extension is extremely specific to the implementation. Usually, the recipe’s PROVIDES , PN , and DEPENDS variables would need to be modified by the extension class. For specific examples, see the OE-Core native , nativesdk , and multilib classes.

3.10 Dependencies

To allow for efficient parallel processing, BitBake handles dependencies at the task level. Dependencies can exist both between tasks within a single recipe and between tasks in different recipes. Following are examples of each:

  • For tasks within a single recipe, a recipe’s do_configure task might need to complete before its do_compile task can run.

  • For tasks in different recipes, one recipe’s do_configure task might require another recipe’s do_populate_sysroot task to finish first such that the libraries and headers provided by the other recipe are available.

This section describes several ways to declare dependencies. Remember, even though dependencies are declared in different ways, they are all simply dependencies between tasks.

3.10.1 Dependencies Internal to the .bb File

BitBake uses the addtask directive to manage dependencies that are internal to a given recipe file. You can use the addtask directive to indicate when a task is dependent on other tasks or when other tasks depend on that recipe. Here is an example:

addtask printdate after do_fetch before do_build

In this example, the do_printdate task depends on the completion of the do_fetch task, and the do_build task depends on the completion of the do_printdate task.


For a task to run, it must be a direct or indirect dependency of some other task that is scheduled to run.

For illustration, here are some examples:

  • The directive addtask mytask before do_configure causes do_mytask to run before do_configure runs. Be aware that do_mytask still only runs if its input checksum has changed since the last time it was run. Changes to the input checksum of do_mytask also indirectly cause do_configure to run.

  • The directive addtask mytask after do_configure by itself never causes do_mytask to run. do_mytask can still be run manually as follows:

    $ bitbake recipe -c mytask

    Declaring do_mytask as a dependency of some other task that is scheduled to run also causes it to run. Regardless, the task runs after do_configure.

3.10.2 Build Dependencies

BitBake uses the DEPENDS variable to manage build time dependencies. The [deptask] varflag for tasks signifies the task of each item listed in DEPENDS that must complete before that task can be executed. Here is an example:

do_configure[deptask] = "do_populate_sysroot"

In this example, the do_populate_sysroot task of each item in DEPENDS must complete before do_configure can execute.

3.10.3 Runtime Dependencies

BitBake uses the PACKAGES, RDEPENDS, and RRECOMMENDS variables to manage runtime dependencies.

The PACKAGES variable lists runtime packages. Each of those packages can have RDEPENDS and RRECOMMENDS runtime dependencies. The [rdeptask] flag for tasks is used to signify the task of each item runtime dependency which must have completed before that task can be executed.

do_package_qa[rdeptask] = "do_packagedata"

In the previous example, the do_packagedata task of each item in RDEPENDS must have completed before do_package_qa can execute. Although RDEPENDS contains entries from the runtime dependency namespace, BitBake knows how to map them back to the build-time dependency namespace, in which the tasks are defined.

3.10.4 Recursive Dependencies

BitBake uses the [recrdeptask] flag to manage recursive task dependencies. BitBake looks through the build-time and runtime dependencies of the current recipe, looks through the task’s inter-task dependencies, and then adds dependencies for the listed task. Once BitBake has accomplished this, it recursively works through the dependencies of those tasks. Iterative passes continue until all dependencies are discovered and added.

The [recrdeptask] flag is most commonly used in high-level recipes that need to wait for some task to finish “globally”. For example, image.bbclass has the following:

do_rootfs[recrdeptask] += "do_packagedata"

This statement says that the do_packagedata task of the current recipe and all recipes reachable (by way of dependencies) from the image recipe must run before the do_rootfs task can run.

BitBake allows a task to recursively depend on itself by referencing itself in the task list:

do_a[recrdeptask] = "do_a do_b"

In the same way as before, this means that the do_a and do_b tasks of the current recipe and all recipes reachable (by way of dependencies) from the recipe must run before the do_a task can run. In this case BitBake will ignore the current recipe’s do_a task circular dependency on itself.

3.10.5 Inter-Task Dependencies

BitBake uses the [depends] flag in a more generic form to manage inter-task dependencies. This more generic form allows for inter-dependency checks for specific tasks rather than checks for the data in DEPENDS. Here is an example:

do_patch[depends] = "quilt-native:do_populate_sysroot"

In this example, the do_populate_sysroot task of the target quilt-native must have completed before the do_patch task can execute.

The [rdepends] flag works in a similar way but takes targets in the runtime namespace instead of the build-time dependency namespace.

3.11 Functions You Can Call From Within Python

BitBake provides many functions you can call from within Python functions. This section lists the most commonly used functions, and mentions where to find others.

3.11.1 Functions for Accessing Datastore Variables

It is often necessary to access variables in the BitBake datastore using Python functions. The BitBake datastore has an API that allows you this access. Here is a list of available operations:



d.getVar("X", expand)

Returns the value of variable “X”. Using “expand=True” expands the value. Returns “None” if the variable “X” does not exist.

d.setVar("X", "value")

Sets the variable “X” to “value”

d.appendVar("X", "value")

Adds “value” to the end of the variable “X”. Acts like d.setVar("X", "value") if the variable “X” does not exist.

d.prependVar("X", "value")

Adds “value” to the start of the variable “X”. Acts like d.setVar("X","value") if the variable “X” does not exist.


Deletes the variable “X” from the datastore. Does nothing if the variable “X” does not exist.

d.renameVar("X", "Y")

Renames the variable “X” to “Y”. Does nothing if the variable “X” does not exist.

d.getVarFlag("X", flag, expand)

Returns the value of variable “X”. Using “expand=True” expands the value. Returns “None” if either the variable “X” or the named flag does not exist.

d.setVarFlag("X", flag, "value")

Sets the named flag for variable “X” to “value”.

d.appendVarFlag("X", flag, "value")

Appends “value” to the named flag on the variable “X”. Acts like d.setVarFlag("X", flag, "value") if the named flag does not exist.

d.prependVarFlag("X", flag, "value")

Prepends “value” to the named flag on the variable “X”. Acts like d.setVarFlag("X", flag, "value") if the named flag does not exist.

d.delVarFlag("X", flag)

Deletes the named flag on the variable “X” from the datastore.

d.setVarFlags("X", flagsdict)

Sets the flags specified in the flagsdict() parameter. setVarFlags does not clear previous flags. Think of this operation as addVarFlags.


Returns a flagsdict of the flags for the variable “X”. Returns “None” if the variable “X” does not exist.


Deletes all the flags for the variable “X”. Does nothing if the variable “X” does not exist.


Expands variable references in the specified string expression. References to variables that do not exist are left as is. For example, d.expand("foo ${X}") expands to the literal string “foo ${X}” if the variable “X” does not exist.

3.11.2 Other Functions

You can find many other functions that can be called from Python by looking at the source code of the bb module, which is in bitbake/lib/bb. For example, bitbake/lib/bb/utils.py includes the commonly used functions bb.utils.contains() and bb.utils.mkdirhier(), which come with docstrings.

3.11.3 Extending Python Library Code

If you wish to add your own Python library code (e.g. to provide functions/classes you can use from Python functions in the metadata) you can do so from any layer using the addpylib directive. This directive is typically added to your layer configuration ( conf/layer.conf) although it will be handled in any .conf file.

Usage is of the form:

addpylib <directory> <namespace>

Where <directory> specifies the directory to add to the library path. The specified <namespace> is imported automatically, and if the imported module specifies an attribute named BBIMPORTS, that list of sub-modules is iterated and imported too.

3.11.4 Testing and Debugging BitBake Python code

The OpenEmbedded build system implements a convenient pydevshell target which you can use to access the BitBake datastore and experiment with your own Python code. See Using a Python Development Shell in the Yocto Project manual for details.

3.12 Task Checksums and Setscene

BitBake uses checksums (or signatures) along with the setscene to determine if a task needs to be run. This section describes the process. To help understand how BitBake does this, the section assumes an OpenEmbedded metadata-based example.

These checksums are stored in STAMP. You can examine the checksums using the following BitBake command:

$ bitbake-dumpsigs

This command returns the signature data in a readable format that allows you to examine the inputs used when the OpenEmbedded build system generates signatures. For example, using bitbake-dumpsigs allows you to examine the do_compile task’s “sigdata” for a C application (e.g. bash). Running the command also reveals that the “CC” variable is part of the inputs that are hashed. Any changes to this variable would invalidate the stamp and cause the do_compile task to run.

The following list describes related variables:

  • BB_HASHCHECK_FUNCTION: Specifies the name of the function to call during the “setscene” part of the task’s execution in order to validate the list of task hashes.

  • BB_SETSCENE_DEPVALID: Specifies a function BitBake calls that determines whether BitBake requires a setscene dependency to be met.

  • BB_TASKHASH: Within an executing task, this variable holds the hash of the task as returned by the currently enabled signature generator.

  • STAMP: The base path to create stamp files.

  • STAMPCLEAN: Again, the base path to create stamp files but can use wildcards for matching a range of files for clean operations.

3.13 Wildcard Support in Variables

Support for wildcard use in variables varies depending on the context in which it is used. For example, some variables and filenames allow limited use of wildcards through the “%” and “*” characters. Other variables or names support Python’s glob syntax, fnmatch syntax, or Regular Expression (re) syntax.

For variables that have wildcard suport, the documentation describes which form of wildcard, its use, and its limitations.