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ajohns edited this page Jun 18, 2019 · 60 revisions


Package definition files ( usually define a commands section. This is a python function that determines how the environment is configured in order to include the package.

Consider the simple example:

def commands():

This is a typical case, where a package adds its source path to PYTHONPATH, and its tools to PATH. The "{root}" string expands to the installation directory of the package.

When a rez environment is configured, every package in the resolve list has its commands section interpreted and converted into shell code (the language - bash or other - depends on the platform and is extensible). The resulting shell code is sourced, and this configures the environment. Within a configured environment, the variable REZ_CONTEXT_FILE points at this shell code, and the command rez-context --interpet prints it.

The python API that you use in the commands section is called rex (Rez EXecution language). It is an API for performing shell operations in a shell-agnostic way. Some common operations you would perform with this API include setting environment variables, and appending/prepending path-like environment variables.

By default, environment variables that are not referenced by any package are left unaltered. There will typically be many system variables that are left unchanged.

If you need to import any python modules to use in a commands section, the import statements must appear inline to that function.

Order Of Command Execution

The order in which package commands are interpreted depends on two factors - the order in which the packages were requested, and dependencies between packages. This order can be defined as:

  • If package A was requested before package B, then A's commands are interpreted before B's;
  • Unless package A requires (depends on) B, in which case B will be interpreted before A.

Consider a package maya_anim_tool. Let us say this is a maya plugin. Naturally it has a dependency on maya, therefore maya's commands will be interpreted first. This is because the maya plugin may depend on certain environment variables that maya sets. For example, maya might initialize the MAYA_PLUG_IN_PATH environment variable, and maya_anim_tool may then append to this variable.

For example, consider the request:

]$ rez-env maya_anim_tool-1.3+ PyYAML-3.10 maya-2015

Assuming that PyYAML depends on python, and maya_anim_tool depends on maya, then the resulting commands execution order would be:

  • maya;
  • maya_anim_tool;
  • python;
  • PyYAML.

Variable Appending And Prepending

Path-like environment variables can be appended and prepended like so:


However, the first append/prepend operation on any given variable actually overwrites the variable, rather than appending. Why does this happen? Consider PYTHONPATH - if an initial overwrite did not happen, then any modules visible on PYTHONPATH before the rez environment was configured would still be there. This would mean you may not have a properly configured environment. If your system PyQt were on PYTHONPATH for example, and you used rez-env to set a different PyQt version, an attempt to import it within the configured environment would still, incorrectly, import the system version.

PATH is a special case. It is not simply overwritten, because if that happened you would lose important system paths and thus utilities like ls and cd. In this case the system paths are appended back to PATH after all commands are interpreted. The system paths are defined as the default value of PATH in a non-interactive shell.

Better control over environment variable initialization is coming. Specifically, you will be able to specify various modes for variables. For example, one mode will append the original (pre-rez) value back to the resulting value.

String Expansion

Object Expansion

Any of the objects available to you in a commands section can be referred to in formatted strings that are passed to rex functions such as setenv and so on. For example, consider the code:

appendenv("PATH", "{root}/bin")

Here, "{root}" will expand out to the value of root, which is the installation path of the package ("this.root" could also have been used).

You don't have to use this feature; it is provided as a convenience. For example, the following code is equivalent to the previous example, and is just as valid (but more verbose):

import os.path
appendenv("PATH", os.path.join(root, "bin"))

Object string expansion is also supported when setting an environment variable via the env object:

env.FOO_LIC = "{this.root}/lic"

Environment Variable Expansion

Environment variable expansion is also supported when passed to rex functions. The syntaxes $FOO and ${FOO} are supported, regardless of the syntax supported by the target shell.

Literal Strings

You can use the literal function to inhibit object- and environment variable- string expansion. For example, the following code will set the environment variable to the literal string:

env.TEST = literal("this {root} will not expand")

There is also an expandable function, which matches the default behavior. You wouldn't typically use this function; however, you can define a string containing literal and expandable parts by chaining together literal and expandable:

env.DESC = literal("the value of {root} is").expandable("{root}")

Explicit String Expansion

Object string expansion usually occurs only when a string is passed to a rex function, or to the env object. For example the simple statement var = "{root}/bin" would not expand "{root}" into var. However, you can use the expandvars function to enable this behavior explicitly:

var = expandvars("{root}/bin")

The expandvars and expandable functions are slightly different - expandable will generate a shell variable assignment that will expand out; expandvars will expand the value immediately.

This table illustrates the difference between literal, expandable and expandvars:

package command equivalent bash command
env.FOO = literal("${USER}") export FOO='${USER}'
env.FOO = expandable("${USER}") export FOO="${USER}"
env.FOO = expandvars("${USER}") export FOO="jbloggs"

Pre And Post Commands

Occasionally it's useful for a package to run commands either before or after all other packages, regardless of the command execution order rules. This can be achieved by defining a pre_commands or post_commands function. A package can have any, all or none of pre_commands, commands and post_commands defined, although it is very common for a package to define just commands.

The order of command execution is:

  • All package pre_commands are executed, in standard execution order;
  • Then, all package commands are executed, in standard execution order;
  • Then, all package post_commands are executed, in standard execution order.

A Largish Example

Here is an example of a package definition with a fairly lengthy commands section:

name = "foo"

version = "1.0.0"

requires = [

def commands():
    import os.path  # imports MUST be inline to the function

    # add python module, executables

    # show include path if a build is occurring
    if building:
        env.FOO_INCLUDE_PATH = "{this.root}/include"

    # debug support to point at local config
    if defined("DEBUG_FOO"):
        conf_file = os.path.expanduser("~/.foo/config")
        conf_file = "{this.root}/config"
    env.FOO_CONFIG_FILE = conf_file

    # if maya is in use then include the maya plugin part of this package
    if "maya" in resolve:

        if resolve.maya.version.minor == "sp3":
            error("known issue with GL renderer in service pack 3, beware")

    # license file per major version
    env.FOO_LIC = "/lic/foo_{this.version.major}.lic"


Various objects and functions are available to use in the commands function (as well as pre_commands and post_commands). For example, env is a dict-like object that represents all the environment variables being constructed in the target environment.

Following is a list of the objects and functions available.



alias("nukex", "Nuke -x")

Create a command alias.

In bash, aliases are implemented as bash functions.



See this.base.



if building:
    env.FOO_INCLUDE_PATH = "{root}/include"

This boolean variable is True if a build is occurring (typically done via the rez-build tool), and False otherwise. Typically a package will use this variable to set environment variables that are only useful during a build - C++ header include paths are a good example.



command("rm -rf ~/.foo_plugin")

Run an arbitrary shell command. Note that you cannot return a value from this function call, because the command has not yet run. All of the packages in a resolve only have their commands executed after all packages have been interpreted and converted to the target shell language. Therefore any value returned from the command, or any side effect the command has, is not visible to any package.

You should prefer to perform simple operations (such as file manipulations and so on) in python where possible instead. Not only does that take effect immediately, but it's also more cross platform. For example, instead of running the command above, we could have done this:

def commands():
    import shutil
    import os.path
    path = os.path.expanduser("~/.foo_plugin")
    if os.path.exists(path):



if "nuke" in resolve:
    comment("note: taking over 'nuke' binary!")
    alias("nuke", "foo_nuke_replacer")

Creates a comment line in the converted shell script code. This is only visible if the user views the current shell's code using the command "rez-context --interpret" or looks at the file referenced by the environment variable REZ_CONTEXT_FILE. You would create a comment for debugging purposes.



if defined("REZ_MAYA_VERSION"):
    env.FOO_MAYA = 1

Use this boolean function to determine whether or not an environment variable is set.


Dict-like object

env.FOO_DEBUG = 1
env["BAH_LICENSE"] = "/lic/bah.lic"

The env object represents the environment dict of the configured environment. Note that this is different from the standard python os.environ dict, which represents the current environment, not the one being configured. If a prior package's commands set a variable via the env object, it will be visible only via env, not os. The os dict hasn't been updated because the target configured environment does not yet exist!

The env object also provides the following functions:




Appends a value to an environment variable. By default this will use the os.pathsep delimiter between list items, but this can be overridden using the config setting env_var_separators. See here for further information on the behavior of this function.




like env.append, but prepends the environment variable instead.



if "PyQt" in resolve:
    error("The floob package has problems running in combo with PyQt")

Prints to standard error.

This function just prints the error, it does not prevent the target environment from being constructed (use the stop command for that).



if getenv("REZ_MAYA_VERSION") == "2016.sp1":

Gets the value of an environment variable; raises RexUndefinedVariableError if not set.


Dict-like object

if "platform" in implicits:

This is similar to the request object, but it contains only the package requests as defined by the implicit_packages configuration setting.



info("floob version is %s" % resolve.floob.version)

Prints to standard out.



env.FOO = literal("this {root} will not expand")

Inhibits expansion of object and environment variable references. You can also chain together literal and expandable functions like so:

env.FOO = literal("the value of {root} is").expandable("{root}")


Dict-like object

if "maya" in request:
    info("maya was asked for!")

A dict representing the list of package requests. Each item is a request string keyed by the package name. For example, consider the package request:

]$ rez-env maya-2015 maya_utils-1.2+<2 !corelib-1.4.4

This request would yield the following request object:

    "maya": "maya-2015",
    "maya_utils": "maya_utils-1.2+<2",
    "corelib": "!corelib-1.4.4"

If multiple requests are present that refer to the same package, the request is combined ahead of time. In other words, if requests foo-4+ and foo-<6 were both present, the single request foo-4+<6 would be present in the request object.


Dict-like object

if "maya" in resolve:
    info("Maya version is %s", resolve.maya.version)
    # ..or resolve["maya"].version

A dict representing the list of packages in the resolved environment. Each item is a Package object, keyed by the package name.



See this.root.



setenv("FOO_PLUGIN_PATH", "{root}/plugins")

This function sets an environment variable to the given value. It is equivalent to setting a variable via the env object (eg, "env.FOO = 'BAH'").




Source a shell script. Note that, similarly to commands, this function cannot return a value, and any side effects that the script sourcing has is not visible to any packages. For example, if the script above contained "export FOO=BAH", a subsequent test for this variable on the env object would yield nothing.



stop("The value should be %s", expected_value)

Raises an exception and stops a resolve from completing. You should use this when an unrecoverable error is detected and it is not possible to configure a valid environment.


System object

if system.platform == "windows":

This object provided system information, such as current platform, arch and os. See the source for more info.


Package object

import os.path
env.PATH.append(os.path.join(this.root, "bin"))

The this object represents the current package. The following attributes are most commonly used in a commands section (though you have access to all package attributes - see here):



Similar to this.root, but does not include the variant subpath, if there is one. Different variants of the same package share the same base directory. See here for more information on package structure in relation to variants.


The name of the package, eg 'houdini'.



The installation directory of the package. If the package contains variants, this path will include the variant subpath. This is the directory that contains the installed package payload. See here for more information on package structure in relation to variants.


Version object

The package version. It can be used as a string, however you can also access specific tokens in the version (such as major version number and so on), as this code snippet demonstrates:

env.FOO_MAJOR = this.version.major  # or, this.version[0]

The available token references are this.version.major, this.version.minor and this.version.patch, but you can also use a standard list index to reference any version token.



if undefined("REZ_MAYA_VERSION"):
    info("maya is not present")

Use this boolean function to determine whether or not an environment variable is set. This is the opposite of defined.




Unsets an environment variable. This function does nothing if the environment variable was not set.


Version object

See this.version.

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