4 File Download Support

BitBake’s fetch module is a standalone piece of library code that deals with the intricacies of downloading source code and files from remote systems. Fetching source code is one of the cornerstones of building software. As such, this module forms an important part of BitBake.

The current fetch module is called “fetch2” and refers to the fact that it is the second major version of the API. The original version is obsolete and has been removed from the codebase. Thus, in all cases, “fetch” refers to “fetch2” in this manual.

4.1 The Download (Fetch)

BitBake takes several steps when fetching source code or files. The fetcher codebase deals with two distinct processes in order: obtaining the files from somewhere (cached or otherwise) and then unpacking those files into a specific location and perhaps in a specific way. Getting and unpacking the files is often optionally followed by patching. Patching, however, is not covered by this module.

The code to execute the first part of this process, a fetch, looks something like the following:

src_uri = (d.getVar('SRC_URI') or "").split()
fetcher = bb.fetch2.Fetch(src_uri, d)

This code sets up an instance of the fetch class. The instance uses a space-separated list of URLs from the SRC_URI variable and then calls the download method to download the files.

The instantiation of the fetch class is usually followed by:

rootdir = l.getVar('WORKDIR')

This code unpacks the downloaded files to the specified by WORKDIR.


For convenience, the naming in these examples matches the variables used by OpenEmbedded. If you want to see the above code in action, examine the OpenEmbedded class file base.bbclass .

The SRC_URI and WORKDIR variables are not hardcoded into the fetcher, since those fetcher methods can be (and are) called with different variable names. In OpenEmbedded for example, the shared state (sstate) code uses the fetch module to fetch the sstate files.

When the download() method is called, BitBake tries to resolve the URLs by looking for source files in a specific search order:

  • Pre-mirror Sites: BitBake first uses pre-mirrors to try and find source files. These locations are defined using the PREMIRRORS variable.

  • Source URI: If pre-mirrors fail, BitBake uses the original URL (e.g from SRC_URI).

  • Mirror Sites: If fetch failures occur, BitBake next uses mirror locations as defined by the MIRRORS variable.

For each URL passed to the fetcher, the fetcher calls the submodule that handles that particular URL type. This behavior can be the source of some confusion when you are providing URLs for the SRC_URI variable. Consider the following two URLs:


In the former case, the URL is passed to the wget fetcher, which does not understand “git”. Therefore, the latter case is the correct form since the Git fetcher does know how to use HTTP as a transport.

Here are some examples that show commonly used mirror definitions:

   bzr://.*/.\*  http://somemirror.org/sources/ \\n \
   cvs://.*/.\*  http://somemirror.org/sources/ \\n \
   git://.*/.\*  http://somemirror.org/sources/ \\n \
   hg://.*/.\*   http://somemirror.org/sources/ \\n \
   osc://.*/.\*  http://somemirror.org/sources/ \\n \
   p4://.*/.\*   http://somemirror.org/sources/ \\n \
  svn://.*/.\*   http://somemirror.org/sources/ \\n"

   ftp://.*/.\*   http://somemirror.org/sources/ \\n \
   http://.*/.\*  http://somemirror.org/sources/ \\n \
   https://.*/.\* http://somemirror.org/sources/ \\n"

It is useful to note that BitBake supports cross-URLs. It is possible to mirror a Git repository on an HTTP server as a tarball. This is what the git:// mapping in the previous example does.

Since network accesses are slow, BitBake maintains a cache of files downloaded from the network. Any source files that are not local (i.e. downloaded from the Internet) are placed into the download directory, which is specified by the DL_DIR variable.

File integrity is of key importance for reproducing builds. For non-local archive downloads, the fetcher code can verify SHA-256 and MD5 checksums to ensure the archives have been downloaded correctly. You can specify these checksums by using the SRC_URI variable with the appropriate varflags as follows:

SRC_URI[md5sum] = "value"
SRC_URI[sha256sum] = "value"

You can also specify the checksums as parameters on the SRC_URI as shown below:

SRC_URI = "http://example.com/foobar.tar.bz2;md5sum=4a8e0f237e961fd7785d19d07fdb994d"

If multiple URIs exist, you can specify the checksums either directly as in the previous example, or you can name the URLs. The following syntax shows how you name the URIs:

SRC_URI = "http://example.com/foobar.tar.bz2;name=foo"
SRC_URI[foo.md5sum] = 4a8e0f237e961fd7785d19d07fdb994d

After a file has been downloaded and has had its checksum checked, a “.done” stamp is placed in DL_DIR. BitBake uses this stamp during subsequent builds to avoid downloading or comparing a checksum for the file again.


It is assumed that local storage is safe from data corruption. If this were not the case, there would be bigger issues to worry about.

If BB_STRICT_CHECKSUM is set, any download without a checksum triggers an error message. The BB_NO_NETWORK variable can be used to make any attempted network access a fatal error, which is useful for checking that mirrors are complete as well as other things.

If BB_CHECK_SSL_CERTS is set to 0 then SSL certificate checking will be disabled. This variable defaults to 1 so SSL certificates are normally checked.

4.2 The Unpack

The unpack process usually immediately follows the download. For all URLs except Git URLs, BitBake uses the common unpack method.

A number of parameters exist that you can specify within the URL to govern the behavior of the unpack stage:

  • unpack: Controls whether the URL components are unpacked. If set to “1”, which is the default, the components are unpacked. If set to “0”, the unpack stage leaves the file alone. This parameter is useful when you want an archive to be copied in and not be unpacked.

  • dos: Applies to .zip and .jar files and specifies whether to use DOS line ending conversion on text files.

  • subdir: Unpacks the specific URL to the specified subdirectory within the root directory.

The unpack call automatically decompresses and extracts files with “.Z”, “.z”, “.gz”, “.xz”, “.zip”, “.jar”, “.ipk”, “.rpm”. “.srpm”, “.deb” and “.bz2” extensions as well as various combinations of tarball extensions.

As mentioned, the Git fetcher has its own unpack method that is optimized to work with Git trees. Basically, this method works by cloning the tree into the final directory. The process is completed using references so that there is only one central copy of the Git metadata needed.

4.3 Fetchers

As mentioned earlier, the URL prefix determines which fetcher submodule BitBake uses. Each submodule can support different URL parameters, which are described in the following sections.

4.3.1 Local file fetcher (file://)

This submodule handles URLs that begin with file://. The filename you specify within the URL can be either an absolute or relative path to a file. If the filename is relative, the contents of the FILESPATH variable is used in the same way PATH is used to find executables. If the file cannot be found, it is assumed that it is available in DL_DIR by the time the download() method is called.

If you specify a directory, the entire directory is unpacked.

Here are a couple of example URLs, the first relative and the second absolute:

SRC_URI = "file://relativefile.patch"
SRC_URI = "file:///Users/ich/very_important_software"

4.3.2 HTTP/FTP wget fetcher (http://, ftp://, https://)

This fetcher obtains files from web and FTP servers. Internally, the fetcher uses the wget utility.

The executable and parameters used are specified by the FETCHCMD_wget variable, which defaults to sensible values. The fetcher supports a parameter “downloadfilename” that allows the name of the downloaded file to be specified. Specifying the name of the downloaded file is useful for avoiding collisions in DL_DIR when dealing with multiple files that have the same name.

Some example URLs are as follows:

SRC_URI = "http://oe.handhelds.org/not_there.aac"
SRC_URI = "ftp://oe.handhelds.org/not_there_as_well.aac"
SRC_URI = "ftp://you@oe.handhelds.org/home/you/secret.plan"


Because URL parameters are delimited by semi-colons, this can introduce ambiguity when parsing URLs that also contain semi-colons, for example:

SRC_URI = "http://abc123.org/git/?p=gcc/gcc.git;a=snapshot;h=a5dd47"

Such URLs should should be modified by replacing semi-colons with ‘&’ characters:

SRC_URI = "http://abc123.org/git/?p=gcc/gcc.git&a=snapshot&h=a5dd47"

In most cases this should work. Treating semi-colons and ‘&’ in queries identically is recommended by the World Wide Web Consortium (W3C). Note that due to the nature of the URL, you may have to specify the name of the downloaded file as well:

SRC_URI = "http://abc123.org/git/?p=gcc/gcc.git&a=snapshot&h=a5dd47;downloadfilename=myfile.bz2"

4.3.3 CVS fetcher ((cvs://)

This submodule handles checking out files from the CVS version control system. You can configure it using a number of different variables:

  • FETCHCMD_cvs: The name of the executable to use when running the cvs command. This name is usually “cvs”.

  • SRCDATE: The date to use when fetching the CVS source code. A special value of “now” causes the checkout to be updated on every build.

  • CVSDIR: Specifies where a temporary checkout is saved. The location is often DL_DIR/cvs.

  • CVS_PROXY_HOST: The name to use as a “proxy=” parameter to the cvs command.

  • CVS_PROXY_PORT: The port number to use as a “proxyport=” parameter to the cvs command.

As well as the standard username and password URL syntax, you can also configure the fetcher with various URL parameters:

The supported parameters are as follows:

  • “method”: The protocol over which to communicate with the CVS server. By default, this protocol is “pserver”. If “method” is set to “ext”, BitBake examines the “rsh” parameter and sets CVS_RSH. You can use “dir” for local directories.

  • “module”: Specifies the module to check out. You must supply this parameter.

  • “tag”: Describes which CVS TAG should be used for the checkout. By default, the TAG is empty.

  • “date”: Specifies a date. If no “date” is specified, the SRCDATE of the configuration is used to checkout a specific date. The special value of “now” causes the checkout to be updated on every build.

  • “localdir”: Used to rename the module. Effectively, you are renaming the output directory to which the module is unpacked. You are forcing the module into a special directory relative to CVSDIR.

  • “rsh”: Used in conjunction with the “method” parameter.

  • “scmdata”: Causes the CVS metadata to be maintained in the tarball the fetcher creates when set to “keep”. The tarball is expanded into the work directory. By default, the CVS metadata is removed.

  • “fullpath”: Controls whether the resulting checkout is at the module level, which is the default, or is at deeper paths.

  • “norecurse”: Causes the fetcher to only checkout the specified directory with no recurse into any subdirectories.

  • “port”: The port to which the CVS server connects.

Some example URLs are as follows:

SRC_URI = "cvs://CVSROOT;module=mymodule;tag=some-version;method=ext"
SRC_URI = "cvs://CVSROOT;module=mymodule;date=20060126;localdir=usethat"

4.3.4 Subversion (SVN) Fetcher (svn://)

This fetcher submodule fetches code from the Subversion source control system. The executable used is specified by FETCHCMD_svn, which defaults to “svn”. The fetcher’s temporary working directory is set by SVNDIR, which is usually DL_DIR/svn.

The supported parameters are as follows:

  • “module”: The name of the svn module to checkout. You must provide this parameter. You can think of this parameter as the top-level directory of the repository data you want.

  • “path_spec”: A specific directory in which to checkout the specified svn module.

  • “protocol”: The protocol to use, which defaults to “svn”. If “protocol” is set to “svn+ssh”, the “ssh” parameter is also used.

  • “rev”: The revision of the source code to checkout.

  • “scmdata”: Causes the “.svn” directories to be available during compile-time when set to “keep”. By default, these directories are removed.

  • “ssh”: An optional parameter used when “protocol” is set to “svn+ssh”. You can use this parameter to specify the ssh program used by svn.

  • “transportuser”: When required, sets the username for the transport. By default, this parameter is empty. The transport username is different than the username used in the main URL, which is passed to the subversion command.

Following are three examples using svn:

SRC_URI = "svn://myrepos/proj1;module=vip;protocol=http;rev=667"
SRC_URI = "svn://myrepos/proj1;module=opie;protocol=svn+ssh"
SRC_URI = "svn://myrepos/proj1;module=trunk;protocol=http;path_spec=${MY_DIR}/proj1"

4.3.5 Git Fetcher (git://)

This fetcher submodule fetches code from the Git source control system. The fetcher works by creating a bare clone of the remote into GITDIR, which is usually DL_DIR/git2. This bare clone is then cloned into the work directory during the unpack stage when a specific tree is checked out. This is done using alternates and by reference to minimize the amount of duplicate data on the disk and make the unpack process fast. The executable used can be set with FETCHCMD_git.

This fetcher supports the following parameters:

  • “protocol”: The protocol used to fetch the files. The default is “git” when a hostname is set. If a hostname is not set, the Git protocol is “file”. You can also use “http”, “https”, “ssh” and “rsync”.

  • “nocheckout”: Tells the fetcher to not checkout source code when unpacking when set to “1”. Set this option for the URL where there is a custom routine to checkout code. The default is “0”.

  • “rebaseable”: Indicates that the upstream Git repository can be rebased. You should set this parameter to “1” if revisions can become detached from branches. In this case, the source mirror tarball is done per revision, which has a loss of efficiency. Rebasing the upstream Git repository could cause the current revision to disappear from the upstream repository. This option reminds the fetcher to preserve the local cache carefully for future use. The default value for this parameter is “0”.

  • “nobranch”: Tells the fetcher to not check the SHA validation for the branch when set to “1”. The default is “0”. Set this option for the recipe that refers to the commit that is valid for a tag instead of the branch.

  • “bareclone”: Tells the fetcher to clone a bare clone into the destination directory without checking out a working tree. Only the raw Git metadata is provided. This parameter implies the “nocheckout” parameter as well.

  • “branch”: The branch(es) of the Git tree to clone. If unset, this is assumed to be “master”. The number of branch parameters much match the number of name parameters.

  • “rev”: The revision to use for the checkout. The default is “master”.

  • “tag”: Specifies a tag to use for the checkout. To correctly resolve tags, BitBake must access the network. For that reason, tags are often not used. As far as Git is concerned, the “tag” parameter behaves effectively the same as the “rev” parameter.

  • “subpath”: Limits the checkout to a specific subpath of the tree. By default, the whole tree is checked out.

  • “destsuffix”: The name of the path in which to place the checkout. By default, the path is git/.

  • “usehead”: Enables local git:// URLs to use the current branch HEAD as the revision for use with AUTOREV. The “usehead” parameter implies no branch and only works when the transfer protocol is file://.

Here are some example URLs:

SRC_URI = "git://git.oe.handhelds.org/git/vip.git;tag=version-1"
SRC_URI = "git://git.oe.handhelds.org/git/vip.git;protocol=http"


When using git as the fetcher of the main source code of your software, S should be set accordingly:

S = "${WORKDIR}/git"


Specifying passwords directly in git:// urls is not supported. There are several reasons: SRC_URI is often written out to logs and other places, and that could easily leak passwords; it is also all too easy to share metadata without removing passwords. SSH keys, ~/.netrc and ~/.ssh/config files can be used as alternatives.

4.3.6 Git Submodule Fetcher (gitsm://)

This fetcher submodule inherits from the Git fetcher and extends that fetcher’s behavior by fetching a repository’s submodules. SRC_URI is passed to the Git fetcher as described in the Git Fetcher (git://) section.


You must clean a recipe when switching between ‘git://’ and ‘gitsm://’ URLs.

The Git Submodules fetcher is not a complete fetcher implementation. The fetcher has known issues where it does not use the normal source mirroring infrastructure properly. Further, the submodule sources it fetches are not visible to the licensing and source archiving infrastructures.

4.3.7 ClearCase Fetcher (ccrc://)

This fetcher submodule fetches code from a ClearCase repository.

To use this fetcher, make sure your recipe has proper SRC_URI, SRCREV, and PV settings. Here is an example:

SRC_URI = "ccrc://cc.example.org/ccrc;vob=/example_vob;module=/example_module"
PV = "${@d.getVar("SRCREV", False).replace("/", "+")}"

The fetcher uses the rcleartool or cleartool remote client, depending on which one is available.

Following are options for the SRC_URI statement:

  • vob: The name, which must include the prepending “/” character, of the ClearCase VOB. This option is required.

  • module: The module, which must include the prepending “/” character, in the selected VOB.


    The module and vob options are combined to create the load rule in the view config spec. As an example, consider the vob and module values from the SRC_URI statement at the start of this section. Combining those values results in the following:

    load /example_vob/example_module
  • proto: The protocol, which can be either http or https.

By default, the fetcher creates a configuration specification. If you want this specification written to an area other than the default, use the CCASE_CUSTOM_CONFIG_SPEC variable in your recipe to define where the specification is written.


the SRCREV loses its functionality if you specify this variable. However, SRCREV is still used to label the archive after a fetch even though it does not define what is fetched.

Here are a couple of other behaviors worth mentioning:

  • When using cleartool, the login of cleartool is handled by the system. The login require no special steps.

  • In order to use rcleartool with authenticated users, an “rcleartool login” is necessary before using the fetcher.

4.3.8 Perforce Fetcher (p4://)

This fetcher submodule fetches code from the Perforce source control system. The executable used is specified by FETCHCMD_p4, which defaults to “p4”. The fetcher’s temporary working directory is set by P4DIR, which defaults to “DL_DIR/p4”. The fetcher does not make use of a perforce client, instead it relies on p4 files to retrieve a list of files and p4 print to transfer the content of those files locally.

To use this fetcher, make sure your recipe has proper SRC_URI, SRCREV, and PV values. The p4 executable is able to use the config file defined by your system’s P4CONFIG environment variable in order to define the Perforce server URL and port, username, and password if you do not wish to keep those values in a recipe itself. If you choose not to use P4CONFIG, or to explicitly set variables that P4CONFIG can contain, you can specify the P4PORT value, which is the server’s URL and port number, and you can specify a username and password directly in your recipe within SRC_URI.

Here is an example that relies on P4CONFIG to specify the server URL and port, username, and password, and fetches the Head Revision:

SRC_URI = "p4://example-depot/main/source/..."
PV = "p4-${SRCPV}"
S = "${WORKDIR}/p4"

Here is an example that specifies the server URL and port, username, and password, and fetches a Revision based on a Label:

P4PORT = "tcp:p4server.example.net:1666"
SRC_URI = "p4://user:passwd@example-depot/main/source/..."
SRCREV = "release-1.0"
PV = "p4-${SRCPV}"
S = "${WORKDIR}/p4"


You should always set S to “${WORKDIR}/p4” in your recipe.

By default, the fetcher strips the depot location from the local file paths. In the above example, the content of example-depot/main/source/ will be placed in ${WORKDIR}/p4. For situations where preserving parts of the remote depot paths locally is desirable, the fetcher supports two parameters:

  • “module”:

    The top-level depot location or directory to fetch. The value of this parameter can also point to a single file within the depot, in which case the local file path will include the module path.

  • “remotepath”:

    When used with the value “keep”, the fetcher will mirror the full depot paths locally for the specified location, even in combination with the module parameter.

Here is an example use of the the module parameter:

SRC_URI = "p4://user:passwd@example-depot/main;module=source/..."

In this case, the content of the top-level directory source/ will be fetched to ${P4DIR}, including the directory itself. The top-level directory will be accesible at ${P4DIR}/source/.

Here is an example use of the the remotepath parameter:

SRC_URI = "p4://user:passwd@example-depot/main;module=source/...;remotepath=keep"

In this case, the content of the top-level directory source/ will be fetched to ${P4DIR}, but the complete depot paths will be mirrored locally. The top-level directory will be accessible at ${P4DIR}/example-depot/main/source/.

4.3.9 Repo Fetcher (repo://)

This fetcher submodule fetches code from google-repo source control system. The fetcher works by initiating and syncing sources of the repository into REPODIR, which is usually ${DL_DIR}/repo.

This fetcher supports the following parameters:

  • “protocol”: Protocol to fetch the repository manifest (default: git).

  • “branch”: Branch or tag of repository to get (default: master).

  • “manifest”: Name of the manifest file (default: default.xml).

Here are some example URLs:

SRC_URI = "repo://REPOROOT;protocol=git;branch=some_branch;manifest=my_manifest.xml"
SRC_URI = "repo://REPOROOT;protocol=file;branch=some_branch;manifest=my_manifest.xml"

4.3.10 Az Fetcher (az://)

This submodule fetches data from an Azure Storage account , it inherits its functionality from the HTTP wget fetcher, but modifies its behavior to accomodate the usage of a Shared Access Signature (SAS) for non-public data.

Such functionality is set by the variable:

  • AZ_SAS: The Azure Storage Shared Access Signature provides secure delegate access to resources, if this variable is set, the Az Fetcher will use it when fetching artifacts from the cloud.

You can specify the AZ_SAS variable as shown below:

AZ_SAS = "se=2021-01-01&sp=r&sv=2018-11-09&sr=c&skoid=<skoid>&sig=<signature>"

Here is an example URL:

SRC_URI = "az://<azure-storage-account>.blob.core.windows.net/<foo_container>/<bar_file>"

It can also be used when setting mirrors definitions using the PREMIRRORS variable.

4.3.11 Other Fetchers

Fetch submodules also exist for the following:

  • Bazaar (bzr://)

  • Mercurial (hg://)

  • npm (npm://)

  • OSC (osc://)

  • Secure FTP (sftp://)

  • Secure Shell (ssh://)

  • Trees using Git Annex (gitannex://)

No documentation currently exists for these lesser used fetcher submodules. However, you might find the code helpful and readable.

4.4 Auto Revisions

We need to document AUTOREV and SRCREV_FORMAT here.