docs/build.tex

   1 One of the biggest challenges to getting started with embedded devices is that you
   2 cannot just install a copy of Linux and expect to be able to compile a firmware.
   3 Even if you did remember to install a compiler and every development tool offered,
   4 you still would not have the basic set of tools needed to produce a firmware image.
   5 The embedded device represents an entirely new hardware platform, which is
   6 most of the time incompatible with the hardware on your development machine, so in a process called
   7 cross compiling you need to produce a new compiler capable of generating code for
   8 your embedded platform, and then use it to compile a basic Linux distribution to
   9 run on your device.
  10
  11 The process of creating a cross compiler can be tricky, it is not something that is
  12 regularly attempted and so there is a certain amount of mystery and black magic
  13 associated with it. In many cases when you are dealing with embedded devices you will
  14 be provided with a binary copy of a compiler and basic libraries rather than
  15 instructions for creating your own -- it is a time saving step but at the same time
  16 often means you will be using a rather dated set of tools. Likewise, it is also common
  17 to be provided with a patched copy of the Linux kernel from the board or chip vendor,
  18 but this is also dated and it can be difficult to spot exactly what has been
  19 modified to make the kernel run on the embedded platform.
  20
  21 \subsection{Building an image}
  22
  23 OpenWrt takes a different approach to building a firmware; downloading, patching
  24 and compiling everything from scratch, including the cross compiler. To put it
  25 in simpler terms, OpenWrt does not contain any executables or even sources, it is an
  26 automated system for downloading the sources, patching them to work with the given
  27 platform and compiling them correctly for that platform. What this means is that
  28 just by changing the template, you can change any step in the process.
  29
  30 As an example, if a new kernel is released, a simple change to one of the Makefiles
  31 will download the latest kernel, patch it to run on the embedded platform and produce
  32 a new firmware image -- there is no work to be done trying to track down an unmodified
  33 copy of the existing kernel to see what changes had been made, the patches are
  34 already provided and the process ends up almost completely transparent. This does not
  35 just apply to the kernel, but to anything included with OpenWrt -- It is this one
  36 simple understated concept which is what allows OpenWrt to stay on the bleeding edge
  37 with the latest compilers, latest kernels and latest applications.
  38
  39 So let's take a look at OpenWrt and see how this all works.
  40
  41
  42 \subsubsection{Download OpenWrt}
  43
  44 This article refers to the "Kamikaze" branch of OpenWrt, which can be downloaded via
  45 subversion using the following command:
  46
  47 \begin{Verbatim}
  48 $ svn checkout https://svn.openwrt.org/openwrt/trunk kamikaze
  49 \end{Verbatim}
  50
  51 Additionally, there is a trac interface on \href{https://dev.openwrt.org/}{https://dev.openwrt.org/}
  52 which can be used to monitor svn commits and browse the source repository.
  53
  54
  55 \subsubsection{The directory structure}
  56
  57 There are four key directories in the base:
  58
  59 \begin{itemize}
  60     \item \texttt{tools}
  61     \item \texttt{toolchain}
  62     \item \texttt{package}
  63     \item \texttt{target}
  64 \end{itemize}
  65
  66 \texttt{tools} and \texttt{toolchain} refer to common tools which will be
  67 used to build the firmware image, the compiler, and the C library.
  68 The result of this is three new directories, \texttt{build\_dir/host}, which is a temporary
  69 directory for building the target independent tools, \texttt{build\_dir/toolchain-\textit{<arch>}*}
  70 which is used for building the toolchain for a specific architecture, and
  71 \texttt{staging\_dir/toolchain-\textit{<arch>}*} where the resulting toolchain is installed.
  72 You will not need to do anything with the toolchain directory unless you intend to
  73 add a new version of one of the components above.
  74
  75 \begin{itemize}
  76     \item \texttt{build\_dir/host}
  77     \item \texttt{build\_dir/toolchain-\textit{<arch>}*}
  78 \end{itemize}
  79
  80 \texttt{package} is for exactly that -- packages. In an OpenWrt firmware, almost everything
  81 is an \texttt{.ipk}, a software package which can be added to the firmware to provide new
  82 features or removed to save space. Note that packages are also maintained outside of the main
  83 trunk and can be obtained from subversion using the package feeds system:
  84
  85 \begin{Verbatim}
  86 $ ./scripts/feeds update
  87 \end{Verbatim}
  88
  89 Those packages can be used to extend the functionality of the build system and need to be
  90 symlinked into the main trunk. Once you do that, the packages will show up in the menu for
  91 configuration. From kamikaze you would do something like this:
  92
  93 \begin{Verbatim}
  94 $ ./scripts/feeds search nmap
  95 Search results in feed 'packages':
  96 nmap       Network exploration and/or security auditing utility
  97
  98 $ ./scripts/feeds install nmap
  99 \end{Verbatim}
 100
 101 To include all packages, issue the following command:
 102
 103 \begin{Verbatim}
 104 $ make package/symlinks
 105 \end{Verbatim}
 106
 107 \texttt{target} refers to the embedded platform, this contains items which are specific to
 108 a specific embedded platform. Of particular interest here is the "\texttt{target/linux}"
 109 directory which is broken down by platform \textit{<arch>} and contains the patches to the
 110 kernel, profile config, for a particular platform. There's also the "\texttt{target/image}" directory
 111 which describes how to package a firmware for a specific platform.
 112
 113 Both the target and package steps will use the directory "\texttt{build\_dir/\textit{<arch>}}"
 114 as a temporary directory for compiling. Additionally, anything downloaded by the toolchain,
 115 target or package steps will be placed in the "\texttt{dl}" directory.
 116
 117 \begin{itemize}
 118     \item \texttt{build\_dir/\textit{<arch>}}
 119     \item \texttt{dl}
 120 \end{itemize}
 121
 122 \subsubsection{Building OpenWrt}
 123
 124 While the OpenWrt build environment was intended mostly for developers, it also has to be
 125 simple enough that an inexperienced end user can easily build his or her own customized firmware.
 126
 127 Running the command "\texttt{make menuconfig}" will bring up OpenWrt's configuration menu
 128 screen, through this menu you can select which platform you're targeting, which versions of
 129 the toolchain you want to use to build and what packages you want to install into the
 130 firmware image. Note that it will also check to make sure you have the basic dependencies for it
 131 to run correctly.  If that fails, you will need to install some more tools in your local environment
 132 before you can begin.
 133
 134 Similar to the linux kernel config, almost every option has three choices,
 135 \texttt{y/m/n} which are represented as follows:
 136
 137 \begin{itemize}
 138     \item{\texttt{<*>} (pressing y)} \\
 139         This will be included in the firmware image
 140     \item{\texttt{<M>} (pressing m)} \\
 141         This will be compiled but not included (for later install)
 142     \item{\texttt{< >} (pressing n)} \\
 143         This will not be compiled
 144 \end{itemize}
 145
 146 After you've finished with the menu configuration, exit and when prompted, save your
 147 configuration changes.
 148
 149 If you want, you can also modify the kernel config for the selected target system.
 150 simply run "\texttt{make kernel\_menuconfig}" and the build system will unpack the kernel sources
 151 (if necessary), run menuconfig inside of the kernel tree, and then copy the kernel config
 152 to \texttt{target/linux/\textit{<platform>}/config} so that it is preserved over
 153 "\texttt{make clean}" calls.
 154
 155 To begin compiling the firmware, type "\texttt{make}". By default
 156 OpenWrt will only display a high level overview of the compile process and not each individual
 157 command.
 158
 159 \subsubsection{Example:}
 160
 161 \begin{Verbatim}
 162 make[2] toolchain/install
 163 make[3] -C toolchain install
 164 make[2] target/compile
 165 make[3] -C target compile
 166 make[4] -C target/utils prepare
 167
 168 [...]
 169 \end{Verbatim}
 170
 171 This makes it easier to monitor which step it's actually compiling and reduces the amount
 172 of noise caused by the compile output. To see the full output, run the command
 173 "\texttt{make V=99}".
 174
 175 During the build process, buildroot will download all sources to the "\texttt{dl}"
 176 directory and will start patching and compiling them in the "\texttt{build\_dir/\textit{<arch>}}"
 177 directory. When finished, the resulting firmware will be in the "\texttt{bin}" directory
 178 and packages will be in the "\texttt{bin/packages}" directory.
 179
 180
 181 \subsection{Creating packages}
 182
 183 One of the things that we've attempted to do with OpenWrt's template system is make it
 184 incredibly easy to port software to OpenWrt. If you look at a typical package directory
 185 in OpenWrt you'll find two things:
 186
 187 \begin{itemize}
 188     \item \texttt{package/\textit{<name>}/Makefile}
 189     \item \texttt{package/\textit{<name>}/patches}
 190     \item \texttt{package/\textit{<name>}/files}
 191 \end{itemize}
 192
 193 The patches directory is optional and typically contains bug fixes or optimizations to
 194 reduce the size of the executable. The package makefile is the important item, provides
 195 the steps actually needed to download and compile the package.
 196
 197 The files directory is also optional and typicall contains package specific startup scripts or default configuration files that can be used out of the box with OpenWrt.
 198
 199 Looking at one of the package makefiles, you'd hardly recognize it as a makefile.
 200 Through what can only be described as blatant disregard and abuse of the traditional
 201 make format, the makefile has been transformed into an object oriented template which
 202 simplifies the entire ordeal.
 203
 204 Here for example, is \texttt{package/bridge/Makefile}:
 205
 206 \begin{Verbatim}[frame=single,numbers=left]
 207
 208 include $(TOPDIR)/rules.mk
 209
 210 PKG_NAME:=bridge
 211 PKG_VERSION:=1.0.6
 212 PKG_RELEASE:=1
 213
 214 PKG_SOURCE:=bridge-utils-$(PKG_VERSION).tar.gz
 215 PKG_SOURCE_URL:=@SF/bridge
 216 PKG_MD5SUM:=9b7dc52656f5cbec846a7ba3299f73bd
 217 PKG_CAT:=zcat
 218
 219 PKG_BUILD_DIR:=$(BUILD_DIR)/bridge-utils-$(PKG_VERSION)
 220
 221 include $(INCLUDE_DIR)/package.mk
 222
 223 define Package/bridge
 224   SECTION:=net
 225   CATEGORY:=Base system
 226   TITLE:=Ethernet bridging configuration utility
 227   URL:=http://bridge.sourceforge.net/
 228 endef
 229
 230 define Package/bridge/description
 231   Manage ethernet bridging:
 232   a way to connect networks together to form a larger network.
 233 endef
 234
 235 define Build/Configure
 236     $(call Build/Configure/Default, \
 237         --with-linux-headers="$(LINUX_DIR)" \
 238     )
 239 endef
 240
 241 define Package/bridge/install
 242     $(INSTALL_DIR) $(1)/usr/sbin
 243     $(INSTALL_BIN) $(PKG_BUILD_DIR)/brctl/brctl $(1)/usr/sbin/
 244 endef
 245
 246 $(eval $(call BuildPackage,bridge))
 247 \end{Verbatim}
 248
 249 As you can see, there's not much work to be done; everything is hidden in other makefiles
 250 and abstracted to the point where you only need to specify a few variables.
 251
 252 \begin{itemize}
 253     \item \texttt{PKG\_NAME} \\
 254         The name of the package, as seen via menuconfig and ipkg
 255     \item \texttt{PKG\_VERSION} \\
 256         The upstream version number that we are downloading
 257     \item \texttt{PKG\_RELEASE} \\
 258         The version of this package Makefile
 259     \item \texttt{PKG\_SOURCE} \\
 260         The filename of the original sources
 261     \item \texttt{PKG\_SOURCE\_URL} \\
 262         Where to download the sources from (no trailing slash), you can add multiple download sources by separating them with a \\ and a carriage return.
 263     \item \texttt{PKG\_MD5SUM} \\
 264         A checksum to validate the download
 265     \item \texttt{PKG\_CAT} \\
 266         How to decompress the sources (zcat, bzcat, unzip)
 267     \item \texttt{PKG\_BUILD\_DIR} \\
 268         Where to compile the package
 269 \end{itemize}
 270
 271 The \texttt{PKG\_*} variables define where to download the package from;
 272 \texttt{@SF} is a special keyword for downloading packages from sourceforge. There is also
 273 another keyword of \texttt{@GNU} for grabbing GNU source releases. If any of the above mentionned download source fails, the OpenWrt mirrors will be used as source.
 274
 275 The md5sum (if present) is used to verify the package was downloaded correctly and
 276 \texttt{PKG\_BUILD\_DIR} defines where to find the package after the sources are
 277 uncompressed into \texttt{\$(BUILD\_DIR)}.
 278
 279 At the bottom of the file is where the real magic happens, "BuildPackage" is a macro
 280 set up by the earlier include statements. BuildPackage only takes one argument directly --
 281 the name of the package to be built, in this case "\texttt{bridge}". All other information
 282 is taken from the define blocks. This is a way of providing a level of verbosity, it's
 283 inherently clear what the contents of the \texttt{description} template in
 284 \texttt{Package/bridge} is, which wouldn't be the case if we passed this information
 285 directly as the Nth argument to \texttt{BuildPackage}.
 286
 287 \texttt{BuildPackage} uses the following defines:
 288
 289 \textbf{\texttt{Package/\textit{<name>}}:} \\
 290     \texttt{\textit{<name>}} matches the argument passed to buildroot, this describes
 291     the package the menuconfig and ipkg entries. Within \texttt{Package/\textit{<name>}}
 292     you can define the following variables:
 293
 294     \begin{itemize}
 295         \item \texttt{SECTION} \\
 296             The type of package (currently unused)
 297         \item \texttt{CATEGORY} \\
 298             Which menu it appears in menuconfig: Network, Sound, Utilities, Multimedia ...
 299         \item \texttt{TITLE} \\
 300             A short description of the package
 301         \item \texttt{URL} \\
 302             Where to find the original software
 303         \item \texttt{MAINTAINER} (optional) \\
 304             Who to contact concerning the package
 305         \item \texttt{DEPENDS} (optional) \\
 306             Which packages must be built/installed before this package. To reference a dependency defined in the
 307                         same Makefile, use \textit{<dependency name>}. If defined as an external package, use
 308                         \textit{+<dependency name>}. For a kernel version dependency use: \textit{@LINUX\_2\_<minor version>}
 309                 \item \texttt{BUILDONLY} (optional) \\
 310                         Set this option to 1 if you do NOT want your package to appear in menuconfig.
 311                         This is useful for packages which are only used as build dependencies.
 312     \end{itemize}
 313
 314 \textbf{\texttt{Package/\textit{<name>}/conffiles} (optional):} \\
 315    A list of config files installed by this package, one file per line.
 316
 317 \textbf{\texttt{Build/Prepare} (optional):} \\
 318    A set of commands to unpack and patch the sources. You may safely leave this
 319    undefined.
 320
 321 \textbf{\texttt{Build/Configure} (optional):} \\
 322    You can leave this undefined if the source doesn't use configure or has a
 323    normal config script, otherwise you can put your own commands here or use
 324    "\texttt{\$(call Build/Configure/Default,\textit{<first list of arguments, second list>})}" as above to
 325    pass in additional arguments for a standard configure script. The first list of arguments will be passed
 326    to the configure script like that: \texttt{--arg 1} \texttt{--arg 2}. The second list contains arguments that should be
 327    defined before running the configure script such as autoconf or compiler specific variables.
 328
 329    To make it easier to modify the configure command line, you can either extend or completely override the following variables:
 330    \begin{itemize}
 331      \item \texttt{CONFIGURE\_ARGS} \\
 332              Contains all command line arguments (format: \texttt{--arg 1} \texttt{--arg 2})
 333      \item \texttt{CONFIGURE\_VARS} \\
 334              Contains all environment variables that are passed to ./configure (format: \texttt{NAME="value"})
 335    \end{itemize}
 336
 337 \textbf{\texttt{Build/Compile} (optional):} \\
 338    How to compile the source; in most cases you should leave this undefined.
 339
 340    As with \texttt{Build/Configure} there are two variables that allow you to override
 341    the make command line environment variables and flags:
 342    \begin{itemize}
 343      \item \texttt{MAKE\_FLAGS} \\
 344            Contains all command line arguments (typically variable overrides like \texttt{NAME="value"}
 345          \item \texttt{MAKE\_VARS} \\
 346            Contains all environment variables that are passed to the make command
 347    \end{itemize}
 348
 349 \textbf{\texttt{Build/InstallDev} (optional):} \\
 350         If your package provides a library that needs to be made available to other packages,
 351         you can use the \texttt{Build/InstallDev} template to copy it into the staging directory
 352         which is used to collect all files that other packages might depend on at build time.
 353         When it is called by the build system, two parameters are passed to it. \texttt{\$(1)} points to
 354         the regular staging dir, typically \texttt{staging\_dir/\textit{ARCH}}, while \texttt{\$(2)} points
 355         to \texttt{staging\_dir/host}. The host staging dir is only used for binaries, which are
 356         to be executed or linked against on the host and its \texttt{bin/} subdirectory is included
 357         in the \texttt{PATH} which is passed down to the build system processes.
 358         Please use \texttt{\$(1)} and \texttt{\$(2)} here instead of the build system variables
 359         \texttt{\$(STAGING\_DIR)} and \texttt{\$(STAGING\_DIR\_HOST)}, because the build system behavior
 360         when staging libraries might change in the future to include automatic uninstallation.
 361
 362 \textbf{\texttt{Package/\textit{<name>}/install}:} \\
 363    A set of commands to copy files out of the compiled source and into the ipkg
 364    which is represented by the \texttt{\$(1)} directory. Note that there are currently
 365    4 defined install macros:
 366    \begin{itemize}
 367        \item \texttt{INSTALL\_DIR} \\
 368            install -d -m0755
 369        \item \texttt{INSTALL\_BIN} \\
 370            install -m0755
 371        \item \texttt{INSTALL\_DATA} \\
 372            install -m0644
 373        \item \texttt{INSTALL\_CONF} \\
 374            install -m0600
 375    \end{itemize}
 376
 377 The reason that some of the defines are prefixed by "\texttt{Package/\textit{<name>}}"
 378 and others are simply "\texttt{Build}" is because of the possibility of generating
 379 multiple packages from a single source. OpenWrt works under the assumption of one
 380 source per package Makefile, but you can split that source into as many packages as
 381 desired. Since you only need to compile the sources once, there's one global set of
 382 "\texttt{Build}" defines, but you can add as many "Package/<name>" defines as you want
 383 by adding extra calls to \texttt{BuildPackage} -- see the dropbear package for an example.
 384
 385 After you have created your \texttt{package/\textit{<name>}/Makefile}, the new package
 386 will automatically show in the menu the next time you run "make menuconfig" and if selected
 387 will be built automatically the next time "\texttt{make}" is run.
 388
 389 \subsection{Creating binary packages}
 390
 391 You might want to create binary packages and include them in the resulting images as packages.
 392 To do so, you can use the following template, which basically sets to nothing the Configure and
 393 Compile templates.
 394
 395 \begin{Verbatim}[frame=single,numbers=left]
 396
 397 include $(TOPDIR)/rules.mk
 398
 399 PKG_NAME:=binpkg
 400 PKG_VERSION:=1.0
 401 PKG_RELEASE:=1
 402
 403 PKG_SOURCE:=binpkg-$(PKG_VERSION).tar.gz
 404 PKG_SOURCE_URL:=http://server
 405 PKG_MD5SUM:=9b7dc52656f5cbec846a7ba3299f73bd
 406 PKG_CAT:=zcat
 407
 408 include $(INCLUDE_DIR)/package.mk
 409
 410 define Package/binpkg
 411   SECTION:=net
 412   CATEGORY:=Network
 413   TITLE:=Binary package
 414 endef
 415
 416 define Package/bridge/description
 417   Binary package
 418 endef
 419
 420 define Build/Configure
 421 endef
 422
 423 define Build/Compile
 424 endef
 425
 426 define Package/bridge/install
 427     $(INSTALL_DIR) $(1)/usr/sbin
 428     $(INSTALL_BIN) $(PKG_BUILD_DIR)/* $(1)/usr/sbin/
 429 endef
 430
 431 $(eval $(call BuildPackage,bridge))
 432 \end{Verbatim}
 433
 434 Provided that the tarball which contains the binaries reflects the final
 435 directory layout (/usr, /lib ...), it becomes very easy to get your package
 436 look like one build from sources.
 437
 438 Note that using the same technique, you can easily create binary pcakages
 439 for your proprietary kernel modules as well.
 440
 441 \subsection{Creating kernel modules packages}
 442
 443 The OpenWrt distribution makes the distinction between two kind of kernel modules, those coming along with the mainline kernel, and the others available as a separate project. We will see later that a common template is used for both of them.
 444
 445 For kernel modules that are part of the mainline kernel source, the makefiles are located in \textit{package/kernel/modules/*.mk} and they appear under the section "Kernel modules"
 446
 447 For external kernel modules, you can add them to the build system just like if they were software packages by defining a KernelPackage section in the package makefile.
 448
 449 Here for instance the Makefile for the I2C subsytem kernel modules :
 450
 451 \begin{Verbatim}[frame=single,numbers=left]
 452
 453 I2CMENU:=I2C Bus
 454
 455 define KernelPackage/i2c-core
 456   TITLE:=I2C support
 457   DESCRIPTION:=Kernel modules for i2c support
 458   SUBMENU:=$(I2CMENU)
 459   KCONFIG:=CONFIG_I2C_CORE CONFIG_I2C_DEV
 460   FILES:=$(MODULES_DIR)/kernel/drivers/i2c/*.$(LINUX_KMOD_SUFFIX)
 461   AUTOLOAD:=$(call AutoLoad,50,i2c-core i2c-dev)
 462 endef
 463 $(eval $(call KernelPackage,i2c-core))
 464 \end{Verbatim}
 465
 466 To group kernel modules under a common description in menuconfig, you might want to define a \textit{<description>MENU} variable on top of the kernel modules makefile.
 467
 468 \begin{itemize}
 469     \item \texttt{TITLE} \\
 470         The name of the module as seen via menuconfig
 471     \item \texttt{DESCRIPTION} \\
 472         The description as seen via help in menuconfig
 473     \item \texttt{SUBMENU} \\
 474         The sub menu under which this package will be seen
 475     \item \texttt{KCONFIG} \\
 476         Kernel configuration option dependency. For external modules, remove it.
 477     \item \texttt{FILES} \\
 478         Files you want to inlude to this kernel module package, separate with spaces.
 479     \item \texttt{AUTOLOAD} \\
 480         Modules that will be loaded automatically on boot, the order you write them is the order they would be loaded.
 481 \end{itemize}
 482
 483 After you have created your \texttt{package/kernel/modules/\textit{<name>}.mk}, the new kernel modules package
 484 will automatically show in the menu under "Kernel modules" next time you run "make menuconfig" and if selected
 485 will be built automatically the next time "\texttt{make}" is run.
 486
 487 \subsection{Conventions}
 488
 489 There are a couple conventions to follow regarding packages:
 490
 491 \begin{itemize}
 492     \item \texttt{files}
 493     \begin{enumerate}
 494         \item configuration files follow the convention \\
 495         \texttt{\textit{<name>}.conf}
 496         \item init files follow the convention \\
 497         \texttt{\textit{<name>}.init}
 498     \end{enumerate}
 499     \item \texttt{patches}
 500     \begin{enumerate}
 501         \item patches are numerically prefixed and named related to what they do
 502     \end{enumerate}
 503 \end{itemize}
 504
 505 \subsection{Troubleshooting}
 506
 507 If you find your package doesn't show up in menuconfig, try the following command to
 508 see if you get the correct description:
 509
 510 \begin{Verbatim}
 511   TOPDIR=$PWD make -C package/<name> DUMP=1 V=99
 512 \end{Verbatim}
 513
 514 If you're just having trouble getting your package to compile, there's a few
 515 shortcuts you can take. Instead of waiting for make to get to your package, you can
 516 run one of the following:
 517
 518 \begin{itemize}
 519     \item \texttt{make package/\textit{<name>}/clean V=99}
 520     \item \texttt{make package/\textit{<name>}/install V=99}
 521 \end{itemize}
 522
 523 Another nice trick is that if the source directory under \texttt{build\_dir/\textit{<arch>}}
 524 is newer than the package directory, it won't clobber it by unpacking the sources again.
 525 If you were working on a patch you could simply edit the sources under the
 526 \texttt{build\_dir/\textit{<arch>}/\textit{<source>}} directory and run the install command above,
 527 when satisfied, copy the patched sources elsewhere and diff them with the unpatched
 528 sources. A warning though - if you go modify anything under \texttt{package/\textit{<name>}}
 529 it will remove the old sources and unpack a fresh copy.
 530
 531 Other useful targets include:
 532
 533 \begin{itemize}
 534     \item \texttt{make package/\textit{<name>}/prepare V=99}
 535     \item \texttt{make package/\textit{<name>}/compile V=99}
 536     \item \texttt{make package/\textit{<name>}/configure V=99}
 537 \end{itemize}
 538
 539
 540 \subsection{Using build environments}
 541 OpenWrt provides a means of building images for multiple configurations
 542 which can use multiple targets in one single checkout. These \emph{environments}
 543 store a copy of the .config file generated by \texttt{make menuconfig} and the contents
 544 of the \texttt{./files} folder.
 545 The script \texttt{./scripts/env} is used to manage these environments, it uses
 546 \texttt{git} (which needs to be installed on your system) as backend for version control.
 547
 548 The command
 549 \begin{Verbatim}
 550   ./scripts/env help
 551 \end{Verbatim}
 552 produces a short help text with a list of commands.
 553
 554 To create a new environment named \texttt{current}, run the following command
 555 \begin{Verbatim}
 556   ./scripts/env new current
 557 \end{Verbatim}
 558 This will move your \texttt{.config} file and \texttt{./files} (if it exists) to
 559 the \texttt{env/} subdirectory and create symlinks in the base folder.
 560
 561 After running make menuconfig or changing things in files/, your current state will
 562 differ from what has been saved before. To show these changes, use:
 563 \begin{Verbatim}
 564   ./scripts/env diff
 565 \end{Verbatim}
 566
 567 If you want to save these changes, run:
 568 \begin{Verbatim}
 569   ./scripts/env save
 570 \end{Verbatim}
 571 If you want to revert your changes to the previously saved copy, run:
 572 \begin{Verbatim}
 573   ./scripts/env revert
 574 \end{Verbatim}
 575
 576 If you want, you can now create a second environment using the \texttt{new} command.
 577 It will ask you whether you want to make it a clone of the current environment (e.g.
 578 for minor changes) or if you want to start with a clean version (e.g. for selecting
 579 a new target).
 580
 581 To switch to a different environment (e.g. \texttt{test1}), use:
 582 \begin{Verbatim}
 583   ./scripts/env switch test1
 584 \end{Verbatim}
 585
 586 To rename the current branch to a new name (e.g. \texttt{test2}), use:
 587 \begin{Verbatim}
 588   ./scripts/env rename test2
 589 \end{Verbatim}
 590
 591 If you want to get rid of environment switching and keep everything in the base directory
 592 again, use:
 593 \begin{Verbatim}
 594   ./scripts/env clear
 595 \end{Verbatim}