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# add_custom_command

Add a custom build rule to the generated build system.

There are two main signatures for add_custom_command The first signature is for adding a custom command to produce an output.

    add_custom_command(OUTPUT output1 [output2 ...]
                       COMMAND command1 [ARGS] [args1...]
                       [COMMAND command2 [ARGS] [args2...] ...]
                       [MAIN_DEPENDENCY depend]
                       [DEPENDS [depends...]]
                       [IMPLICIT_DEPENDS <lang1> depend1
                                        [<lang2> depend2] ...]
                       [WORKING_DIRECTORY dir]
                       [COMMENT comment] [VERBATIM] [APPEND])

This defines a command to generate specified OUTPUT file(s). A target created in the same directory (CMakeLists.txt file) that specifies any output of the custom command as a source file is given a rule to generate the file using the command at build time. Do not list the output in more than one independent target that may build in parallel or the two instances of the rule may conflict (instead use add_custom_target to drive the command and make the other targets depend on that one). If an output name is a relative path it will be interpreted relative to the build tree directory corresponding to the current source directory. Note that MAIN_DEPENDENCY is completely optional and is used as a suggestion to visual studio about where to hang the custom command. In makefile terms this creates a new target in the following form:

    OUTPUT: MAIN_DEPENDENCY DEPENDS
            COMMAND

If more than one command is specified they will be executed in order. The optional ARGS argument is for backward compatibility and will be ignored.

The second signature adds a custom command to a target such as a library or executable. This is useful for performing an operation before or after building the target. The command becomes part of the target and will only execute when the target itself is built. If the target is already built, the command will not execute.

    add_custom_command(TARGET target
                       PRE_BUILD | PRE_LINK | POST_BUILD
                       COMMAND command1 [ARGS] [args1...]
                       [COMMAND command2 [ARGS] [args2...] ...]
                       [WORKING_DIRECTORY dir]
                       [COMMENT comment] [VERBATIM])

This defines a new command that will be associated with building the specified target. When the command will happen is determined by which of the following is specified:

    PRE_BUILD - run before all other dependencies
    PRE_LINK - run after other dependencies
    POST_BUILD - run after the target has been built

Note that the PRE_BUILD option is only supported on Visual Studio 7 or later. For all other generators PRE_BUILD will be treated as PRE_LINK.

If WORKING_DIRECTORY is specified the command will be executed in the directory given. If it is a relative path it will be interpreted relative to the build tree directory corresponding to the current source directory. If COMMENT is set, the value will be displayed as a message before the commands are executed at build time. If APPEND is specified the COMMAND and DEPENDS option values are appended to the custom command for the first output specified. There must have already been a previous call to this command with the same output. The COMMENT, WORKING_DIRECTORY, and MAIN_DEPENDENCY options are currently ignored when APPEND is given, but may be used in the future.

If VERBATIM is given then all arguments to the commands will be escaped properly for the build tool so that the invoked command receives each argument unchanged. Note that one level of escapes is still used by the CMake language processor before add_custom_command even sees the arguments. Use of VERBATIM is recommended as it enables correct behavior. When VERBATIM is not given the behavior is platform specific because there is no protection of tool-specific special characters.

If the output of the custom command is not actually created as a file on disk it should be marked as SYMBOLIC with SET_SOURCE_FILES_PROPERTIES.

The IMPLICIT_DEPENDS option requests scanning of implicit dependencies of an input file. The language given specifies the programming language whose corresponding dependency scanner should be used. Currently only C and CXX language scanners are supported. The language has to be specified for every file in the IMPLICIT_DEPENDS list. Dependencies discovered from the scanning are added to those of the custom command at build time. Note that the IMPLICIT_DEPENDS option is currently supported only for Makefile generators and will be ignored by other generators.

If COMMAND specifies an executable target (created by ADD_EXECUTABLE) it will automatically be replaced by the location of the executable created at build time. Additionally a target-level dependency will be added so that the executable target will be built before any target using this custom command. However this does NOT add a file-level dependency that would cause the custom command to re-run whenever the executable is recompiled.

Arguments to COMMAND may use “generator expressions” with the syntax `$<...>`. See the `cmake-generator-expressions(7)` manual for available expressions.

Note that tgt is not added as a dependency of the target this expression is evaluated on.

    $<TARGET_POLICY:pol>          = '1' if the policy was NEW when the 'head' target was created, else '0'.  If the policy was not set, the warning message for the policy will be emitted.  This generator expression only works for a subset of policies.
    $<INSTALL_PREFIX>         = Content of the install prefix when the target is exported via INSTALL(EXPORT) and empty otherwise.

Boolean expressions:

    $<AND:?[,?]...>           = '1' if all '?' are '1', else '0'
    $<OR:?[,?]...>            = '0' if all '?' are '0', else '1'
    $<NOT:?>                  = '0' if '?' is '1', else '1'

where ‘?’ is always either ‘0’ or ‘1’.

Expressions with an implicit ‘this’ target:

    $<TARGET_PROPERTY:prop>   = The value of the property prop on the target on which the generator expression is evaluated.

References to target names in generator expressions imply target-level dependencies, but NOT file-level dependencies. List target names with the DEPENDS option to add file dependencies.

The DEPENDS option specifies files on which the command depends. If any dependency is an OUTPUT of another custom command in the same directory (CMakeLists.txt file) CMake automatically brings the other custom command into the target in which this command is built. If DEPENDS is not specified the command will run whenever the OUTPUT is missing; if the command does not actually create the OUTPUT then the rule will always run. If DEPENDS specifies any target (created by an ADD_* command) a target-level dependency is created to make sure the target is built before any target using this custom command. Additionally, if the target is an executable or library a file-level dependency is created to cause the custom command to re-run whenever the target is recompiled.

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