# `ninja.build` reference

Illustrative file:

``` ninja
# Variables
CXX = g++
CXXFLAGS = -Wall -Wextra -pedantic-errors

# Command definitions
rule link
    command = $CXX $CXXFLAGS $in -o $out

rule compile
    command = $CXX $CXXFLAGS -c $in -o $out

# Targets
build main.exe: link main.o Product.o

build main.o: compile main.cpp

build Product.o: compile Product.cpp | Product.h
# the above target rule uses | to specify implicit dependencies, in this case Product.h

# Miscellaneous rules
default main.exe
# Don't forget a newline at the end!

```

## Reference

- `#`: Used at every line beginning to signify a line comment

- `variableName = value`: how you store a string value inside a variable, used for substitution.

    > Example:
    >
    > ```
    > CXXFLAGS = -Wall -Wextra -pedantic-errors
    > ```
    >
    > could be used for defining C++ compiler flags

- `rule`: used to define commands (note that a tab is required before the `command` keyword)

    > Example:
    > ``` ninja
    > rule compile
    >     command = $CXX $CXXFLAGS -c $in -o $out
    > ```
    > could be used to define how an object file is compiled with a given `$CXX` compiler

- `build`: used to define a build target

    > Example:
    > `build main.exe: link main.o Product.o`
    > uses the `link` command on input files `main.o` and `Product.o` to produce the output file `main.exe`.
    >
    > Note that this means `$in = main.o Product.o` and `$out = main.exe`.
    >
    > IMPORTANT: the `|` symbol can be used to signify **implicit dependencies**. Example:
    > `build Product.o: compile Product.cpp | Product.h`
    > implies that `Product.h` is indirectly involved in the production of `Product.o`, in that it is not an input file to be used for the `compile` command (`$in = Product.cpp` only), but its modification requires `Product.o` to be updated. This may happen when `Product.h` is `#include`d in `Product.cpp`.

- `default`: used to signify a default build target, i.e. what happens when `ninja` is executed with no build target specified.

    > Example:
    > `default main.exe`
    > could be used to specify that `main.exe` is the default build target.

- `ninja.build` only uses linefeeds, no carriage returns allowed

- A newline is required at EOF