ninja

Q: How do I install ninja?

Run npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill ninja in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/ninja/ (personal) or .claude/skills/ninja/ (project).

by mohitmishra786

Ninja build system skill. Use when diagnosing Ninja build failures, understanding Ninja's role as a low-level build executor generated by CMake or other meta-build systems, tuning parallelism, interpreting Ninja output, or working with build.ninja files. Activates on queries about ninja errors, ninja parallelism, ninja verbose output, build.ninja format, or ninja as a CMake generator.

Installs: 629
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Security: Not audited

Install

Works with every agent the skills CLI supports.

npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill ninja

01
Install
Run the command in your project. The CLI asks which agents to install for.
02
Describe the task
Ask your agent for something this skill covers. It loads ninja automatically when relevant.
03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.

SKILL.md

The instructions your agent reads when the skill is triggered.

Ninja

Purpose

Guide agents through Ninja as a build executor: diagnosing failures, controlling parallelism, generating from CMake, and understanding the .ninja file format when needed.

Triggers

"Ninja is failing — how do I get more output?"
"How do I use Ninja with CMake?"
"How many parallel jobs does Ninja use?"
"How do I add a custom build step in Ninja?"
"What is a build.ninja file?"

Workflow

1. Ninja as a CMake generator

The most common use of Ninja is as the build executor for CMake:

# Configure with Ninja
cmake -S . -B build -G Ninja
cmake --build build                    # uses ninja internally

# Or invoke ninja directly
cd build && ninja

# Specify parallelism
ninja -j4
ninja -j$(nproc)

# Build specific target
ninja myapp
ninja install

CMake also supports Ninja Multi-Config:

cmake -S . -B build -G "Ninja Multi-Config"
cmake --build build --config Release
cmake --build build --config Debug

2. Verbose output and diagnostics

# Show full commands (not just [CC] foo.c)
ninja -v

# Dry run (show what would be built)
ninja -n

# Show why a target needs rebuilding
ninja -d explain myapp

# Print all targets
ninja -t targets all

# Print targets grouped by rule
ninja -t targets rule cc

# Dependency graph (graphviz)
ninja -t graph myapp | dot -Tsvg -o deps.svg

3. Common Ninja flags

Flag	Effect
`-j N`	Parallel jobs (default: CPUs + 2)
`-l N`	Don't start new jobs if load average > N
`-k N`	Keep going after N failures (default 1)
`-v`	Verbose: show full command lines
`-n`	Dry run
`-C dir`	Change to `dir` before doing anything
`-t tool`	Run a sub-tool (`clean`, `query`, `targets`, `graph`, `compdb`)

4. Cleaning

ninja -t clean          # remove build outputs
ninja -t clean -g       # also remove generated files

Or via CMake:

cmake --build build --target clean

5. compile_commands.json

Ninja (via CMake) can generate a compile_commands.json for IDE integration and clang-tidy:

cmake -S . -B build -G Ninja -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
ln -sf build/compile_commands.json .

6. build.ninja format (reference)

Rarely hand-written, but useful to understand for debugging:

# Variable
cflags = -Wall -O2

# Rule
rule cc
  command = gcc $cflags -c $in -o $out
  description = CC $in

# Build edge
build foo.o: cc foo.c

# Phony target
build all: phony foo.o

# Default target
default all

Key concepts:

rule: defines how to produce outputs from inputs
build: instantiates a rule with specific files
$in / $out: automatic variables for inputs/outputs
phony: a target that is always considered out of date (like .PHONY in make)

7. Ninja sub-tools

# List all build targets
ninja -t targets

# Query dependencies of a target
ninja -t query myapp

# Clean (already mentioned)
ninja -t clean

# Generate compile_commands.json (if supported by generator)
ninja -t compdb cc cxx > compile_commands.json

# List rules
ninja -t rules

8. Common issues

Issue	Cause	Fix
`ninja: error: 'foo.o', needed by 'prog', missing and no known rule to make it`	Missing build rule	Regenerate with CMake; check `add_executable` source list
Build not picking up changes	Stale `build.ninja`	Re-run `cmake -S . -B build`
Very slow parallel build	`-j` too high for I/O-bound build	Use `-l$(nproc)` to limit by load
Circular dependency	Rule depends on itself	Check CMake target dependencies

For the full Ninja command reference, build.ninja format details, and CMake integration patterns, see references/cheatsheet.md.

Related skills

Use skills/build-systems/cmake for CMake configuration that generates Ninja files
Use skills/build-systems/make for Make-based projects

FAQ

What does the ninja skill do?

How do I install ninja?

Run npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill ninja in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/ninja/ (personal) or .claude/skills/ninja/ (project).

Which AI agents can use ninja?

Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.

Is ninja safe to install?

No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.

Who maintains ninja?

It is published by mohitmishra786 in mohitmishra786/low-level-dev-skills. It has 629 recorded installs.