Using the compiler

    The run command compiles the source file hello_world.cr to a binary executable in a temporary location and immediately executes it.

    The crystal build command builds a binary executable. The output file has the same name as the source file minus the extension .cr.

    1. $ crystal build hello_world.cr
    2. $ ./hello_world
    3. Hello World!

    By default, the generated executables are not fully optimized. The --release flag can be used to enable optimizations.

    1. $ crystal build hello_world.cr --release

    Compiling without release mode is much faster and the resulting binaries still offer pretty good performance.

    Building in release mode should be used for production-ready executables and when performing benchmarks. For simple development builds, there is usually no reason to do so.

    To reduce the binary size for distributable files, the --no-debug flag can be used. This removes debug symbols reducing file size, but obviously making debugging more difficult.

    Creating a statically-linked executable

    The --static flag can be used to build a statically-linked executable:

    1. $ crystal build hello_world.cr --release --static

    Note

    Building fully statical linked executables is currently only supported on Alpine Linux.

    More information about statically linking can be found in the Static Linking guide.

    The compiler uses the CRYSTAL_LIBRARY_PATH environment variable as a first lookup destination for static and dynamic libraries that are to be linked. This can be used to provide static versions of libraries that are also available as dynamic libraries.

    Creating a Crystal project

    The crystal init command helps to initialize a Crystal project folder, setting up a basic project structure. crystal init app <name> is used for an application, crystal init lib <name> for a library.

    1. $ crystal init app myapp
    2.     create  myapp/.gitignore
    3.     create  myapp/.editorconfig
    4.     create  myapp/LICENSE
    5.     create  myapp/README.md
    6.     create  myapp/shard.yml
    7.     create  myapp/src/myapp.cr
    8.     create  myapp/spec/spec_helper.cr
    9.     create  myapp/spec/myapp_spec.cr
    10. Initialized empty Git repository in /home/crystal/myapp/.git/

    Not all of these files are required for every project, and some might need more customization, but crystal init creates a good default environment for developing Crystal applications and libraries.

    • : generate a new project
    • crystal build: build an executable
    • : generate documentation
    • crystal env: print Crystal environment information
    • : eval code from args or standard input
    • crystal play: starts crystal playground server
    • : build and run program
    • crystal spec: build and run specs
    • : run a compiler tool
    • crystal help: show help about commands and options
    • crystal version: show version

    To see the available options for a particular command, use --help after a command:

    crystal run

    The run command compiles a source file to a binary executable and immediately runs it.

    1. crystal [run] [<options>] <programfile> [-- <argument>...]

    Arguments to the compiled binary can be separated with double dash -- from the compiler arguments. The binary executable is stored in a temporary location between compiling and running.

    Example:

    1. $ echo 'puts "Hello #{ARGV[0]?}!"' > hello_world.cr
    2. $ crystal run hello_world.cr -- Crystal

    Common options:

    • --release: Compile in release mode, doing extra work to apply optimizations to the generated code.
    • --progress: Show progress during compilation.
    • --static: Link statically.

    The crystal build command builds a dynamically-linked binary executable.

    1. crystal build [<options>] <programfile>

    Unless specified, the resulting binary will have the same name as the source file minus the extension .cr.

    Example:

    Common options:

    • --cross-compile: Generate a .o file, and print the command to generate an executable to stdout.
    • -D FLAG, --define FLAG: Define a compile-time flag.
    • -o <output_file>: Define the name of the binary executable.
    • --release: Compile in release mode, doing extra work to apply optimizations to the generated code.
    • --link-flags FLAGS: Additional flags to pass to the linker.
    • --no-debug: Skip any symbolic debug info, reducing the output file size.
    • --progress: Show progress during compilation.
    • --static: Link statically.
    • --verbose: Display executed commands.

    More options are described in the integrated help: crystal build --help or man page man crystal.

    crystal eval

    The crystal eval command reads Crystal source code from command line or stdin, compiles it to a binary executable and immediately runs it.

    1. crystal eval [<options>] [<source>]

    If no source argument is provided, the Crystal source is read from standard input. The binary executable is stored in a temporary location between compiling and running.

    Example:

    1. $ crystal eval 'puts "Hello World"'
    2. Hello World!
    3. $ echo 'puts "Hello World"' | crystal eval

    Note

    When running interactively, stdin can usually be closed by typing the end of transmission character (Ctrl+D).

    Common options:

    • -o <output_file>: Define the name of the binary executable.
    • --release: Compile in release mode, doing extra work to apply optimizations to the generated code.
    • --no-debug: Skip any symbolic debug info, reducing the output file size.
    • --progress: Show progress during compilation.
    • --static: Link statically.

    More options are described in the integrated help: crystal eval --help or man page man crystal.

    crystal version

    The crystal version command prints the Crystal version, LLVM version and default target triple.

    1. crystal version

    Example:

    1. $ crystal version
    2. Crystal 1.5.0 [994c70b10] (2022-07-06)
    4. LLVM: 10.0.0
    5. Default target: x86_64-unknown-linux-gnu

    crystal init

    The crystal init command initializes a Crystal project folder.

    1. crystal init (lib|app) <name> [<dir>]

    The first argument is either lib or app. A lib is a reusable library whereas app describes an application not intended to be used as a dependency. A library doesn’t have a shard.lock file in its repository and no build target in shard.yml, but instructions for using it as a dependency.

    Example:

    1. $ crystal init lib my_cool_lib
    2.     create  my_cool_lib/.gitignore
    3.     create  my_cool_lib/.editorconfig
    4.     create  my_cool_lib/LICENSE
    5.     create  my_cool_lib/README.md
    6.     create  my_cool_lib/shard.yml
    7.     create  my_cool_lib/src/my_cool_lib.cr
    8.     create  my_cool_lib/spec/spec_helper.cr
    9.     create  my_cool_lib/spec/my_cool_lib_spec.cr
    10. Initialized empty Git repository in ~/my_cool_lib/.git/

    The crystal docs command generates API documentation from inline docstrings in Crystal files (see ).

    1. crystal docs [--output=<output_dir>] [--canonical-base-url=<url>] [<source_file>...]

    By default, all Crystal files in ./src will be appended (i.e. src/**/*.cr). In order to account for load-order dependencies, source_file can be used to specify one (or multiple) entrypoints for the docs generator.

    Common options:

    • --project-name=NAME: Set the project name. The default value is extracted from shard.yml if available. In case no default can be found, this option is mandatory.
    • --project-version=VERSION: Set the project version. The default value is extracted from current git commit or shard.yml if available. In case no default can be found, this option is mandatory.
    • --output=DIR, -o DIR: Set the output directory (default: ./docs)
    • --canonical-base-url=URL, -b URL: Set the canonical base url

    For the above example to output the docs at public with custom canonical base url, and entrypoint src/my_app.cr, the following arguments can be used:

    1. crystal docs --output public --canonical-base-url http://example.com/ src/my_app.cr

    crystal env

    The crystal env command prints environment variables used by Crystal.

    1. crystal env [<var>...]

    By default, it prints information as a shell script. If one or more var arguments are provided, the value of each named variable is printed on its own line.

    Example:

    1. CRYSTAL_CACHE_DIR="/home/crystal/.cache/crystal"
    2. CRYSTAL_PATH="/usr/bin/../share/crystal/src:lib"
    3. CRYSTAL_VERSION="0.28.0"
    4. CRYSTAL_LIBRARY_PATH="/usr/bin/../lib/crystal/lib"
    5. $ crystal env CRYSTAL_VERSION
    6. 0.28.0

    crystal spec

    The crystal spec command compiles and runs a Crystal spec suite.

    1. crystal spec [<options>] [<file>...] [-- [<runner_options>]]

    All files arguments are concatenated into a single Crystal source. If an argument points to a folder, all spec files inside that folder are appended. If no files argument is provided, the default is . A filename can be suffixed by : and a line number, providing this location to the --location runner option (see below).

    Run crystal spec --options for available options.

    Runner options:

    runner_options are provided to the compiled binary executable which runs the specs. They should be separated from the other arguments by a double dash (--).

    • --verbose: Prints verbose output, including all example names.
    • --profile: Prints the 10 slowest specs.
    • --fail-fast: Abort the spec run on first failure.
    • --junit_output <output_dir>: Generates JUnit XML output.

    The following options can be combined to filter the list of specs to run.

    • --example <name>: Runs examples whose full nested names include name.
    • --line <line>: Runs examples whose line matches line.
    • --location <file>:<line>: Runs example(s) at line in file (multiple options allowed).
    • --tag <tag>: Runs examples with the specified tag, or excludes examples by adding ~ before the tag (multiple options allowed).
      • --tag a --tag b will include specs tagged with a OR b.
      • --tag ~a --tag ~b will include specs not tagged with a AND not tagged with b.
      • --tag a --tag ~b will include specs tagged with a, but not tagged with b

    Example:

    1. $ crystal spec
    2. F
    4. Failures:
    6.   1) Myapp works
    7.      Failure/Error: false.should eq(true)
    9.        Expected: true
    10.             got: false
    12.      # spec/myapp_spec.cr:7
    14. Finished in 880 microseconds
    15. 1 examples, 1 failures, 0 errors, 0 pending
    17. Failed examples:
    19. crystal spec spec/myapp_spec.cr:6 # Myapp works

    crystal play

    The crystal play command starts a webserver serving an interactive Crystal playground.

    1. crystal play [--port <port>] [--binding <host>] [--verbose] [file]

    • crystal tool context: Show context for given location
    • crystal tool expand: Show macro expansion for given location
    • crystal tool format: Format Crystal files
    • crystal tool hierarchy: Show type hierarchy
    • crystal tool implementations: Show implementations for given call in location
    • crystal tool types: Show types of main variables

    crystal tool format

    The crystal tool format command applies default format to Crystal source files.

    1. crystal tool format [--check] [<path>...]

    path can be a file or folder name and include all Crystal files in that folder tree. Omitting path is equal to specifying the current working directory.

    The formatter also applies to Crystal code blocks in comments (see Documenting Code).

    • CRYSTAL_CACHE_DIR: Defines path where Crystal caches partial compilation results for faster subsequent builds. This path is also used to temporarily store executables when Crystal programs are run with rather than crystal build. Default value is the first directory that either exists or can be created of ${XDG_CACHE_HOME}/crystal (if XDG_CACHE_HOME is defined), ${HOME}/.cache/crystal, ${HOME}/.crystal, ./.crystal. If CRYSTAL_CACHE_DIR is set but points to a path that is not writeable, the default values are used instead.
    • CRYSTAL_PATH: Defines paths where Crystal searches for required files.
    • CRYSTAL_LIBRARY_PATH: The compiler uses the paths in this variable as a first lookup destination for static and dynamic libraries that are to be linked. For example, if static libraries are put in build/libs, setting the environment variable accordingly will tell the compiler to look for libraries there.

    The compiler conforms to and turns off ANSI color escapes in the terminal when the environment variable is present (has a value other than the empty string).