Using the compiler

Compiling and running at once

To compile and run a program in a single shot, invoke crystal run with a single filename:

$ echo 'puts "Hello World!"' > hello_world.cr
$ crystal run hello_world.cr
Hello World!

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

Creating an executable

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

$ crystal build hello_world.cr
$ ./hello_world
Hello World!

Release builds

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

$ 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:

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

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.

$ crystal init app myapp
    create  myapp/.gitignore
    create  myapp/.editorconfig
    create  myapp/LICENSE
    create  myapp/README.md
    create  myapp/.travis.yml
    create  myapp/shard.yml
    create  myapp/src/myapp.cr
    create  myapp/src/myapp/version.cr
    create  myapp/spec/spec_helper.cr
    create  myapp/spec/myapp_spec.cr
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.

Compiler commands

• crystal init: generate a new project
• crystal build: build an executable
• crystal docs: generate documentation
• crystal env: print Crystal environment information
• crystal eval: eval code from args or standard input
• crystal play: starts crystal playground server
• crystal run: build and run program
• crystal spec: build and run specs
• crystal tool: 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.

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:

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

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

crystal build

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

crystal build [<options>] <programfile>

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

Example:

$ echo 'puts "Hello #{ARGV[0]?}!"' > hello_world.cr
$ crystal build hello_world.cr
$ ./hello_world Crystal
Hello Crystal!

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.
• --lto=thin: Use ThinLTO, improving performance on release builds.
• --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.

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:

$ crystal eval 'puts "Hello World"'
Hello World!
$ echo 'puts "Hello World"' | crystal eval
Hello World!

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.
• --lto=thin: Use ThinLTO, improves performance.
• --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.

crystal version

Example:

$ crystal version
Crystal 1.1.1 [6d9a1d583] (2021-07-26)

LLVM: 10.0.1
Default target: x86_64-unknown-linux-gnu

crystal init

The crystal init command initializes a Crystal project folder.

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:

$ crystal init lib my_cool_lib
    create  my_cool_lib/.gitignore
    create  my_cool_lib/.editorconfig
    create  my_cool_lib/LICENSE
    create  my_cool_lib/README.md
    create  my_cool_lib/.travis.yml
    create  my_cool_lib/shard.yml
    create  my_cool_lib/src/my_cool_lib.cr
    create  my_cool_lib/spec/spec_helper.cr
    create  my_cool_lib/spec/my_cool_lib_spec.cr
Initialized empty Git repository in ~/my_cool_lib/.git/

crystal docs

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

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

The command creates a static website in output_dir (default ./docs), consisting of HTML files for each Crystal type, in a folder structure mirroring the Crystal namespaces. The entrypoint docs/index.html can be opened by any web browser. The entire API docs are also stored as a JSON document in $output_dir/index.json.

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.

crystal docs src/my_app.cr

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:

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.

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:

$ crystal env
CRYSTAL_CACHE_DIR="/home/crystal/.cache/crystal"
CRYSTAL_PATH="/usr/bin/../share/crystal/src:lib"
CRYSTAL_VERSION="0.28.0"
CRYSTAL_LIBRARY_PATH="/usr/bin/../lib/crystal/lib"
$ crystal env CRYSTAL_VERSION
0.28.0

crystal spec

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

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 ./spec. 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:

$ crystal spec
F

Failures:

  1) Myapp works
     Failure/Error: false.should eq(true)

       Expected: true
            got: false

     # spec/myapp_spec.cr:7

Finished in 880 microseconds
1 examples, 1 failures, 0 errors, 0 pending

Failed examples:

crystal spec spec/myapp_spec.cr:6 # Myapp works

crystal play

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

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

crystal tool

• 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.

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).

Environment variables

The following environment variables are used by the Crystal compiler if set in the environment. Otherwise the compiler will populate them with default values. Their values can be inspected using crystal env.

• 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 crystal run 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_VERSION is only available as output of crystal env. The compiler neither sets nor reads it.
• 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.