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:

  1. $ echo 'puts "Hello World!"' > hello_world.cr
  2. $ crystal run hello_world.cr
  3. 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.

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

Release builds

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 statically 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.

Compiler commands

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
  3. Hello Crystal!

Common options:

  • -O LEVEL: Define optimization level: 0 (default), 1, 2, 3. See Optimizations for details.
  • --release: Compile in release mode. Equivalent to -O3 --single-module.
  • --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.

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

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

Example:

  1. $ echo 'puts "Hello #{ARGV[0]?}!"' > hello_world.cr
  2. $ crystal build hello_world.cr
  3. $ ./hello_world Crystal
  4. 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.
  • -O LEVEL: Define optimization level: 0 (default), 1, 2, 3. See Optimizations for details.
  • --release: Compile in release mode. Equivalent to -O3 --single-module.
  • --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
  4. Hello World!

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.
  • -O LEVEL: Define optimization level: 0 (default), 1, 2, 3. See Optimizations for details.
  • --release: Compile in release mode. Equivalent to -O3 --single-module.
  • --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 1.14.0 [dacd97bcc] (2024-10-09)
  4. LLVM: 18.1.6
  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/

crystal docs

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

  1. 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.

  1. 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:

  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 env
  2. CRYSTAL_CACHE_DIR=/home/crystal/.cache/crystal
  3. CRYSTAL_PATH=lib:/usr/bin/../share/crystal/src
  4. CRYSTAL_VERSION=1.9.0
  5. CRYSTAL_LIBRARY_PATH=/usr/bin/../lib/crystal
  6. CRYSTAL_LIBRARY_RPATH=''
  7. CRYSTAL_OPTS=''
  8. $ crystal env CRYSTAL_VERSION
  9. 1.9.0

crystal spec

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

  1. crystal spec [<options>] [<file>[:line] | <folder>]... [-- [<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 (and its recursive subfolders) named *_spec.cr are appended. If no files argument is provided, the default is the ./spec folder. 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 preceding 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, -v: Prints verbose output, including all example names.
  • --profile, -p: Prints the 10 slowest specs.
  • --fail-fast: Abort the spec run on first failure.
  • --junit_output <output_dir>: Generates JUnit XML output.
  • --tap: Generates output for the Test Anything Protocol (TAP).
  • --(no-)color: Enables ANSI colored output. The default mode automatically enables color if STDOUT is a TTY.
  • --order <mode>: Run examples in the given order. <mode> is either default (definition order), random, or a numeric seed value. Default value is default.
  • --list-tags: Lists all defined tags and exits.
  • --dry-run: Passes all tests without actually executing them.
  • --help, -h: Prints help and exits.

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

  • --example <name>, -e <name>: Runs examples whose full nested names include <name>.
  • --line <line>, -l <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]

Screenshot of Crystal playground

crystal tool

  • crystal tool context: Show context for given location
  • crystal tool dependencies: Show tree of required source files
  • crystal tool expand: Show macro expansion for given location
  • crystal tool flags: Print all macro flag? values
  • 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 unreachable: Show methods that are never called.

crystal tool dependencies

Show tree of required source files.

  1. crystal tool dependencies [options] [programfile]

Options:

  • -D FLAG, --define FLAG: Define a compile-time flag. This is useful to conditionally define types, methods, or commands based on flags available at compile time. The default flags are from the target triple given with --target-triple or the hosts default, if none is given.
  • -f FORMAT, --format FORMAT: Output format tree (default), flat, dot, or mermaid.
  • -i PATH, --include PATH: Include path in output.
  • -e PATH, --exclude PATH: Exclude path in output.
  • --verbose: Show skipped and heads of filtered paths
  • --error-trace: Show full error trace.
  • -h, --help: Show this message
  • --prelude PATH: Specify prelude to use. The default one initializes the garbage collector. You can also use --prelude=empty to use no preludes. This can be useful for checking code generation for a specific source code file.
  • -s, --stats: Enable statistics output
  • -p, --progress: Enable progress output
  • -t, --time: Enable execution time output
  • --stdin-filename: Source file name to be read from STDIN

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 tool unreachable

Show methods that are never called.

  1. crystal tool unreachable [options] [programfile]

The text output is a list of lines with columns separated by tab.

Output fields:

  • count: sum of all calls to this method (only with --tallies option; otherwise skipped)
  • location: pathname, line and column, all separated by colon
  • name
  • lines: length of the def in lines
  • annotations

Options:

  • -D FLAG, --define FLAG: Define a compile-time flag
  • -f FORMAT, --format FORMAT: Output format text (default), json, or csv
  • --tallies: Print reachable methods and their call counts as well.
  • --check: Exit with error if there is any unreachable code.
  • --error-trace: Show full error trace
  • -h, --help: Show this message
  • -i PATH, --include PATH: Include path
  • -e PATH, --exclude PATH: Exclude path (default: lib)
  • --no-color: Disable colored output
  • --prelude PATH: Use given file as prelude
  • -s, --stats: Enable statistics output
  • -p, --progress: Enable progress output
  • -t, --time: Enable execution time output
  • --stdin-filename: Source file name to be read from STDIN

crystal clear_cache

Clears the compiler cache located at CRYSTAL_CACHE_DIR.

Optimizations

The optimization level specifies the codegen effort for producing optimal code. It’s a trade-off between compilation performance (decreasing per optimization level) and runtime performance (increasing per optimization level).

Production builds should usually have the highest optimization level. Best results are achieved with --release which also implies --single-module.

  • -O0: No optimization (default)
  • -O1: Low optimization
  • -O2: Middle optimization
  • -O3: High optimization

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.

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