just provides a few built-in functions that might be useful when writing recipes.

All functions ending in _directory can be abbreviated to _dir. So home_directory() can also be written as home_dir(). In addition, invocation_directory_native() can be abbreviated to invocation_dir_native().

System Information

  • arch() — Instruction set architecture. Possible values are: "aarch64", "arm", "asmjs", "hexagon", "mips", "msp430", "powerpc", "powerpc64", "s390x", "sparc", "wasm32", "x86", "x86_64", and "xcore".
  • num_cpus()1.15.0 - Number of logical CPUs.
  • os() — Operating system. Possible values are: "android", "bitrig", "dragonfly", "emscripten", "freebsd", "haiku", "ios", "linux", "macos", "netbsd", "openbsd", "solaris", and "windows".
  • os_family() — Operating system family; possible values are: "unix" and "windows".

For example:

  @echo "This is an {{arch()}} machine".
$ just system-info
This is an x86_64 machine

The os_family() function can be used to create cross-platform justfiles that work on various operating systems. For an example, see cross-platform.just file.

External Commands

  • shell(command, args...)1.27.0 returns the standard output of shell script command with zero or more positional arguments args. The shell used to interpret command is the same shell that is used to evaluate recipe lines, and can be changed with set shell := […].

    command is passed as the first argument, so if the command is 'echo $@', the full command line, with the default shell command shell -cu and args 'foo' and 'bar' will be:

    'shell' '-cu' 'echo $@' 'echo $@' 'foo' 'bar'

    This is so that $@ works as expected, and $1 refers to the first argument. $@ does not include the first positional argument, which is expected to be the name of the program being run.

# arguments can be variables or expressions
file := '/sys/class/power_supply/BAT0/status'
bat0stat := shell('cat $1', file)

# commands can be variables or expressions
command := 'wc -l'
output := shell(command + ' "$1"', 'main.c')

# arguments referenced by the shell command must be used
empty := shell('echo', 'foo')
full := shell('echo $1', 'foo')
error := shell('echo $1')
# Using python as the shell. Since `python -c` sets `sys.argv[0]` to `'-c'`,
# the first "real" positional argument will be `sys.argv[2]`.
set shell := ["python3", "-c"]
olleh := shell('import sys; print(sys.argv[2][::-1])', 'hello')

Environment Variables

  • env_var(key) — Retrieves the environment variable with name key, aborting if it is not present.
home_dir := env_var('HOME')

  echo "{{home_dir}}"
$ just
  • env_var_or_default(key, default) — Retrieves the environment variable with name key, returning default if it is not present.
  • env(key)1.15.0 — Alias for env_var(key).
  • env(key, default)1.15.0 — Alias for env_var_or_default(key, default).

Invocation Information

  • is_dependency() - Returns the string true if the current recipe is being run as a dependency of another recipe, rather than being run directly, otherwise returns the string false.

Invocation Directory

  • invocation_directory() - Retrieves the absolute path to the current directory when just was invoked, before just changed it (chdir’d) prior to executing commands. On Windows, invocation_directory() uses cygpath to convert the invocation directory to a Cygwin-compatible /-separated path. Use invocation_directory_native() to return the verbatim invocation directory on all platforms.

For example, to call rustfmt on files just under the “current directory” (from the user/invoker’s perspective), use the following rule:

  find {{invocation_directory()}} -name \*.rs -exec rustfmt {} \;

Alternatively, if your command needs to be run from the current directory, you could use (e.g.):

  cd {{invocation_directory()}}; ./some_script_that_needs_to_be_run_from_here
  • invocation_directory_native() - Retrieves the absolute path to the current directory when just was invoked, before just changed it (chdir’d) prior to executing commands.

Justfile and Justfile Directory

  • justfile() - Retrieves the path of the current justfile.

  • justfile_directory() - Retrieves the path of the parent directory of the current justfile.

For example, to run a command relative to the location of the current justfile:


Source and Source Directory

  • source_file()1.27.0 - Retrieves the path of the current source file.

  • source_directory()1.27.0 - Retrieves the path of the parent directory of the current source file.

source_file() and source_directory() behave the same as justfile() and justfile_directory() in the root justfile, but will return the path and directory, respectively, of the current import or mod source file when called from within an import or submodule.

Just Executable

  • just_executable() - Absolute path to the just executable.

For example:

  @echo The executable is at: {{just_executable()}}
$ just
The executable is at: /bin/just

Just Process ID

  • just_pid() - Process ID of the just executable.

For example:

  @echo The process ID is: {{ just_pid() }}
$ just
The process ID is: 420

String Manipulation

  • append(suffix, s)1.27.0 Append suffix to whitespace-separated strings in s. append('/src', 'foo bar baz')'foo/src bar/src baz/src'
  • prepend(prefix, s)1.27.0 Prepend prefix to whitespace-separated strings in s. prepend('src/', 'foo bar baz')'src/foo src/bar src/baz'
  • encode_uri_component(s)1.27.0 - Percent-encode characters in s except [A-Za-z0-9_.!~*'()-], matching the behavior of the JavaScript encodeURIComponent function.
  • quote(s) - Replace all single quotes with '\'' and prepend and append single quotes to s. This is sufficient to escape special characters for many shells, including most Bourne shell descendants.
  • replace(s, from, to) - Replace all occurrences of from in s to to.
  • replace_regex(s, regex, replacement) - Replace all occurrences of regex in s to replacement. Regular expressions are provided by the Rust regex crate. See the syntax documentation for usage examples. Capture groups are supported. The replacement string uses Replacement string syntax.
  • trim(s) - Remove leading and trailing whitespace from s.
  • trim_end(s) - Remove trailing whitespace from s.
  • trim_end_match(s, pat) - Remove suffix of s matching pat.
  • trim_end_matches(s, pat) - Repeatedly remove suffixes of s matching pat.
  • trim_start(s) - Remove leading whitespace from s.
  • trim_start_match(s, pat) - Remove prefix of s matching pat.
  • trim_start_matches(s, pat) - Repeatedly remove prefixes of s matching pat.

Case Conversion

  • capitalize(s)1.7.0 - Convert first character of s to uppercase and the rest to lowercase.
  • kebabcase(s)1.7.0 - Convert s to kebab-case.
  • lowercamelcase(s)1.7.0 - Convert s to lowerCamelCase.
  • lowercase(s) - Convert s to lowercase.
  • shoutykebabcase(s)1.7.0 - Convert s to SHOUTY-KEBAB-CASE.
  • shoutysnakecase(s)1.7.0 - Convert s to SHOUTY_SNAKE_CASE.
  • snakecase(s)1.7.0 - Convert s to snake_case.
  • titlecase(s)1.7.0 - Convert s to Title Case.
  • uppercamelcase(s)1.7.0 - Convert s to UpperCamelCase.
  • uppercase(s) - Convert s to uppercase.

Path Manipulation

  • absolute_path(path) - Absolute path to relative path in the working directory. absolute_path("./bar.txt") in directory /foo is /foo/bar.txt.
  • canonicalize(path)1.24.0 - Canonicalize path by resolving symlinks and removing ., .., and extra /s where possible.
  • extension(path) - Extension of path. extension("/foo/bar.txt") is txt.
  • file_name(path) - File name of path with any leading directory components removed. file_name("/foo/bar.txt") is bar.txt.
  • file_stem(path) - File name of path without extension. file_stem("/foo/bar.txt") is bar.
  • parent_directory(path) - Parent directory of path. parent_directory("/foo/bar.txt") is /foo.
  • without_extension(path) - path without extension. without_extension("/foo/bar.txt") is /foo/bar.

These functions can fail, for example if a path does not have an extension, which will halt execution.

  • clean(path) - Simplify path by removing extra path separators, intermediate . components, and .. where possible. clean("foo//bar") is foo/bar, clean("foo/..") is ., clean("foo/./bar") is foo/bar.
  • join(a, b…) - This function uses / on Unix and \ on Windows, which can be lead to unwanted behavior. The / operator, e.g., a / b, which always uses /, should be considered as a replacement unless \s are specifically desired on Windows. Join path a with path b. join("foo/bar", "baz") is foo/bar/baz. Accepts two or more arguments.

Filesystem Access

  • path_exists(path) - Returns true if the path points at an existing entity and false otherwise. Traverses symbolic links, and returns false if the path is inaccessible or points to a broken symlink.
Error Reporting
  • error(message) - Abort execution and report error message to user.

UUID and Hash Generation

  • blake3(string)1.25.0 - Return BLAKE3 hash of string as hexadecimal string.
  • blake3_file(path)1.25.0 - Return BLAKE3 hash of file at path as hexadecimal string.
  • sha256(string) - Return the SHA-256 hash of string as hexadecimal string.
  • sha256_file(path) - Return SHA-256 hash of file at path as hexadecimal string.
  • uuid() - Generate a random version 4 UUID.


  • choose(n, alphabet)1.27.0 - Generate a string of n randomly selected characters from alphabet, which may not contain repeated characters. For example, choose('64', HEX) will generate a random 64-character lowercase hex string.


  • datetime(format)1.30.0 - Return local time with format.
  • datetime_utc(format)1.30.0 - Return UTC time with format.

The arguments to datetime and datetime_utc are strftime-style format strings, see the chrono library docs for details.

Semantic Versions

  • semver_matches(version, requirement)1.16.0 - Check whether a semantic version, e.g., "0.1.0" matches a requirement, e.g., ">=0.1.0", returning "true" if so and "false" otherwise.
XDG Directories1.23.0

These functions return paths to user-specific directories for things like configuration, data, caches, executables, and the user’s home directory. These functions follow the XDG Base Directory Specification, and are implemented with the dirs crate.

  • cache_directory() - The user-specific cache directory.
  • config_directory() - The user-specific configuration directory.
  • config_local_directory() - The local user-specific configuration directory.
  • data_directory() - The user-specific data directory.
  • data_local_directory() - The local user-specific data directory.
  • executable_directory() - The user-specific executable directory.
  • home_directory() - The user’s home directory.