Elixir v1.0.5 System

The System module provides access to variables used or maintained by the VM and to functions that interact directly with the VM or the host system.



List command line arguments

Modify command line arguments

Registers a program exit handler function

Elixir build information

Executes the given command with args

Current working directory

Current working directory, exception on error

Deletes an environment variable

Locates an executable on the system

System environment variables

Environment variable value

Erlang VM process identifier

Halt the Erlang runtime system

Set multiple environment variables

Set an environment variable value

Last exception stacktrace

Writable temporary directory

Writable temporary directory, exception on error

User home directory

User home directory, exception on error

Elixir version information




argv :: [String.t]

List command line arguments.

Returns the list of command line arguments passed to the program.



argv([String.t]) :: :ok

Modify command line arguments.

Changes the list of command line arguments. Use it with caution, as it destroys any previous argv information.


Registers a program exit handler function.

Registers a function that will be invoked at the end of program execution. Useful for invoking a hook in “script” mode.

The handler always executes in a different process from the one it was registered in. As a consequence, any resources managed by the calling process (ETS tables, open files, etc.) won’t be available by the time the handler function is invoked.

The function must receive the exit status code as an argument.



build_info :: %{}

Elixir build information.

Returns a keyword list with Elixir version, git tag info and compilation date.

cmd(command, args, opts \\ [])


cmd(binary, [binary], Keyword.t) :: {Collectable.t, exit_status :: non_neg_integer}

Executes the given command with args.

command is expected to be an executable available in PATH unless an absolute path is given.

args must be a list of strings which are not expanded in any way. For example, this means wildcard expansion will not happen unless Path.wildcard/2 is used. On Windows though, wildcard expansion is up to the program.

A set of options are also supported and described below.


  • :into - injects the result into the given collectable, defaults to ""
  • :cd - the directory to run the command in
  • :env - an enumerable of tuples containing environment key-value as binary
  • :arg0 - set the command arg0
  • :stderr_to_stdout - redirects stderr to stdout when true
  • :parallelism - when true, the VM will schedule port tasks to improve parallelism in the system. If set to false, the VM will try to perform commands immediately, improving latency at the expense of parallelism. The default can be set on system startup by passing the “+spp” argument to --erl.

Error reasons

If invalid arguments are given, ArgumentError is raised by System.cmd/3. System.cmd/3 also expects a strict set of options and will raise if unknown or invalid options are given.

Furthermore, System.cmd/3 may fail with one of the POSIX reasons detailed below:

  • :system_limit - all available ports in the Erlang emulator are in use

  • :enomem - there was not enough memory to create the port

  • :eagain - there are no more available operating system processes

  • :enametoolong - the external command given was too long

  • :emfile - there are no more available file descriptors (for the operating system process that the Erlang emulator runs in)

  • :enfile - the file table is full (for the entire operating system)

  • :eacces - the command does not point to an executable file

  • :enoent - the command does not point to an existing file

Shell commands

If you desire to execute a trusted command inside a shell, with pipes, redirecting and so on, please check Erlang’s :os.cmd/1 function.


Current working directory.

Returns the current working directory or nil if one is not available.


Current working directory, exception on error.

Returns the current working directory or raises RuntimeError.



delete_env(String.t) :: :ok

Deletes an environment variable.

Removes the variable varname from the environment.



find_executable(binary) :: binary | nil

Locates an executable on the system.

This function looks up an executable program given its name using the environment variable PATH on Unix and Windows. It also considers the proper executable extension for each OS, so for Windows it will try to lookup files with .com, .cmd or similar extensions.



get_env :: %{String.t => String.t}

System environment variables.

Returns a list of all environment variables. Each variable is given as a {name, value} tuple where both name and value are strings.



get_env(binary) :: binary | nil

Environment variable value.

Returns the value of the environment variable varname as a binary, or nil if the environment variable is undefined.



get_pid :: binary

Erlang VM process identifier.

Returns the process identifier of the current Erlang emulator in the format most commonly used by the operating system environment.

See http://www.erlang.org/doc/man/os.html#getpid-0 for more info.

halt(status \\ 0)


halt(non_neg_integer | binary | :abort) :: no_return

Halt the Erlang runtime system.

Halts the Erlang runtime system where the argument status must be a non-negative integer, the atom :abort or a binary.

  • If an integer, the runtime system exits with the integer value which is returned to the operating system.

  • If :abort, the runtime system aborts producing a core dump, if that is enabled in the operating system.

  • If a string, an erlang crash dump is produced with status as slogan, and then the runtime system exits with status code 1.

Note that on many platforms, only the status codes 0-255 are supported by the operating system.

For more information, check: http://www.erlang.org/doc/man/erlang.html#halt-1




put_env(Dict.t) :: :ok

Set multiple environment variables.

Sets a new value for each environment variable corresponding to each key in dict.

put_env(varname, value)


put_env(binary, binary) :: :ok

Set an environment variable value.

Sets a new value for the environment variable varname.


Last exception stacktrace.

Note that the Erlang VM (and therefore this function) does not return the current stacktrace but rather the stacktrace of the latest exception.

Inlined by the compiler into :erlang.get_stacktrace/0.


Writable temporary directory.

Returns a writable temporary directory. Searches for directories in the following order:

  1. the directory named by the TMPDIR environment variable
  2. the directory named by the TEMP environment variable
  3. the directory named by the TMP environment variable
  4. C:\TMP on Windows or /tmp on Unix
  5. as a last resort, the current working directory

Returns nil if none of the above are writable.


Writable temporary directory, exception on error.

Same as tmp_dir/0 but raises RuntimeError instead of returning nil if no temp dir is set.


User home directory.

Returns the user home directory (platform independent). Returns nil if no user home is set.


User home directory, exception on error.

Same as user_home/0 but raises RuntimeError instead of returning nil if no user home is set.



version :: String.t

Elixir version information.

Returns Elixir’s version as binary.