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.
Summary↑
| argv() | Lists command line arguments |
| argv(args) | Modifies command line arguments |
| at_exit(fun) | Registers a program exit handler function |
| build_info() | Elixir build information |
| cmd(command, args, opts \\ []) | Executes the given |
| cwd!() | Current working directory, exception on error |
| cwd() | Current working directory |
| delete_env(varname) | Deletes an environment variable |
| find_executable(program) | Locates an executable on the system |
| get_env() | System environment variables |
| get_env(varname) | Environment variable value |
| get_pid() | Erlang VM process identifier |
| halt(status \\ 0) | Halts the Erlang runtime system |
| put_env(dict) | Sets multiple environment variables |
| put_env(varname, value) | Sets an environment variable value |
| stacktrace() | Last exception stacktrace |
| tmp_dir!() | Writable temporary directory, exception on error |
| tmp_dir() | Writable temporary directory |
| user_home!() | User home directory, exception on error |
| user_home() | User home directory |
| version() | Elixir version information |
Functions
Specs:
- argv :: [String.t]
Lists command line arguments.
Returns the list of command line arguments passed to the program.
Specs:
- argv([String.t]) :: :ok
Modifies 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.
Specs:
- build_info :: %{}
Elixir build information.
Returns a keyword list with Elixir version, git tag info and compilation date.
Specs:
- 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.
This function returns a tuple containing the collected result and the command exit status.
Examples
iex> System.cmd "echo", ["hello"]
{"hello\n", 0}
iex> System.cmd "echo", ["hello"], env: [{"MIX_ENV", "test"}]
{"hello\n", 0}
iex> System.cmd "echo", ["hello"], into: IO.stream(:stdio, :line)
hello
{%IO.Stream{}, 0}Options
: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.
Specs:
- delete_env(String.t) :: :ok
Deletes an environment variable.
Removes the variable varname from the environment.
Specs:
- 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.
Specs:
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.
Specs:
- 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.
Specs:
- 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.
Specs:
- halt(non_neg_integer | binary | :abort) :: no_return
Halts 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
Examples
System.halt(0)
System.halt(1)
System.halt(:abort)Specs:
- put_env(Dict.t) :: :ok
Sets multiple environment variables.
Sets a new value for each environment variable corresponding
to each key in dict.
Specs:
- put_env(binary, binary) :: :ok
Sets 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.
Specs:
- version :: String.t
Elixir version information.
Returns Elixir’s version as binary.