System

The System module provides functions that interact directly with the VM or the host system.

Time

The System module also provides functions that work with time, returning different times kept by the system with support for different time units.

One of the complexities in relying on system times is that they may be adjusted. For example, when you enter and leave daylight saving time, the system clock will be adjusted, often adding or removing one hour. We call such changes “time warps”. In order to understand how such changes may be harmful, imagine the following code:

## DO NOT DO THIS
prev = System.os_time()
# ... execute some code ...
next = System.os_time()
diff = next - prev

If, while the code is executing, the system clock changes, some code that executed in 1 second may be reported as taking over 1 hour! To address such concerns, the VM provides a monotonic time via System.monotonic_time/0 which never decreases and does not leap:

## DO THIS
prev = System.monotonic_time()
# ... execute some code ...
next = System.monotonic_time()
diff = next - prev

Generally speaking, the VM provides three time measurements:

• os_time/0 - the time reported by the operating system (OS). This time may be adjusted forwards or backwards in time with no limitation;

• system_time/0 - the VM view of the os_time/0. The system time and operating system time may not match in case of time warps although the VM works towards aligning them. This time is not monotonic (i.e., it may decrease) as its behaviour is configured by the VM time warp mode;

• monotonic_time/0 - a monotonically increasing time provided by the Erlang VM.

The time functions in this module work in the :native unit(unless specified otherwise), which is operating system dependent. Most of the time, all calculations are done in the :native unit, to avoid loss of precision, with convert_time_unit/3 being invoked at the end to convert to a specific time unit like :millisecond or :microsecond. See the t:time_unit/0 type for more information.

For a more complete rundown on the VM support for different times, see the chapter on time and time correction in the Erlang docs.

Function argv/0

Lists command line arguments.

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

Function argv/1

Modifies command line arguments.

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

Function at_exit/1

Registers a program exit handler function.

Registers a function that will be invoked at the end of an Elixir script. A script is typically started via the command line via the elixir and mix executables.

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, and others) won’t be available by the time the handler function is invoked.

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

If the VM terminates programmatically, via System.stop/1, System.halt/1, or exit signals, the at_exit/1 callbacks are not executed.

Function build_info/0

Elixir build information.

Returns a map with the Elixir version, the Erlang/OTP release it was compiled with, a short Git revision hash and the date and time it was built.

Every value in the map is a string, and these are:

• :build - the Elixir version, short Git revision hash and Erlang/OTP release it was compiled with
• :date - a string representation of the ISO8601 date and time it was built
• :otp_release - OTP release it was compiled with
• :revision - short Git revision hash. If Git was not available at building time, it is set to ""
• :version - the Elixir version

One should not rely on the specific formats returned by each of those fields.Instead one should use specialized functions, such as version/0 to retrieve the Elixir version and otp_release/0 to retrieve the Erlang/OTP release.

Examples

System.build_info()

Function cmd/3

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 binaries which the executable will receive as its arguments as is. This means that:

• environment variables will not be interpolated
• wildcard expansion will not happen (unless Path.wildcard/2 is used explicitly)
• arguments do not need to be escaped or quoted for shell safety

This function returns a tuple containing the collected resultand the command exit status.

Internally, this function uses a Port for interacting with the outside world. However, if you plan to run a long-running program, ports guarantee stdin/stdout devices will be closed but it does not automatically terminate the program. The documentation for the Port module describes this problem and possible solutions under the “Zombie processes” section.

Examples

System.cmd("echo", ["hello"])

System.cmd("echo", ["hello"], env: [{"MIX_ENV", "test"}])

If you want to stream the output to Standard IO as it arrives:

System.cmd("echo", ["hello"], into: IO.stream())

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. The child process inherits all environment variables from its parent process, the Elixir application, except those overwritten or cleared using this option. Specify a value of nil to clear (unset) an environment variable, which is useful for preventing credentials passed to the application from leaking into child processes.
• :arg0 - sets 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 shell/2.

Function compiled_endianness/0

Returns the endianness the system was compiled with.

Function convert_time_unit/3

Converts time from time unit from_unit to time unit to_unit.

The result is rounded via the floor function.

convert_time_unit/3 accepts an additional time unit (other than the ones in the t:time_unit/0 type) called :native. :native is the time unit used by the Erlang runtime system. It’s determined when the runtime starts and stays the same until the runtime is stopped, but could differ the next time the runtime is started on the same machine. For this reason, you should use this function to convert :native time units to a predictable unit before you display them to humans.

To determine how many seconds the :native unit represents in your current runtime, you can call this function to convert 1 second to the :native time unit: System.convert_time_unit(1, :second, :native).

Function cwd/0

Current working directory.

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

Function cwd!/0

Current working directory, exception on error.

Returns the current working directory or raises RuntimeError.

Function delete_env/1

Deletes an environment variable.

Removes the variable varname from the environment.

Function endianness/0

Returns the endianness.

Function fetch_env/1

Returns the value of the given environment variable or :error if not found.

If the environment variable varname is set, then {:ok, value} is returned where value is a string. If varname is not set, :error is returned.

Examples

System.fetch_env("PORT")

System.fetch_env("NOT_SET")

Function fetch_env!/1

Returns the value of the given environment variable or raises if not found.

Same as get_env/1 but raises instead of returning nil when the variable is not set.

Examples

System.fetch_env!("PORT")

System.fetch_env!("NOT_SET")

Function find_executable/1

Locates an executable on the system.

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

Function get_env/0

Returns all system environment variables.

The returned value is a map containing name-value pairs. Variable names and their values are strings.

Function get_env/2

Returns the value of the given environment variable.

The returned value of the environment variable varname is a string. If the environment variable is not set, returns the string specified in default or nil if none is specified.

Examples

System.get_env("PORT")

System.get_env("NOT_SET")

System.get_env("NOT_SET", "4001")

Function get_pid/0

Erlang VM process identifier.

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

For more information, see :os.getpid/0.

Function halt/1

Immediately halts the Erlang runtime system.

Terminates the Erlang runtime system without properly shutting down applications and ports. Please see stop/1 for a careful shutdown of the system.

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 supportedby the operating system.

For more information, see :erlang.halt/1.

Examples

System.halt(0)
System.halt(1)
System.halt(:abort)

Function monotonic_time/0

Returns the current monotonic time in the :native time unit.

This time is monotonically increasing and starts in an unspecified point in time.

Inlined by the compiler.

Function monotonic_time/1

Returns the current monotonic time in the given time unit.

This time is monotonically increasing and starts in an unspecified point in time.

Function no_halt/0

Checks if the system will halt or not at the end of ARGV processing.

Function no_halt/1

Marks if the system should halt or not at the end of ARGV processing.

Function os_time/0

Returns the current operating system (OS) time.

The result is returned in the :native time unit.

This time may be adjusted forwards or backwards in time with no limitation and is not monotonic.

Inlined by the compiler.

Function os_time/1

Returns the current operating system (OS) time in the given time unit.

This time may be adjusted forwards or backwards in time with no limitation and is not monotonic.

Function otp_release/0

Returns the Erlang/OTP release number.

Function pid/0

Returns the operating system PID for the current Erlang runtime system instance.

Returns a string containing the (usually) numerical identifier for a process. On Unix-like operating systems, this is typically the return value of the getpid() system call. On Windows, the process ID as returned by the GetCurrentProcessId() system call is used.

Examples

System.pid()

Function put_env/1

Sets multiple environment variables.

Sets a new value for each environment variable corresponding to each {key, value} pair in enum.

Function put_env/2

Sets an environment variable value.

Sets a new value for the environment variable varname.

Function restart/0

Restarts all applications in the Erlang runtime system.

All applications are taken down smoothly, all code is unloaded, and all ports are closed before the system starts all applications once again.

Examples

System.restart()

Function schedulers/0

Returns the number of schedulers in the VM.

Function schedulers_online/0

Returns the number of schedulers online in the VM.

Function shell/2

Executes the given command in the OS shell.

It uses sh for Unix-like systems and cmd for Windows.

Important: Use this function with care. In particular, never pass untrusted user input to this function, as the user would be able to perform “command injection attacks” by executing any code directly on the machine. Generally speaking, prefer to use cmd/3 over this function.

Examples

System.shell("echo hello")

If you want to stream the output to Standard IO as it arrives:

System.shell("echo hello", into: IO.stream())

Options

It accepts the same options as cmd/3, except for arg0.

Function stacktrace/0

Deprecated mechanism to retrieve the last exception stacktrace.

Starting from Erlang/OTP 23, this function will always return an empty list.

Function stop/1

Carefully stops the Erlang runtime system.

All applications are taken down smoothly, all code is unloaded, and all ports are closed before the system terminates by calling halt/1.

status must be a non-negative integer or a binary.

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

• If a binary, 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 supportedby the operating system.

Examples

System.stop(0)
System.stop(1)

Function system_time/0

Returns the current system time in the :native time unit.

It is the VM view of the os_time/0. They may not match in case of time warps although the VM works towards aligning them. This time is not monotonic.

Inlined by the compiler.

Function system_time/1

Returns the current system time in the given time unit.

It is the VM view of the os_time/0. They may not match in case of time warps although the VM works towards aligning them. This time is not monotonic.

Function time_offset/0

Returns the current time offset between the Erlang VM monotonic time and the Erlang VM system time.

The result is returned in the :native time unit.

See time_offset/1 for more information.

Inlined by the compiler.

Function time_offset/1

Returns the current time offset between the Erlang VM monotonic time and the Erlang VM system time.

The result is returned in the given time unit unit. The returned offset, added to an Erlang monotonic time (for instance, one obtained with monotonic_time/1), gives the Erlang system time that corresponds to that monotonic time.

Function tmp_dir/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-like operating systems
5. as a last resort, the current working directory

Returns nil if none of the above are writable.

Function tmp_dir!/0

Writable temporary directory, exception on error.

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

Function trap_signal/3

Traps the given signal to execute the fun.

Important: Trapping signals may have strong implications on how a system shuts down and behave in production and therefore it is extremely discouraged for libraries to set their own traps. Instead, they should redirect users to configure them themselves. The only cases where it is acceptable for libraries to set their own traps is when using Elixir in script mode, such as in .exs files and via Mix tasks.

An optional id that uniquely identifies the function can be given, otherwise a unique one is automatically generated. If a previously registered id is given, this function returns an error tuple. The id can be used to remove a registered signal by calling untrap_signal/2.

The given fun receives no arguments and it must return :ok.

It returns {:ok, id} in case of success, {:error, :already_registered} in case the id has already been registered for the given signal, or {:error, :not_sup} in case trapping exists is not supported by the current OS.

The first time a signal is trapped, it will override the default behaviour from the operating system. If the same signal is trapped multiple times, subsequent functions given to trap_signal will execute first. In other words, you can consider each function is prepended to the signal handler.

By default, the Erlang VM register traps to the three signals:

• :sigstop - gracefully shuts down the VM with stop/0
• :sigquit - halts the VM via halt/0
• :sigusr1 - halts the VM via status code of 1

Therefore, if you add traps to the signals above, thedefault behaviour above will be executed after all user signals.

Implementation notes

All signals run from a single process. Therefore, blocking the fun will block subsequent traps. It is also not possible to add or remove traps from within a trap itself.

Internally, this functionality is built on top of :os.set_signal/2. When you register a trap, Elixir automatically sets it to :handle and it reverts it back to :default once all traps are removed (except for :sigquit, :sigterm, and :sigusr1 which are always handled). If you or a library call :os.set_signal/2 directly, it may disable Elixir traps (or Elixir may override your configuration).

Function unique_integer/1

Generates and returns an integer that is unique in the current runtime instance.

“Unique” means that this function, called with the same list of modifiers, will never return the same integer more than once on the current runtime instance.

If modifiers is [], then a unique integer (that can be positive or negative) is returned. Other modifiers can be passed to change the properties of the returned integer:

• :positive - the returned integer is guaranteed to be positive.
• :monotonic - the returned integer is monotonically increasing. This means that, on the same runtime instance (but even on different processes), integers returned using the :monotonic modifier will always be strictly less than integers returned by successive calls with the :monotonic modifier.

All modifiers listed above can be combined; repeated modifiers in modifierswill be ignored.

Inlined by the compiler.

Function untrap_signal/2

Removes a previously registered signal with id.

Function user_home/0

User home directory.

Returns the user home directory (platform independent).

Function user_home!/0

User home directory, exception on error.

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

Function version/0

Elixir version information.

Returns Elixir’s version as binary.

Type time_unit

The time unit to be passed to functions like monotonic_time/1 and others.

The :second, :millisecond, :microsecond and :nanosecond time units controls the return value of the functions that accept a time unit.

A time unit can also be a strictly positive integer. In this case, it represents the “parts per second”: the time will be returned in 1 / parts_per_second seconds. For example, using the :millisecond time unit is equivalent to using 1000 as the time unit (as the time will be returned in 1/1000 seconds - milliseconds).