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# os (kernel v10.4.1)

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Operating system-specific functions.

The functions in this module are operating system-specific. Careless use of these functions results in programs that will only run on a specific platform. On the other hand, with careful use, these functions can be of help in enabling a program to run on most platforms.

#### Note

The functions in this module will raise a `badarg` exception if their arguments contain invalid characters according to the description in the "Data Types" section.

# Summary

## Types

env_var_name()

A string containing valid characters on the specific OS for environment variable names using `file:native_name_encoding()` encoding.

env_var_name_value()

Assuming that environment variables has been correctly set, a strings containing valid characters on the specific OS for environment variable names and values using `file:native_name_encoding()` encoding.

env_var_value()

A string containing valid characters on the specific OS for environment variable values using `file:native_name_encoding()` encoding.

os_command()

All characters needs to be valid characters on the specific OS using `file:native_name_encoding()` encoding. Null characters (integer value zero) are not allowed.

os_command_opts()

Options for `os:cmd/2`.

## Functions

cmd(Command)

Equivalent to `cmd(Command, #{})`.

cmd(Command, Options)

Executes `Command` in a command shell of the target OS, captures the standard output and standard error of the command, and returns this result as a string.

env()

Returns a list of all environment variables. Each environment variable is expressed as a tuple `{VarName,Value}`, where `VarName` is the name of the variable and `Value` its value.

find_executable(Name)

Equivalent to `find_executable(Name, Path)` where `Path` is the current execution path (that is, the environment variable `PATH` on Unix and Windows).

find_executable(Name, Path)

Look up an executable program, with the specified name and a search path, in the same way as the underlying OS.

getenv()

Returns a list of all environment variables. Each environment variable is expressed as a single string on the format `"VarName=Value"`, where `VarName` is the name of the variable and `Value` its value.

getenv(VarName)

Returns the `Value` of the environment variable `VarName`, or `false` if the environment variable is undefined.

getenv(VarName, DefaultValue)

Returns the `Value` of the environment variable `VarName`, or `DefaultValue` if the environment variable is undefined.

getpid()

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

perf_counter()

Returns the current performance counter value in `perf_counter` time unit. This is a highly optimized call that might not be traceable.

perf_counter(Unit)

Returns a performance counter that can be used as a very fast and high resolution timestamp.

putenv(VarName, Value)

Sets a new `Value` for environment variable `VarName`.

set_signal(Signal, Option)

Enables or disables OS signals.

system_time()

Returns the current OS system time in `native` time unit.

system_time(Unit)

Returns the current OS system time converted into the `Unit` passed as argument.

timestamp()

Returns the current OS system time in the same format as `erlang:timestamp/0`.

type()

Returns the `Osfamily` and, in some cases, the `Osname` of the current OS.

unsetenv(VarName)

Deletes the environment variable `VarName`.

version()

Returns the OS version. On most systems, this function returns a tuple, but a string is returned instead if the system has versions that cannot be expressed as three numbers.

# Types

# env_var_name()

    -type env_var_name() :: nonempty_string().

A string containing valid characters on the specific OS for environment variable names using `file:native_name_encoding()` encoding.

Null characters (integer value zero) are not allowed. On Unix, `=` characters are not allowed. On Windows, a `=` character is only allowed as the very first character in the string.

# env_var_name_value()

    -type env_var_name_value() :: nonempty_string().

Assuming that environment variables has been correctly set, a strings containing valid characters on the specific OS for environment variable names and values using `file:native_name_encoding()` encoding.

The first `=` characters appearing in the string separates environment variable name (on the left) from environment variable value (on the right).

# env_var_value()

    -type env_var_value() :: string().

A string containing valid characters on the specific OS for environment variable values using `file:native_name_encoding()` encoding.

Null characters (integer value zero) are not allowed.

# os_command()

    -type os_command() :: atom() | io_lib:chars().

All characters needs to be valid characters on the specific OS using `file:native_name_encoding()` encoding. Null characters (integer value zero) are not allowed.

# os_command_opts()

    -type os_command_opts() ::
              #{max_size => non_neg_integer() | infinity, exception_on_failure => boolean()}.

Options for `os:cmd/2`.

- `max_size` - The maximum size of the data returned by the `os:cmd/2` call. See the `os:cmd/2` documentation for more details.
- `exception_on_failure` - If set to true, `cmd/2` will throw an error exception if the command exits with a non-zero exit code.

# Functions

# cmd(Command)

    -spec cmd(Command) -> string() when Command :: os_command().

Equivalent to `cmd(Command, #{})`.

# cmd(Command, Options)

(since OTP 20.2.3)

    -spec cmd(Command, Options) -> string() when Command :: os_command(), Options :: os_command_opts().

Executes `Command` in a command shell of the target OS, captures the standard output and standard error of the command, and returns this result as a string.

Examples:

    LsOut = os:cmd("ls"), % on unix platform
    DirOut = os:cmd("dir"), % on Win32 platform

Notice that in some cases, standard output of a command when called from another program can differ, compared with the standard output of the command when called directly from an OS command shell.

The possible options are:

- `max_size` - The maximum size of the data returned by the `os:cmd/2` call. This option is a safety feature that should be used when the command executed can return a very large, possibly infinite, result.

  Example:

      > os:cmd("cat /dev/zero", #{ max_size => 20 }).
      [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]

- `exception_on_failure` - If set to true, `os:cmd/2` will throw an error exception if the command exits with a non-zero exit code. The exception reason looks like this: `{command_failed, ResultBeforeFailure, ExitCode}` where `ResultBeforeFailure` is the result written to stdout by the command before the error happened and `ExitCode` is the exit code from the command.

  Example:

      > catch os:cmd("echo hello && exit 123", #{ exception_on_failure => true }).
      {'EXIT',{{command_failed,"hello\n",123},
               [{os,cmd,2,[{file,"os.erl"},{line,579}]},
      ...

The command shell can be set using the kernel configuration parameter, by default the shell is detected upon system startup.

# env()

(since OTP 24.0)

    -spec env() -> [{env_var_name(), env_var_value()}].

Returns a list of all environment variables. Each environment variable is expressed as a tuple `{VarName,Value}`, where `VarName` is the name of the variable and `Value` its value.

If Unicode filename encoding is in effect (see the `erl` manual page), the strings can contain characters with codepoints > 255.

# find_executable(Name)

    -spec find_executable(Name) -> Filename | false when Name :: string(), Filename :: string().

Equivalent to `find_executable(Name, Path)` where `Path` is the current execution path (that is, the environment variable `PATH` on Unix and Windows).

# find_executable(Name, Path)

    -spec find_executable(Name, Path) -> Filename | false
                             when Name :: string(), Path :: string(), Filename :: string().

Look up an executable program, with the specified name and a search path, in the same way as the underlying OS.

`Path` is to conform to the syntax of execution paths on the OS. Returns the absolute filename of the executable program `Name`, or `false` if the program is not found.

# getenv()

    -spec getenv() -> [env_var_name_value()].

Returns a list of all environment variables. Each environment variable is expressed as a single string on the format `"VarName=Value"`, where `VarName` is the name of the variable and `Value` its value.

If Unicode filename encoding is in effect (see the `erl` manual page), the strings can contain characters with codepoints > 255.

Consider using `env/0` for a nicer 2-tuple format.

# getenv(VarName)

    -spec getenv(VarName) -> Value | false when VarName :: env_var_name(), Value :: env_var_value().

Returns the `Value` of the environment variable `VarName`, or `false` if the environment variable is undefined.

If Unicode filename encoding is in effect (see the `erl` manual page), the strings `VarName` and `Value` can contain characters with codepoints > 255.

# getenv(VarName, DefaultValue)

(since OTP 18.0)

    -spec getenv(VarName, DefaultValue) -> Value
                    when
                        VarName :: env_var_name(), DefaultValue :: env_var_value(), Value :: env_var_value().

Returns the `Value` of the environment variable `VarName`, or `DefaultValue` if the environment variable is undefined.

If Unicode filename encoding is in effect (see the `erl` manual page), the strings `VarName` and `Value` can contain characters with codepoints > 255.

# getpid()

    -spec getpid() -> Value when Value :: string().

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

Returns `Value` as a string containing the (usually) numerical identifier for a process.

- On Unix, this is typically the return value of the `getpid/0` system call.
- On Windows, the process id as returned by the `GetCurrentProcessId()` system call is used.

# perf_counter()

(since OTP 19.0)

    -spec perf_counter() -> Counter when Counter :: integer().

Returns the current performance counter value in `perf_counter` time unit. This is a highly optimized call that might not be traceable.

# perf_counter(Unit)

(since OTP 19.0)

    -spec perf_counter(Unit) -> integer() when Unit :: erlang:time_unit().

Returns a performance counter that can be used as a very fast and high resolution timestamp.

This counter is read directly from the hardware or operating system with the same guarantees. This means that two consecutive calls to the function are not guaranteed to be monotonic, though it most likely will be. The performance counter will be converted to the resolution passed as an argument.

    1> T1 = os:perf_counter(1000),receive after 10000 -> ok end,T2 = os:perf_counter(1000).
    176525861
    2> T2 - T1.
    10004

# putenv(VarName, Value)

    -spec putenv(VarName, Value) -> true when VarName :: env_var_name(), Value :: env_var_value().

Sets a new `Value` for environment variable `VarName`.

If Unicode filename encoding is in effect (see the `erl` manual page), the strings `VarName` and `Value` can contain characters with codepoints > 255.

On Unix platforms, the environment is set using UTF-8 encoding if Unicode filename translation is in effect. On Windows, the environment is set using wide character interfaces.

# set_signal(Signal, Option)

(since OTP 20.0)

    -spec set_signal(Signal, Option) -> ok
                        when
                            Signal ::
                                sighup | sigquit | sigabrt | sigalrm | sigterm | sigusr1 | sigusr2 |
                                sigchld | sigstop | sigtstp | sigcont | sigwinch | siginfo,
                            Option :: default | handle | ignore.

Enables or disables OS signals.

Each signal my be set to one of the following options:

- `ignore` - This signal will be ignored.

- `default` - This signal will use the default signal handler for the operating system.

- `handle` - This signal will notify `erl_signal_server` when it is received by the Erlang runtime system.

# system_time()

(since OTP 18.0)

    -spec system_time() -> integer().

Returns the current OS system time in `native` time unit.

#### Note

This time is not a monotonically increasing time.

# system_time(Unit)

(since OTP 18.0)

    -spec system_time(Unit) -> integer() when Unit :: erlang:time_unit().

Returns the current OS system time converted into the `Unit` passed as argument.

Calling `os:system_time(Unit)` is equivalent to `erlang:convert_time_unit`(`os:system_time()``, native, Unit)`.

#### Note

This time is not a monotonically increasing time.

# timestamp()

    -spec timestamp() -> Timestamp when Timestamp :: erlang:timestamp().

Returns the current OS system time in the same format as `erlang:timestamp/0`.

The tuple can be used together with function `calendar:now_to_universal_time/1` or `calendar:now_to_local_time/1` to get calendar time. Using the calendar time, together with the `MicroSecs` part of the return tuple from this function, allows you to log time stamps in high resolution and consistent with the time in the rest of the OS.

Example of code formatting a string in format "DD Mon YYYY HH:MM:SS.mmmmmm", where DD is the day of month, Mon is the textual month name, YYYY is the year, HH:MM:SS is the time, and mmmmmm is the microseconds in six positions:

    -module(print_time).
    -export([format_utc_timestamp/0]).
    format_utc_timestamp() ->
        TS = {_,_,Micro} = os:timestamp(),
        {{Year,Month,Day},{Hour,Minute,Second}} =
    calendar:now_to_universal_time(TS),
        Mstr = element(Month,{"Jan","Feb","Mar","Apr","May","Jun","Jul",
        "Aug","Sep","Oct","Nov","Dec"}),
        io_lib:format("~2w ~s ~4w ~2w:~2..0w:~2..0w.~6..0w",
        [Day,Mstr,Year,Hour,Minute,Second,Micro]).

This module can be used as follows:

    1> io:format("~s~n",[print_time:format_utc_timestamp()]).
    29 Apr 2009  9:55:30.051711

OS system time can also be retrieved by `system_time/0` and `system_time/1`.

# type()

    -spec type() -> {Osfamily, Osname} when Osfamily :: unix | win32, Osname :: atom().

Returns the `Osfamily` and, in some cases, the `Osname` of the current OS.

On Unix, `Osname` has the same value as `uname -s` returns, but in lower case. For example, on Solaris 1 and 2, it is `sunos`.

On Windows, `Osname` is `nt`.

#### Note

Think twice before using this function. Use module `filename` if you want to inspect or build filenames in a portable way. Avoid matching on atom `Osname`.

# unsetenv(VarName)

(since OTP R16B03)

    -spec unsetenv(VarName) -> true when VarName :: env_var_name().

Deletes the environment variable `VarName`.

If Unicode filename encoding is in effect (see the `erl` manual page), the string `VarName` can contain characters with codepoints > 255.

# version()

    -spec version() -> VersionString | {Major, Minor, Release}
                     when
                         VersionString :: string(),
                         Major :: non_neg_integer(),
                         Minor :: non_neg_integer(),
                         Release :: non_neg_integer().

Returns the OS version. On most systems, this function returns a tuple, but a string is returned instead if the system has versions that cannot be expressed as three numbers.

#### Note

Think twice before using this function. If you still need to use it, always `call os:type()` first.