os(3)

NAME
os - Operating system-specific functions.

DESCRIPTION

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.

DATA TYPES

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() :: 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() :: 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() :: 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() :: #{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.

        Since OTP 20.2.3
    exception_on_failure
        If set to true, cmd/2 will throw an error exception if the command exits with a non-zero exit code.

        Since OTP 28.0

FUNCTIONS

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]

        Since OTP 20.2.3
    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}]},

        Since OTP 28.0

    The command shell can be set using the kernel configuration parameter, by default the shell is detected upon system startup.
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) -> 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) -> 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() -> [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) -> 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) -> 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() -> 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() -> 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) -> 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) -> 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) -> 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() -> integer().

    Returns the current OS system time in native time unit.

    NOTE: This time is not a monotonically increasing time.
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() -> 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() -> {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) -> 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() -> 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.