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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_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.

       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).

       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 => integer() >= 0 | infinity}

              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.

EXPORTS

       cmd(Command) -> string()

       cmd(Command, Options) -> string()

              Types:

                 Command = os_command()
                 Options = os_command_opts()

              Executes  Command in a command shell of the target OS, captures the standard output
              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  (for  example,  os:cmd/1) can differ, compared with the standard output of
              the command when called directly from an OS command shell.

              os:cmd/2 was added in kernel-5.5 (OTP-20.2.1). It makes  it  possible  to  pass  an
              options map as the second argument in order to control the behaviour of os:cmd. The
              possible options are:

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

                > 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]

       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

       find_executable(Name, Path) -> Filename | false

              Types:

                 Name = Path = Filename = string()

              These  two  functions  look up an executable program, with the specified name and a
              search path, in the same way as  the  underlying  OS.  find_executable/1  uses  the
              current  execution  path  (that  is,  the  environment  variable  PATH  on Unix and
              Windows).

              Path, if specified, 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

              Types:

                 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

              Types:

                 VarName = env_var_name()
                 DefaultValue = 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

              Types:

                 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() system call. On Windows, the process id as  returned  by  the
              GetCurrentProcessId() system call is used.

       putenv(VarName, Value) -> true

              Types:

                 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

              Types:

                 Signal =
                     sighup | sigquit | sigabrt | sigalrm | sigterm | sigusr1 |
                     sigusr2 | sigchld | sigstop | sigtstp
                 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()

              Types:

                 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

              Types:

                 Timestamp = erlang:timestamp()
                   Timestamp = {MegaSecs, Secs, MicroSecs}

              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.

       perf_counter() -> Counter

              Types:

                 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()

              Types:

                 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

       type() -> {Osfamily, Osname}

              Types:

                 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

              Types:

                 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}

              Types:

                 VersionString = string()
                 Major = Minor = Release = integer() >= 0

              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.