os(3)



os(3erl)                   Erlang Module Definition                   os(3erl)

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 spe-
       cific  platform.  On  the other hand, with careful use, these functions
       can be of help in enabling a program to run on most platforms.

   Note:
       File operations used to accept  filenames  containing  null  characters
       (integer  value zero). This caused the name to be truncated and in some
       cases arguments to primitive operations to be mixed up. Filenames  con-
       taining  null  characters inside the filename are now rejected and will
       cause primitive file operations to fail.

       Also environment variable operations used to accept names and values of
       environment  variables containing null characters (integer value zero).
       This caused operations to silently produce erroneous results.  Environ-
       ment  variable  names  and values containing null characters inside the
       name or value are now rejected and will cause environment variable  op-
       erations to fail.

DATA TYPES
       env_var_name() = nonempty_string()

              A  string containing valid characters on the specific OS for en-
              vironment variable names using  file:native_name_encoding()  en-
              coding.  Note  that  specifically null characters (integer value
              zero) and $= characters are not allowed. However, note that  not
              all invalid characters necessarily will cause the primitiv oper-
              ations to fail, but may instead produce invalid results.

       env_var_value() = string()

              A string containing valid characters on the specific OS for  en-
              vironment  variable values using file:native_name_encoding() en-
              coding. Note that specifically null  characters  (integer  value
              zero)  are not allowed. However, note that not all invalid char-
              acters necessarily will cause the primitiv operations  to  fail,
              but may instead produce invalid results.

       env_var_name_value() = nonempty_string()

              Assuming  that  environment  variables has been correctly set, a
              strings containing valid characters on the specific OS for envi-
              ronment  variable names and values using file:native_name_encod-
              ing() encoding. The first $= characters appearing in the  string
              separates  environment variable name (on the left) from environ-
              ment 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. Note that specifi-
              cally null characters (integer value zero) are not allowed. How-
              ever,  note that not all invalid characters not necessarily will
              cause os:cmd/1 to fail, but may instead produce invalid results.

       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  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.

          Warning:
              Previous  implementation used to allow all characters as long as
              they were integer values greater than or  equal  to  zero.  This
              sometimes  lead to unwanted results since null characters (inte-
              ger value zero) often are interpreted as string termination. The
              current implementation rejects these.

              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 di-
              rectly from an OS command shell.

              os:cmd/2 was added in kernel-5.5 (OTP-20.2.1). It makes it  pos-
              sible  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]

       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  under-
              lying  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 "Var-
              Name=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.

       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 De-
              faultValue 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.

          Note:
              VarName  is not allowed to contain an $= character. Previous im-
              plementations used to just let the $=  character  through  which
              silently  caused  erroneous results. Current implementation will
              instead throw a badarg exception.

       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 op-
                  erating system.

                handle:
                   This signal will notify erl_signal_server when  it  is  re-
                  ceived 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:con-
              vert_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 er-
              lang: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  tex-
              tual month name, YYYY is the year, HH:MM:SS is the time, and mm-
              mmmm 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 retreived by system_time/0  and  sys-
              tem_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 cur-
              rent 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  ver-
              sions 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.

Ericsson AB                       kernel 7.0                          os(3erl)

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