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  <header>
    <copyright>
      <year>1997</year><year>2012</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>os</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>os</module>
  <modulesummary>Operating System Specific Functions</modulesummary>
  <description>
    <p>The functions in this module are operating system specific.
      Careless use of these functions will result 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.</p>
  </description>
  <funcs>
    <func>
      <name name="cmd" arity="1"/>
      <fsummary>Execute a command in a shell of the target OS</fsummary>
      <desc>
        <p>Executes <c><anno>Command</anno></c> in a command shell of the target OS,
          captures the standard output of the command and returns this
          result as a string. This function is a replacement of
          the previous <c>unix:cmd/1</c>; on a Unix platform they are
          equivalent.</p>
        <p>Examples:</p>
        <code type="none">
LsOut = os:cmd("ls"), % on unix platform
DirOut = os:cmd("dir"), % on Win32 platform</code>
        <p>Note that in some cases, standard output of a command when
          called from another program (for example, <c>os:cmd/1</c>)
          may differ, compared to the standard output of the command
          when called directly from an OS command shell.</p>
      </desc>
    </func>
    <func>
      <name name="find_executable" arity="1"/>
      <name name="find_executable" arity="2"/>
      <fsummary>Absolute filename of a program</fsummary>
      <desc>
        <p>These two functions look up an executable program given its
          name and a search path, in the same way as the underlying
          operating system. <c>find_executable/1</c> uses the current
          execution path (that is, the environment variable PATH on
          Unix and Windows).</p>
        <p><c><anno>Path</anno></c>, if given, should conform to the syntax of
          execution paths on the operating system. The absolute
          filename of the executable program <c><anno>Name</anno></c> is returned,
          or <c>false</c> if the program was not found.</p>
      </desc>
    </func>
    <func>
      <name name="getenv" arity="0"/>
      <fsummary>List all environment variables</fsummary>
      <desc>
        <p>Returns a list of all environment variables.
          Each environment variable is given as a single string on
          the format <c>"VarName=Value"</c>, where <c>VarName</c> is
          the name of the variable and <c>Value</c> its value.</p>
	<p>If Unicode file name encoding is in effect (see the <seealso
	marker="erts:erl#file_name_encoding">erl manual
	page</seealso>), the strings may contain characters with
	codepoints > 255.</p>
      </desc>
    </func>
    <func>
      <name name="getenv" arity="1"/>
      <fsummary>Get the value of an environment variable</fsummary>
      <desc>
        <p>Returns the <c><anno>Value</anno></c> of the environment variable
          <c><anno>VarName</anno></c>, or <c>false</c> if the environment variable
          is undefined.</p>
	<p>If Unicode file name encoding is in effect (see the <seealso
	marker="erts:erl#file_name_encoding">erl manual
	page</seealso>), the strings (both <c><anno>VarName</anno></c> and
	<c><anno>Value</anno></c>) may contain characters with codepoints > 255.</p>
      </desc>
    </func>
    <func>
      <name name="getpid" arity="0"/>
      <fsummary>Return the process identifier of the emulator process</fsummary>
      <desc>
        <p>Returns the process identifier of the current Erlang emulator
          in the format most commonly used by the operating system
          environment. <c><anno>Value</anno></c> is returned as a string containing
          the (usually) numerical identifier for a process. On Unix,
          this is typically the return value of the <c>getpid()</c>
          system call. On Windows,
          the process id as returned by the <c>GetCurrentProcessId()</c>
          system call is used.</p>
      </desc>
    </func>
    <func>
      <name name="putenv" arity="2"/>
      <fsummary>Set a new value for an environment variable</fsummary>
      <desc>
        <p>Sets a new <c><anno>Value</anno></c> for the environment variable
          <c><anno>VarName</anno></c>.</p>
	<p>If Unicode filename encoding is in effect (see the <seealso
	marker="erts:erl#file_name_encoding">erl manual
	page</seealso>), the strings (both <c><anno>VarName</anno></c> and
	<c><anno>Value</anno></c>) may contain characters with codepoints > 255.</p>
	<p>On Unix platforms, the environment will be set using UTF-8 encoding
	if Unicode file name translation is in effect. On Windows the 
	environment is set using wide character interfaces.</p>
      </desc>
    </func>
    <func>
      <name name="timestamp" arity="0"/>
      <type_desc variable="Timestamp">Timestamp = {MegaSecs, Secs, MicroSecs}</type_desc>
      <fsummary>Returna a timestamp from the OS in the erlang:now/0 format</fsummary>
      <desc>
      <p>Returns a tuple in the same format as <seealso marker="erts:erlang#now/0">erlang:now/0</seealso>. The difference is that this function returns what the operating system thinks (a.k.a. the wall clock time) without any attempts at time correction. The result of two different calls to this function is <em>not</em> guaranteed to be different.</p>
      <p>The most obvious use for this function is logging. The tuple can be used together with the function <seealso marker="stdlib:calendar#now_to_universal_time/1">calendar:now_to_universal_time/1</seealso> 
or <seealso marker="stdlib:calendar#now_to_local_time/1">calendar:now_to_local_time/1</seealso> to get calendar time. Using the calendar time together with the <c>MicroSecs</c> part of the return tuple from this function allows you to log timestamps in high resolution and consistent with the time in the rest of the operating system.</p>
      <p>Example of code formatting a string in the format &quot;DD Mon YYYY HH:MM:SS.mmmmmm&quot;, 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:</p>
<code>
-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]).
</code>

       <p>The module above could be used in the following way:</p>
<pre>
1> <input>io:format("~s~n",[print_time:format_utc_timestamp()]).</input>
29 Apr 2009  9:55:30.051711
</pre>
      </desc>
    </func>
    <func>
      <name name="type" arity="0"/>
      <fsummary>Return the OS family and, in some cases, OS name of the current operating system</fsummary>
      <desc>
        <p>Returns the <c><anno>Osfamily</anno></c> and, in some cases, <c><anno>Osname</anno></c>
          of the current operating system.</p>
        <p>On Unix, <c><anno>Osname</anno></c> will have same value as
          <c>uname -s</c> returns, but in lower case. For example, on
          Solaris 1 and 2, it will be <c>sunos</c>.</p>
        <p>In Windows, <c><anno>Osname</anno></c> will be either <c>nt</c> (on
          Windows NT), or <c>windows</c> (on Windows 95).</p>
        <note>
          <p>Think twice before using this function. Use the
            <c>filename</c> module if you want to inspect or build
            file names in a portable way.
            Avoid matching on the <c><anno>Osname</anno></c> atom.</p>
        </note>
      </desc>
    </func>
    <func>
      <name name="unsetenv" arity="1"/>
      <fsummary>Delete an environment variable</fsummary>
      <desc>
	<p>Deletes the environment variable <c><anno>VarName</anno></c>.</p>
	<p>If Unicode filename encoding is in effect (see the <seealso
	marker="erts:erl#file_name_encoding">erl manual
	page</seealso>), the string (<c><anno>VarName</anno></c>) may
	contain characters with codepoints > 255.</p>
      </desc>
    </func>
    <func>
      <name name="version" arity="0"/>
      <fsummary>Return the Operating System version</fsummary>
      <desc>
        <p>Returns the operating system version.
          On most systems, this function returns a tuple, but a string
          will be returned instead if the system has versions which
          cannot be expressed as three numbers.</p>
        <note>
          <p>Think twice before using this function. If you still need
            to use it, always <c>call os:type()</c> first.</p>
        </note>
      </desc>
    </func>
  </funcs>
</erlref>

