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Version 2.16.1

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Operating System Specific Functions


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.


cmd(Command) -> string()


Command = atom() | io_lib:chars()

Executes Command 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 unix:cmd/1; on a Unix platform they are equivalent.


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

Note that in some cases, standard output of a command when called from another program (for example, os:cmd/1) may differ, compared to the standard output of the command when called directly from an OS command shell.

find_executable(Name) -> Filename | false
find_executable(Name, Path) -> Filename | false


Name = Path = Filename = string()

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

Path, if given, should conform to the syntax of execution paths on the operating system. The absolute filename of the executable program Name is returned, or false if the program was not found.

getenv() -> [string()]

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

If Unicode file name encoding is in effect (see the erl manual page), the strings may contain characters with codepoints > 255.

getenv(VarName) -> Value | false


VarName = Value = string()

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

If Unicode file name encoding is in effect (see the erl manual page), the strings (both VarName and Value) may contain characters with codepoints > 255.

getpid() -> Value


Value = string()

Returns the process identifier of the current Erlang emulator in the format most commonly used by the operating system environment. Value is returned 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


VarName = Value = string()

Sets a new Value for the environment variable VarName.

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

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.

timestamp() -> Timestamp


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

Returns a tuple in the same format as erlang:now/0. 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 not guaranteed to be different.

The most obvious use for this function is logging. The tuple can be used together with the 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 timestamps in high resolution and consistent with the time in the rest of the operating system.

Example of code formatting a string in the 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:

format_utc_timestamp() ->
    TS = {_,_,Micro} = os:timestamp(),
    {{Year,Month,Day},{Hour,Minute,Second}} = 
    Mstr = element(Month,{"Jan","Feb","Mar","Apr","May","Jun","Jul",
    io_lib:format("~2w ~s ~4w ~2w:~2..0w:~2..0w.~6..0w",

The module above could be used in the following way:

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

type() -> {Osfamily, Osname}


Osfamily = unix | win32
Osname = atom()

Returns the Osfamily and, in some cases, Osname of the current operating system.

On Unix, Osname will have same value as uname -s returns, but in lower case. For example, on Solaris 1 and 2, it will be sunos.

In Windows, Osname will be either nt (on Windows NT), or windows (on Windows 95).


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

version() -> VersionString | {Major, Minor, Release}


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

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.


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