-spec cmd(Command) -> string() when Command :: os_command().

Equivalent to cmd(Command, #{}).

-spec 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.

The command shell can be set using the kernel configuration parameter, by default the shell is detected upon system startup.

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 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]

-spec 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.

-spec 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).

-spec 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.

-spec 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.

-spec 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.

-spec 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.

-spec 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.

-spec 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.

-spec 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

-spec 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.

-spec set_signal(Signal, Option) -> ok
                    when
                        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.

-spec system_time() -> integer().

Returns the current OS system time in native time unit.

Note

This time is not a monotonically increasing time.

-spec 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.

-spec 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.

-spec 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.

-spec 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.

-spec 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.