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.

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]

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.

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.

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.

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.

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.

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.

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.

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.

Returns the current OS system time in native 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).

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.

Returns the current performance counter value in perf_counter time unit. This is a highly optimized call that might not be traceable.

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

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.

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.

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.