Kernel

Reference Manual

Version 6.5.1

Table of Contents

os

Module

os

Module Summary

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 specific 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 containing 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. Environment variable names and values containing null characters inside the name or value are now rejected and will cause environment variable operations to fail.

Data Types

env_var_name() = nonempty_string()

A string containing valid characters on the specific OS for environment variable names using file:native_name_encoding() encoding. 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 operations to fail, but may instead produce invalid results.

env_var_value() = string()

A string containing valid characters on the specific OS for environment variable values using file:native_name_encoding() encoding. Note that specifically null characters (integer value zero) are not allowed. However, note that not all invalid characters 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 environment variable names and values using file:native_name_encoding() encoding. The first $= characters appearing in the string separates environment variable name (on the left) from environment variable value (on the right).

All characters needs to be valid characters on the specific OS using file:native_name_encoding() encoding. Note that specifically null characters (integer value zero) are not allowed. However, 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()
OTP 20.2.3

Types

Command = os_command()

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 (integer 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 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]

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 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.

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 erlmanual page), the strings can contain characters with codepoints > 255.

getenv(VarName) -> Value | false

Types

VarName = env_var_name()

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 erlmanual page), the strings VarName and Value can contain characters with codepoints > 255.

getenv(VarName, DefaultValue) -> Value
OTP 18.0

Types

VarName = env_var_name()
DefaultValue = 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 erlmanual 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()

Sets a new Value for environment variable VarName.

If Unicode filename encoding is in effect (see the erlmanual 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 implementations 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
OTP 20.0

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 operating system.
handle
This signal will notify erl_signal_server when it is received by the Erlang runtime system.

system_time() -> integer()
OTP 18.0

Returns the current OS system time in native time unit.

Note

This time is not a monotonically increasing time.

system_time(Unit) -> integer()
OTP 18.0

Types

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.

timestamp() -> Timestamp

Types

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

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 retreived by system_time/0 and system_time/1.

perf_counter() -> Counter
OTP 19.0

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()
OTP 19.0

Types

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 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.

unsetenv(VarName) -> true
OTP R16B03

Types

VarName = env_var_name()

Deletes the environment variable VarName.

If Unicode filename encoding is in effect (see the erlmanual 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 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.