os(3erl) Erlang Module Definition os(3erl)
NAME
os - 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 spe-
cific 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 con-
taining 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. Environ-
ment variable names and values containing null characters inside the
name or value are now rejected and will cause environment variable op-
erations to fail.
DATA TYPES
env_var_name() = nonempty_string()
A string containing valid characters on the specific OS for en-
vironment variable names using file:native_name_encoding() en-
coding. 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 oper-
ations to fail, but may instead produce invalid results.
env_var_value() = string()
A string containing valid characters on the specific OS for en-
vironment variable values using file:native_name_encoding() en-
coding. Note that specifically null characters (integer value
zero) are not allowed. However, note that not all invalid char-
acters 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 envi-
ronment variable names and values using file:native_name_encod-
ing() encoding. The first $= characters appearing in the string
separates environment variable name (on the left) from environ-
ment variable value (on the right).
os_command() = atom() | io_lib:chars()
All characters needs to be valid characters on the specific OS
using file:native_name_encoding() encoding. Note that specifi-
cally null characters (integer value zero) are not allowed. How-
ever, 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()
Types:
Command = os_command()
Options = os_command_opts()
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 (inte-
ger 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 di-
rectly from an OS command shell.
os:cmd/2 was added in kernel-5.5 (OTP-20.2.1). It makes it pos-
sible 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 under-
lying 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 "Var-
Name=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.
getenv(VarName) -> Value | false
Types:
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.
getenv(VarName, DefaultValue) -> Value
Types:
VarName = env_var_name()
DefaultValue = Value = env_var_value()
Returns the Value of the environment variable VarName, or De-
faultValue 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.
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()
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.
Note:
VarName is not allowed to contain an $= character. Previous im-
plementations 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
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 op-
erating system.
handle:
This signal will notify erl_signal_server when it is re-
ceived by the Erlang runtime system.
system_time() -> integer()
Returns the current OS system time in native time unit.
Note:
This time is not a monotonically increasing time.
system_time(Unit) -> integer()
Types:
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:con-
vert_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 er-
lang: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 tex-
tual month name, YYYY is the year, HH:MM:SS is the time, and mm-
mmmm 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 sys-
tem_time/1.
perf_counter() -> Counter
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()
Types:
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
type() -> {Osfamily, Osname}
Types:
Osfamily = unix | win32
Osname = atom()
Returns the Osfamily and, in some cases, the Osname of the cur-
rent 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
Types:
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.
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 ver-
sions 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.
Ericsson AB kernel 7.0 os(3erl)