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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 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).
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 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()
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 (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 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 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.
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 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
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()
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: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
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 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
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 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.
Ericsson AB kernel 6.5.1 os(3erl)