| os(3erl) | Erlang Module Definition | os(3erl) |
os - Operating system-specific functions.
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.
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.
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
cmd(Command) -> string()
cmd(Command, Options) -> string()
Types:
Executes Command in a command shell of the target OS, captures the standard output 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:
> 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:
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:
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:
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:
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:
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.
set_signal(Signal, Option) -> ok
Types:
Enables or disables OS signals.
Each signal my be set to one of the following options:
system_time() -> integer()
Returns the current OS system time in native time unit.
system_time(Unit) -> integer()
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).
timestamp() -> Timestamp
Types:
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:
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:
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:
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.
unsetenv(VarName) -> true
Types:
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:
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.
| kernel 7.2 | Ericsson AB |