## 4.35 Operating System Interaction

The predicates in this section provide basic access to the operating system that has been part of the Prolog legacy tradition. Note that more advanced access to low-level OS features is provided by several libaries from the `clib` package, notably library `library(process)`, `library(socket)`, `library(unix)` and `library(filesex)`.

shell(+Command)
  Equivalent to ``shell(Command, 0)`'. See shell/2 for details.
shell(+Command, -Status)
  Execute Command on the operating system. Command is given to the Bourne shell (/bin/sh). Status is unified with the exit status of the command.

  On Windows, shell/[1,2] executes the command using the CreateProcess() API and waits for the command to terminate. If the command ends with a `&` sign, the command is handed to the WinExec() API, which does not wait for the new task to terminate. See also win_exec/2 and win_shell/2. Please note that the CreateProcess() API does not imply the Windows command interpreter (cmd.exe and therefore commands that are built in the command interpreter can only be activated using the command interpreter. For example, a file can be compied using the command below.

      ?- shell('cmd.exe /C copy file1.txt file2.txt').

  Note that many of the operations that can be achieved using the shell built-in commands can easily be achieved using Prolog primitives. See make_directory/1, delete_file/1, rename_file/2, etc. The clib package provides `library(filesex)`, implementing various high level file operations such as copy_file/2. Using Prolog primitives instead of shell commands improves the portability of your program.

  The library `library(process)` provides process_create/3 and several related primitives that support more fine-grained interaction with processes, including I/O redirection and management of asynchronous processes.

getenv(+Name, -Value)
  Get environment variable. Fails silently if the variable does not exist. Please note that environment variable names are case-sensitive on Unix systems and case-insensitive on Windows.
setenv(+Name, +Value)
  Set an environment variable. Name and Value must be instantiated to atoms or integers. The environment variable will be passed to shell/[0-2] and can be requested using getenv/2. They also influence expand_file_name/2. Environment variables are shared between threads. Depending on the underlying C library, setenv/2 and unsetenv/1 may not be thread-safe and may cause memory leaks. Only changing the environment once and before starting threads is safe in all versions of SWI-Prolog.
unsetenv(+Name)
  Remove an environment variable from the environment. Some systems lack the underlying unsetenv() library function. On these systems unsetenv/1 sets the variable to the empty string.
setlocale(+Category, -Old, +New)
  Set/Query the locale setting which tells the C library how to interpret text files, write numbers, dates, etc. Category is one of `all`, `collate`, `ctype`, `messages`, `monetary`, `numeric` or `time`. For details, please consult the C library locale documentation. See also section 2.19.1. Please note that the locale is shared between all threads and thread-safe usage of setlocale/3 is in general not possible. Do locale operations before starting threads or thoroughly study threading aspects of locale support in your environment before using in multithreaded environments. Locale settings are used by format_time/3, collation_key/2 and locale_sort/2.
unix(+Command)
  This predicate comes from the Quintus compatibility library and provides a partial implementation thereof. It provides access to some operating system features and unlike the name suggests, is not operating system specific. Defined Command's are below.
  system(+Command)
    Equivalent to calling shell/1. Use for compatibility only.
  shell(+Command)
    Equivalent to calling shell/1. Use for compatibility only.
  shell
    Equivalent to calling shell/0. Use for compatibility only.
  cd
    Equivalent to calling working_directory/2 to the expansion (see expand_file_name/2) of ``~``. For compatibility only.
  cd(+Directory)
    Equivalent to calling working_directory/2. Use for compatibility only.
  argv(-Argv)
    Unify Argv with the list of command line arguments provided to this Prolog run. Please note that Prolog system arguments and application arguments are separated by `--`. Integer arguments are passed as Prolog integers, float arguments and Prolog floating point numbers and all other arguments as Prolog atoms. New applications should use the Prolog flag argv. See also the Prolog flag argv.

### 4.35.1 Windows-specific Operating System Interaction

The predicates in this section are only available on the Windows version of SWI-Prolog. Their use is discouraged if there are portably alternatives. For example, win_exec/2 and win_shell/2 can often be replaced by the more portable shell/2 or the more powerful process_create/3.

win_exec(+Command, +Show)
  Windows only. Spawns a Windows task without waiting for its completion. Show is one of the Win32 `SW_*` constants written in lowercase without the `SW_*`: `hide` `maximize` `minimize` `restore` `show` `showdefault` `showmaximized` `showminimized` `showminnoactive` `showna` `shownoactive` `shownormal`. In addition, `iconic` is a synonym for `minimize` and `normal` for `shownormal`.
win_shell(+Operation, +File, +Show)
  Windows only. Opens the document File using the Windows shell rules for doing so. Operation is one of `open`, `print` or `explore` or another operation registered with the shell for the given document type. On modern systems it is also possible to pass a URL as File, opening the URL in Windows default browser. This call interfaces to the Win32 API ShellExecute(). The Show argument determines the initial state of the opened window (if any). See win_exec/2 for defined values.
win_shell(+Operation, +File)
  Same as `win_shell(Operation, File, normal)`
win_registry_get_value(+Key, +Name, -Value)
  Windows only. Fetches the value of a Windows registry key. Key is an atom formed as a path name describing the desired registry key. Name is the desired attribute name of the key. Value is unified with the value. If the value is of type `DWORD`, the value is returned as an integer. If the value is a string, it is returned as a Prolog atom. Other types are currently not supported. The default `root' is `HKEY_CURRENT_USER`. Other roots can be specified explicitly as `HKEY_CLASSES_ROOT`, `HKEY_CURRENT_USER`, `HKEY_LOCAL_MACHINE` or `HKEY_USERS`. The example below fetches the extension to use for Prolog files (see `README.TXT` on the Windows version):

      ?- win_registry_get_value(
             'HKEY_LOCAL_MACHINE/Software/SWI/Prolog',
             fileExtension,
             Ext).

      Ext = pl

win_folder(?Name, -Directory)
  True if Name is the Windows `CSIDL' of Directory. If Name is unbound, all known Windows special paths are generated. Name is the CSIDL after deleting the leading `CSIDL_` and mapping the constant to lowercase. Check the Windows documentation for the function SHGetSpecialFolderPath() for a description of the defined constants. This example extracts the `My Documents' folder:

      ?- win_folder(personal, MyDocuments).

      MyDocuments = 'C:/Documents and Settings/jan/My Documents'

win_add_dll_directory(+AbsDir)
  This predicate adds a directory to the search path for dependent DLL files. If possible, this is achieved with win_add_dll_directory/2. Otherwise, `%PATH%` is extended with the provided directory. AbsDir may be specified in the Prolog canonical syntax. See prolog_to_os_filename/2. Note that use_foreign_library/1 passes an absolute path to the DLL if the destination DLL can be located from the specification using absolute_file_name/3.
win_add_dll_directory(+AbsDir, -Cookie)
  This predicate adds a directory to the search path for dependent DLL files. If the call is successful it unifies Cookie with a handle that must be passed to win_remove_dll_directory/1 to remove the directory from the search path. Error conditions:

  - This predicate is available in the Windows port of SWI-Prolog starting from 6.3.8/6.2.6.
  - This predicate fails if Windows does not yet support the underlying primitives. These are available in recently patched Windows 7 systems and later.
  - This predicate throws an acception if the provided path is invalid or the underlying Windows API returns an error.

  If open_shared_object/2 is passed an absolute path to a DLL on a Windows installation that supports AddDllDirectory() and friends,133Windows 7 with up-to-date patches or Windows 8. SWI-Prolog uses LoadLibraryEx() with the flags `LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR` and `LOAD_LIBRARY_SEARCH_DEFAULT_DIRS`. In this scenario, directories from `%PATH%` and not searched. Additional directories can be added using win_add_dll_directory/2.

win_remove_dll_directory(-Cookie)
  Remove a DLL search directory installed using win_add_dll_directory/2.

### 4.35.2 Dealing with time and date

Representing time in a computer system is surprisingly complicated. There are a large number of time representations in use, and the correct choice depends on factors such as compactness, resolution and desired operations. Humans tend to think about time in hours, days, months, years or centuries. Physicists think about time in seconds. But, a month does not have a defined number of seconds. Even a day does not have a defined number of seconds as sometimes a leap-second is introduced to synchronise properly with our earth's rotation. At the same time, resolution demands a range from better than pico-seconds to millions of years. Finally, civilizations have a wide range of calendars. Although there exist libraries dealing with most if this complexity, our desire to keep Prolog clean and lean stops us from fully supporting these.

For human-oriented tasks, time can be broken into years, months, days, hours, minutes, seconds and a timezone. Physicists prefer to have time in an arithmetic type representing seconds or fraction thereof, so basic arithmetic deals with comparison and durations. An additional advantage of the physicist's approach is that it requires much less space. For these reasons, SWI-Prolog uses an arithmetic type as its prime time representation.

Many C libraries deal with time using fixed-point arithmetic, dealing with a large but finite time interval at constant resolution. In our opinion, using a floating point number is a more natural choice as we can use a natural unit and the interface does not need to be changed if a higher resolution is required in the future. Our unit of choice is the second as it is the scientific unit.134Using Julian days is a choice made by the Eclipse team. As conversion to dates is needed for a human readable notation of time and Julian days cannot deal naturally with leap seconds, we decided for the second as our unit. We have placed our origin at 1970-1-1T0:0:0Z for compatibility with the POSIX notion of time as well as with older time support provided by SWI-Prolog.

Where older versions of SWI-Prolog relied on the POSIX conversion functions, the current implementation uses libtai to realise conversion between time-stamps and calendar dates for a period of 10 million years.

#### 4.35.2.1 Time and date data structures

We use the following time representations

TimeStamp
  A TimeStamp is a floating point number expressing the time in seconds since the Epoch at 1970-1-1.
date(Y,M,D,H,Mn,S,Off,TZ,DST)
  We call this term a date-time structure. The first 5 fields are integers expressing the year, month (1..12), day (1..31), hour (0..23) and minute (0..59). The S field holds the seconds as a floating point number between 0.0 and 60.0. Off is an integer representing the offset relative to UTC in seconds, where positive values are west of Greenwich. If converted from local time (see stamp_date_time/3), TZ holds the name of the local timezone. If the timezone is not known, TZ is the atom ``-``. DST is `true` if daylight saving time applies to the current time, `false` if daylight saving time is relevant but not effective, and ``-`` if unknown or the timezone has no daylight saving time.
date(Y,M,D)
  Date using the same values as described above. Extracted using date_time_value/3.
time(H,Mn,S)
  Time using the same values as described above. Extracted using date_time_value/3.

#### 4.35.2.2 Time and date predicates

get_time(-TimeStamp)
  Return the current time as a TimeStamp. The granularity is system-dependent. See section 4.35.2.1.
stamp_date_time(+TimeStamp, -DateTime, +TimeZone)
  Convert a TimeStamp to a DateTime in the given timezone. See section 4.35.2.1 for details on the data types. TimeZone describes the timezone for the conversion. It is one of `local` to extract the local time, `'UTC'` to extract a UTC time or an integer describing the seconds west of Greenwich.
date_time_stamp(+DateTime, -TimeStamp)
  Compute the timestamp from a date/9 term. Values for month, day, hour, minute or second need not be normalized. This flexibility allows for easy computation of the time at any given number of these units from a given timestamp. Normalization can be achieved following this call with stamp_date_time/3. This example computes the date 200 days after 2006-7-14:

      ?- date_time_stamp(date(2006,7,214,0,0,0,0,-,-), Stamp),
         stamp_date_time(Stamp, D, 0),
         date_time_value(date, D, Date).
      Date = date(2007, 1, 30)

  When computing a time stamp from a local time specification, the UTC offset (arg 7), TZ (arg 8) and DST (arg 9) argument may be left unbound and are unified with the proper information. The example below, executed in Amsterdam, illustrates this behaviour. On the 25th of March at 01:00, DST does not apply. At 02.00, the clock is advanced by one hour and thus both 02:00 and 03:00 represent the same time stamp.

      1 ?- date_time_stamp(date(2012,3,25,1,0,0,UTCOff,TZ,DST),
                           Stamp).
      UTCOff = -3600,
      TZ = 'CET',
      DST = false,
      Stamp = 1332633600.0.

      2 ?- date_time_stamp(date(2012,3,25,2,0,0,UTCOff,TZ,DST),
                           Stamp).
      UTCOff = -7200,
      TZ = 'CEST',
      DST = true,
      Stamp = 1332637200.0.

      3 ?- date_time_stamp(date(2012,3,25,3,0,0,UTCOff,TZ,DST),
                           Stamp).
      UTCOff = -7200,
      TZ = 'CEST',
      DST = true,
      Stamp = 1332637200.0.

  Note that DST and offset calculation are based on the POSIX function mktime(). If mktime() returns an error, a representation_error `dst` is generated.

date_time_value(?Key, +DateTime, ?Value)
  Extract values from a date/9 term. Provided keys are:

  key | value
  `year` | Calendar year as an integer
  `month` | Calendar month as an integer 1..12
  `day` | Calendar day as an integer 1..31
  `hour` | Clock hour as an integer 0..23
  `minute` | Clock minute as an integer 0..59
  `second` | Clock second as a float 0.0..60.0
  `utc_offset` | Offset to UTC in seconds (positive is west)
  `time_zone` | Name of timezone; fails if unknown
  `daylight_saving` | Bool `daylight_saving`true) if dst is in effect
  `date` | Term `date(Y,M,D)`
  `time` | Term `time(H,M,S)`

format_time(+Out, +Format, +StampOrDateTime)
  Modelled after POSIX strftime(), using GNU extensions. Out is a destination as specified with with_output_to/2. Format is an atom or string with the following conversions. Conversions start with a percent (%) character.135Descriptions taken from Linux Programmer's Manual StampOrDateTime is either a numeric time-stamp, a term `date(Y,M,D,H,M,S,O,TZ,DST)` or a term `date(Y,M,D)`.

  - `a`
    The abbreviated weekday name according to the current locale. Use format_time/4 for POSIX locale.
  - `A`
    The full weekday name according to the current locale. Use format_time/4 for POSIX locale.
  - `b`
    The abbreviated month name according to the current locale. Use format_time/4 for POSIX locale.
  - `B`
    The full month name according to the current locale. Use format_time/4 for POSIX locale.
  - `c`
    The preferred date and time representation for the current locale.
  - `C`
    The century number (year/100) as a 2-digit integer.
  - `d`
    The day of the month as a decimal number (range 01 to 31).
  - `D`
    Equivalent to %m/%d/%y. (For Americans only. Americans should note that in other countries %d/%m/%y is rather common. This means that in an international context this format is ambiguous and should not be used.)
  - `e`
    Like %d, the day of the month as a decimal number, but a leading zero is replaced by a space.
  - `E`
    Modifier. Not implemented.
  - `f`
    Number of microseconds. The `f` can be prefixed by an integer to print the desired number of digits. E.g., `%3f` prints milliseconds. This format is not covered by any standard, but available with different format specifiers in various incarnations of the strftime() function.
  - `F`
    Equivalent to %Y-%m-%d (the ISO 8601 date format).
  - `g`
    Like %G, but without century, i.e., with a 2-digit year (00-99).
  - `G`
    The ISO 8601 year with century as a decimal number. The 4-digit year corresponding to the ISO week number (see %V). This has the same format and value as %y, except that if the ISO week number belongs to the previous or next year, that year is used instead.
  - `V`
    The ISO 8601:1988 week number of the current year as a decimal number, range 01 to 53, where week 1 is the first week that has at least 4 days in the current year, and with Monday as the first day of the week. See also %U and %W.
  - `h`
    Equivalent to %b.
  - `H`
    The hour as a decimal number using a 24-hour clock (range 00 to 23).
  - `I`
    The hour as a decimal number using a 12-hour clock (range 01 to 12).
  - `j`
    The day of the year as a decimal number (range 001 to 366).
  - `k`
    The hour (24-hour clock) as a decimal number (range 0 to 23); single digits are preceded by a blank. (See also %H.)
  - `l`
    The hour (12-hour clock) as a decimal number (range 1 to 12); single digits are preceded by a blank. (See also %I.)
  - `m`
    The month as a decimal number (range 01 to 12).
  - `M`
    The minute as a decimal number (range 00 to 59).
  - `n`
    A newline character.
  - `O`
    Modifier to select locale-specific output. Not implemented.
  - `p`
    Either `AM' or `PM' according to the given time value, or the corresponding strings for the current locale. Noon is treated as `pm' and midnight as `am'.136Despite the above clain, some locales yield `am` or `pm` in lower case.
  - `P`
    Like %p but in lowercase: `am' or `pm' or a corresponding string for the current locale.
  - `r`
    The time in a.m. or p.m. notation. In the POSIX locale this is equivalent to `%I:%M:%S %p'.
  - `R`
    The time in 24-hour notation (%H:%M). For a version including the seconds, see %T below.
  - `s`
    The number of seconds since the Epoch, i.e., since 1970-01-01 00:00:00 UTC.
  - `S`
    The second as a decimal number (range 00 to 60). (The range is up to 60 to allow for occasional leap seconds.)
  - `t`
    A tab character.
  - `T`
    The time in 24-hour notation (%H:%M:%S).
  - `u`
    The day of the week as a decimal, range 1 to 7, Monday being 1. See also %w.
  - `U`
    The week number of the current year as a decimal number, range 00 to 53, starting with the first Sunday as the first day of week 01. See also %V and %W.
  - `w`
    The day of the week as a decimal, range 0 to 6, Sunday being 0. See also %u.
  - `W`
    The week number of the current year as a decimal number, range 00 to 53, starting with the first Monday as the first day of week 01.
  - `x`
    The preferred date representation for the current locale without the time.
  - `X`
    The preferred time representation for the current locale without the date.
  - `y`
    The year as a decimal number without a century (range 00 to 99).
  - `Y`
    The year as a decimal number including the century.
  - `z`
    The timezone as hour offset from GMT using the format HHmm. Required to emit RFC822-conforming dates (using `'%a, %d %b %Y %T %z'`). Our implementation supports `%:z`, which modifies the output to HH:mm as required by XML-Schema. Note that both notations are valid in ISO 8601. The sequence `%:z` is compatible to the GNU date(1) command.
  - `Z`
    The timezone or name or abbreviation.
  - ``+``
    The date and time in date(1) format.
  - `%`
    A literal `%' character.

  The table below gives some format strings for popular time representations. RFC1123 is used by HTTP. The full implementation of http_timestamp/2 as available from `library(http/http_header)` is here.

      http_timestamp(Time, Atom) :-
              stamp_date_time(Time, Date, 'UTC'),
              format_time(atom(Atom),
                          '%a, %d %b %Y %T GMT',
                          Date, posix).

  Standard | Format string
  xsd | `'%FT%T%:z'`
  ISO8601 | `'%FT%T%z'`
  RFC822 | `'%a, %d %b %Y %T %z'`
  RFC1123 | `'%a, %d %b %Y %T GMT'`

format_time(+Out, +Format, +StampOrDateTime, +Locale)
  Format time given a specified Locale. This predicate is a work-around for lacking proper portable and thread-safe time and locale handling in current C libraries. In its current implementation the only value allowed for Locale is `posix`, which currently only modifies the behaviour of the `a`, `A`, `b` and `B` format specifiers. The predicate is used to be able to emit POSIX locale week and month names for emitting standardised time-stamps such as RFC1123.
parse_time(+Text, -Stamp)
  Same as `parse_time(Text, _Format, Stamp)`. See parse_time/3.
parse_time(+Text, ?Format, -Stamp)
  Parse a textual time representation, producing a time-stamp. Supported formats for Text are in the table below. If the format is known, it may be given to reduce parse time and avoid ambiguities. Otherwise, Format is unified with the format encountered.

  Name | Example
  rfc_1123 | `Fri, 08 Dec 2006 15:29:44 GMT`
  | `Fri, 08 Dec 2006 15:29:44 +0000`
  iso_8601 | `2006-12-08T17:29:44+02:00`
  | `20061208T172944+0200`
  | `2006-12-08T15:29Z`
  | `2006-12-08`
  | `20061208`
  | `2006-12`
  | `2006-W49-5`
  | `2006-342`

day_of_the_week(+Date,-DayOfTheWeek)
  Computes the day of the week for a given date. `Date = date(Year,Month,Day)`. Days of the week are numbered from one to seven: Monday = 1, Tuesday = 2, ... , Sunday = 7.

### 4.35.3 Controlling the swipl-win.exe console window

The Windows executable swipl-win.exe console has a number of predicates to control the appearance of the console. Being totally non-portable, we do not advise using it for your own application, but use XPCE or another portable GUI platform instead. We give the predicates for reference here.

window_title(-Old, +New)
  Unify Old with the title displayed in the console and change the title to New.bugThis predicate should have been called `win_window_title` for consistent naming.
win_window_pos(+ListOfOptions)
  Interface to the MS-Windows SetWindowPos() function, controlling size, position and stacking order of the window. ListOfOptions is a list that may hold any number of the terms below:
  size(W, H)
    Change the size of the window. W and H are expressed in character units.
  position(X, Y)
    Change the top-left corner of the window. The values are expressed in pixel units.
  zorder(ZOrder)
    Change the location in the window stacking order. Values are `bottom`, `top`, `topmost` and `notopmost`. Topmost windows are displayed above all other windows.
  show(Bool)
    If `true`, show the window, if `false` hide the window.
  activate
    If present, activate the window.
win_has_menu
  True if win_insert_menu/2 and win_insert_menu_item/4 are present.
win_insert_menu(+Label, +Before)
  Insert a new entry (pulldown) in the menu. If the menu already contains this entry, nothing is done. The Label is the label and, using the Windows convention, a letter prefixed with `&` is underlined and defines the associated accelerator key. Before is the label before which this one must be inserted. Using ``-`` adds the new entry at the end (right). For example, the call below adds an Application entry just before the Help menu.

      win_insert_menu('&Application', '&Help')

win_insert_menu_item(+Pulldown, +Label, +Before, :Goal)
  Add an item to the named Pulldown menu. Label and Before are handled as in win_insert_menu/2, but the label ``-`` inserts a separator. Goal is called if the user selects the item.