## 4.34 Operating System Interaction

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 Win32 systems, 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 (command.exe on Windows 95/98 and cmd.exe on Windows-NT) and therefore commands built-in to the command-interpreter can only be activated using the command interpreter. For example: `'command.exe /C copy file1.txt file2.txt'`

shell(+Command)
  Equivalent to ``shell(Command, 0)`'.
shell
  Start an interactive Unix shell. Default is `/bin/sh`, the environment variable `SHELL` overrides this default. Not available for Win32 platforms.
win_exec(+Command, +Show)
  Win32 systems 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)
  Win32 systems 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)
  Win32 systems only. Fetches the value of a Win32 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)
  Is 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'

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 2.17.1. 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 use in multi-threaded 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 provides 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 prolog Prolog flag argv.

    A stand-alone program could use the following skeleton to handle command-line arguments. See also section 2.10.2.4.

        main :-
                current_prolog_flag(argv, Argv),
                append(_PrologArgs, [--|AppArgs], Argv), !,
                main(AppArgs).

### 4.34.1 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 range from better then 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 deal with comparison and durations. An additional advantage of the physicists 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.58Using 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 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.

#### 4.34.1.1 Time and date data-structures

We use the following time representations

TimeStamp
  A TimeStamp is a floating point number expression 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), 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.

#### 4.34.1.2 Time and date predicates

get_time(-TimeStamp)
  Return the current time as a TimeStamp. The granularity is system dependent. See section 4.34.1.1.
stamp_date_time(+TimeStamp, -DateTime, +TimeZone)
  Convert a TimeStamp to a DateTime in the given time zone. See section 4.34.1.1 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 at 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)

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 effective
  `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 tilde (%) character.59Descriptions taken from Linux Programmer's Manual

  - `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. (Yecch — for Americans only. Americans should note that in other countries %d/%m/%y is rather common. This means that in 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. 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'.
  - `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 time-zone as hour offset from GMT. Required to emit RFC822-conforming dates (using "%a, %d %b %Y %H:%M:%S %z").
  - `Z`
    The time zone or name or abbreviation.
  - ``+``
    The date and time in date(1) format.
  - `%`
    A literal `%' character.
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 or 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)
  Parse a textual time representation, producing a time-stamp. Supported formats for Text are:

  Name | Example
  RFC 1123 | `Fri, 08 Dec 2006 15:29:44 GMT`

### 4.34.2 Controlling the PLWIN.EXE console window

The Windows executable PLWIN.EXE console has a number of predicates to control the appearance of the console. Being totally non-portable, we do not advice 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 conventions, 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 a 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.