6.9 – Operating System Facilities

This library is implemented through table os.

os.clock ()

Returns an approximation of the amount in seconds of CPU time used by the program, as returned by the underlying ISO C function clock.

os.date ([format [, time]])

Returns a string or a table containing date and time, formatted according to the given string format.

If the time argument is present, this is the time to be formatted (see the os.time function for a description of this value). Otherwise, date formats the current time.

If format starts with ‘!‘, then the date is formatted in Coordinated Universal Time. After this optional character, if format is the string “*t“, then date returns a table with the following fields: year, month (1–12), day (1–31), hour (0–23), min (0–59), sec (0–61, due to leap seconds), wday (weekday, 1–7, Sunday is 1), yday (day of the year, 1–366), and isdst (daylight saving flag, a boolean). This last field may be absent if the information is not available.

If format is not “*t“, then date returns the date as a string, formatted according to the same rules as the ISO C function strftime.

If format is absent, it defaults to “%c“, which gives a human-readable date and time representation using the current locale.

On non-POSIX systems, this function may be not thread safe because of its reliance on C function gmtime and C function localtime.

os.difftime (t2, t1)

Returns the difference, in seconds, from time t1 to time t2 (where the times are values returned by os.time). In POSIX, Windows, and some other systems, this value is exactly t2-t1.

os.execute ([command])

This function is equivalent to the ISO C function system. It passes command to be executed by an operating system shell. Its first result is true if the command terminated successfully, or fail otherwise. After this first result the function returns a string plus a number, as follows:

• “exit“: the command terminated normally; the following number is the exit status of the command.
• “signal“: the command was terminated by a signal; the following number is the signal that terminated the command.

When called without a command, os.execute returns a boolean that is true if a shell is available.

os.exit ([code [, close]])

Calls the ISO C function exit to terminate the host program. If code is true, the returned status is EXIT_SUCCESS; if code is false, the returned status is EXIT_FAILURE; if code is a number, the returned status is this number. The default value for code is true.

If the optional second argument close is true, closes the Lua state before exiting.

os.getenv (varname)

Returns the value of the process environment variable varname or fail if the variable is not defined.

os.remove (filename)

Deletes the file (or empty directory, on POSIX systems) with the given name. If this function fails, it returns fail plus a string describing the error and the error code. Otherwise, it returns true.

os.rename (oldname, newname)

Renames the file or directory named oldname to newname. If this function fails, it returns fail, plus a string describing the error and the error code. Otherwise, it returns true.

os.setlocale (locale [, category])

Sets the current locale of the program. locale is a system-dependent string specifying a locale; category is an optional string describing which category to change: "all", "collate", "ctype", "monetary", "numeric", or "time"; the default category is "all". The function returns the name of the new locale, or fail if the request cannot be honored.

If locale is the empty string, the current locale is set to an implementation-defined native locale. If locale is the string “C“, the current locale is set to the standard C locale.

When called with nil as the first argument, this function only returns the name of the current locale for the given category.

This function may be not thread safe because of its reliance on C function setlocale.

os.time ([table])

Returns the current time when called without arguments, or a time representing the local date and time specified by the given table. This table must have fields year, month, and day, and may have fields hour (default is 12), min (default is 0), sec (default is 0), and isdst (default is nil). Other fields are ignored. For a description of these fields, see the os.date function.

When the function is called, the values in these fields do not need to be inside their valid ranges. For instance, if sec is -10, it means 10 seconds before the time specified by the other fields; if hour is 1000, it means 1000 hours after the time specified by the other fields.

The returned value is a number, whose meaning depends on your system. In POSIX, Windows, and some other systems, this number counts the number of seconds since some given start time (the “epoch”). In other systems, the meaning is not specified, and the number returned by time can be used only as an argument to os.date and os.difftime.

When called with a table, os.time also normalizes all the fields documented in the os.date function, so that they represent the same time as before the call but with values inside their valid ranges.

os.tmpname ()

Returns a string with a file name that can be used for a temporary file. The file must be explicitly opened before its use and explicitly removed when no longer needed.

In POSIX systems, this function also creates a file with that name, to avoid security risks. (Someone else might create the file with wrong permissions in the time between getting the name and creating the file.) You still have to open the file to use it and to remove it (even if you do not use it).

When possible, you may prefer to use io.tmpfile, which automatically removes the file when the program ends.