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Table of Contents

The Operating System Library in Lua 5.1.x

os.clock ()

Returns an approximation of the amount in seconds of CPU time used by the program.

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 (four digits), month (1–12), day (1–31), hour (0–23), min (0–59), sec (0–61), wday (weekday, Sunday is 1), yday (day of the year), and isdst (daylight saving flag, a boolean). Example for the previous sentence: os.date(“*t”).year = 2006

If format is not “*t”, then date returns the date as a string, formatted according to the same rules as the C function strftime. The acceptable formats are as the following:

  • ”%a” The abbreviated weekday name. Example: Thu.
  • ”%A” The full weekday name. Example: Thursday.
  • ”%b” The abbreviated month name. Example: Sep.
  • ”%B” The full month name. Example: September.
  • ”%d” The two-digit day of the month padded with leading zeroes if applicable. Example: 09.
  • ”%e” The day of the month space padded if applicable. Example: 9.
  • ”%H” The two-digit military time hour padded with a zero if applicable. Example: 16.
  • ”%I” The two-digit hour on a 12-hour clock padded with a zero if applicable. Example: 04.
  • ”%j” The three-digit day of the year padded with leading zeroes if applicable. Example: 040.
  • ”%k” The two-digit military time hour padded with a space if applicable. Example: 9.
  • ”%l” The hour on a 12-hour clock padded with a space if applicable. Example: 4.
  • ”%m” The two-digit month padded with a leading zero if applicable. Example: 09.
  • ”%M” The two-digits minute padded with a leading zero if applicable. Example: 02.
  • ”%p” Either AM or PM. Language dependent.
  • ”%S” The two-digit second padded with a zero if applicable. Example: 04.
  • ”%w” The numeric day of the week ranging from 0 to 6 where 0 is Sunday. Example: 0.
  • ”%x” The language-aware standard date representation. For most languages, this is just the same as %B %d, %Y. Example: September 06, 2002.
  • ”%X” The language-aware time representation. For most languages, this is just the same as %I:%M %p. Example: 04:31 PM.
  • ”%y” The two-digit year padded with a leading zero if applicable. Example: 01.
  • ”%Y” The four-digit year. Example: 2001.

When called without arguments, date returns a reasonable date and time representation that depends on the host system and on the current locale (that is, os.date() is equivalent to os.date(”%c”)).

os.difftime (t2, t1)

Returns the number of seconds from time t1 to time t2. In POSIX, Windows, and some other systems, this value is exactly t2-t1.

os.execute ([command])

This function is equivalent to the C function system. It passes command to be executed by an operating system shell. It returns a status code, which is system-dependent. If command is absent, then it returns nonzero if a shell is available and zero otherwise.

os.exit ([code])

Calls the C function exit, with an optional code, to terminate the host program. The default value for code is the success code.

os.getenv (varname)

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

os.remove (filename)

Deletes the file or directory with the given name. Directories must be empty to be removed. If this function fails, it returns nil, plus a string describing the error.

os.rename (oldname, newname)

Renames file or directory named oldname to newname. If this function fails, it returns nil, plus a string describing the error.

os.setlocale (locale [, category])

Sets the current locale of the program. locale is a 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 nil if the request cannot be honored.

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

os.time ([table])

Returns the current time when called without arguments, or a time representing the date and time specified by the given table. This table must have fields year, month, and day, and may have fields hour, min, sec, and isdst (for a description of these fields, see the os.date function).

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 date and difftime.

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.


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