Lua io standard library

Version: stable

FUNCTIONS
file:close() closes a file
file:flush() flushes outstanding data to disk
file:lines() returns an iterator function for reading the file line-by-line
file:read() reads the file according to the specified formats
file:seek() sets and gets the current file position
file:setvbuf() sets the buffering mode for an output file
file:write() writes to a file
io.close() closes a file
io.flush() flushes outstanding data to disk for the default output file
io.input() opens filename for input in text mode
io.lines() returns an iterator function for reading a named file line-by-line
io.open() opens a file
io.output() opens a file for output
io.popen() creates a pipe and executes a command
io.read() reads from the default input file
io.tmpfile() returns a handle to a temporary file
io.type() returns type of file handle
io.write() writes to the default output file

Functions

file:close()

file:close()

Closes file. Note that files are automatically closed when their handles are garbage collected, but that takes an unpredictable amount of time to happen.

PARAMETERS

None

file:flush()

file:flush()

Saves any written data to file.

PARAMETERS

None

file:lines()

file:lines()

Returns an iterator function that, each time it is called, returns a new line from the file. Therefore, the construction 

for line in file:lines() do body end
will iterate over all lines of the file. (Unlike io.lines, this function does not close the file when the loop ends.)

PARAMETERS

None

file:read()

file:read(...)

Reads the file file, according to the given formats, which specify what to read. For each format, the function returns a string (or a number) with the characters read, or nil if it cannot read data with the specified format. When called without formats, it uses a default format that reads the entire next line (see below). The available formats are

"*n"
reads a number; this is the only format that returns a number instead of a string.
"*a"
reads the whole file, starting at the current position. On end of file, it returns the empty string.
"*l"
reads the next line (skipping the end of line), returning nil on end of file. This is the default format.
number
reads a string with up to this number of characters, returning nil on end of file. If number is zero, it reads nothing and returns an empty string, or nil on end of file.

PARAMETERS

...

file:seek()

file:seek([whence],[offset])

Sets and gets the file position, measured from the beginning of the file, to the position given by offset plus a base specified by the string whence, as follows:

"set"
base is position 0 (beginning of the file);
"cur"
base is current position;
"end"
base is end of file;
In case of success, function seek returns the final file position, measured in bytes from the beginning of the file. If this function fails, it returns nil, plus a string describing the error. The default value for whence is "cur", and for offset is 0. Therefore, the call file:seek() returns the current file position, without changing it; the call file:seek("set") sets the position to the beginning of the file (and returns 0); and the call file:seek("end") sets the position to the end of the file, and returns its size.

PARAMETERS

[whence]
[offset]

file:setvbuf()

file:setvbuf(mode,[size])

Sets the buffering mode for an output file. There are three available modes:

"no"
no buffering; the result of any output operation appears immediately.
"full"
full buffering; output operation is performed only when the buffer is full (or when you explicitly flush the file ).
"line"
line buffering; output is buffered until a newline is output or there is any input from some special files (such as a terminal device).
For the last two cases, size specifies the size of the buffer, in bytes. The default is an appropriate size.

PARAMETERS

mode
[size]

file:write()

file:write(...)

Writes the value of each of its arguments to the file. The arguments must be strings or numbers. To write other values, use tostring or string.format before write.

PARAMETERS

...

io.close()

io.close([file])

Equivalent to file:close(). Without a file, closes the default output file.

PARAMETERS

[file]

io.flush()

io.flush()

Equivalent to file:flush over the default output file.

PARAMETERS

None

io.input()

io.input([file])

When called with a file name, it opens the named file (in text mode), and sets its handle as the default input file. When called with a file handle, it simply sets this file handle as the default input file. When called without parameters, it returns the current default input file. In case of errors this function raises the error, instead of returning an error code.

PARAMETERS

[file]

io.lines()

io.lines([filename])

Opens the given file name in read mode and returns an iterator function that, each time it is called, returns a new line from the file. Therefore, the construction 

for line in io.lines(filename) do body end
will iterate over all lines of the file. When the iterator function detects the end of file, it returns nil (to finish the loop) and automatically closes the file. The call io.lines() (with no file name) is equivalent to io.input():lines(); that is, it iterates over the lines of the default input file. In this case it does not close the file when the loop ends.

PARAMETERS

[filename]

io.open()

io.open(filename,[mode])

This function opens a file, in the mode specified in the string mode. It returns a new file handle, or, in case of errors, nil plus an error message. The mode string can be any of the following:

"r"
read mode (the default);
"w"
write mode;
"a"
append mode;
"r+"
update mode, all previous data is preserved;
"w+"
update mode, all previous data is erased;
"a+"
append update mode, previous data is preserved, writing is only allowed at the end of file.
The mode string can also have a 'b' at the end, which is needed in some systems to open the file in binary mode. This string is exactly what is used in the standard C function fopen.

PARAMETERS

filename
[mode]

io.output()

io.output([file])

Similar to io.input, but operates over the default output file.

PARAMETERS

[file]

io.popen()

io.popen(prog,[mode])

Starts program prog in a separated process and returns a file handle that you can use to read data from this program (if mode is "r", the default) or to write data to this program (if mode is "w"). This function is system dependent and is not available on all platforms.

PARAMETERS

prog
[mode]

io.read()

io.read(...)

Equivalent to io.input():read.

PARAMETERS

...

io.tmpfile()

io.tmpfile()

Returns a handle for a temporary file. This file is opened in update mode and it is automatically removed when the program ends.

PARAMETERS

None

io.type()

io.type(obj)

Checks whether obj is a valid file handle. Returns the string "file" if obj is an open file handle, "closed file" if obj is a closed file handle, or nil if obj is not a file handle.

PARAMETERS

obj

io.write()

io.write(...)

Equivalent to io.output():write.

PARAMETERS

...