5.7 – Input and Output Facilities

The I/O library provides two different styles for file manipulation. The first one uses implicit file descriptors; that is, there are operations to set a default input file and a default output file, and all input/output operations are over these default files. The second style uses explicit file descriptors.

When using implicit file descriptors, all operations are supplied by table io. When using explicit file descriptors, the operation io.open returns a file descriptor and then all operations are supplied as methods of the file descriptor.

The table io also provides three predefined file descriptors with their usual meanings from C: io.stdin, io.stdout, and io.stderr. The I/O library never closes these files.

Unless otherwise stated, all I/O functions return nil on failure (plus an error message as a second result and a system-dependent error code as a third result) and some value different from nil on success.

io.close ([file])

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

io.flush ()

Equivalent to file:flush over the default output file.

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.

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.

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.

io.output ([file])

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

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.

io.read (···)

Equivalent to io.input():read.

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.

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.

io.write (···)

Equivalent to io.output():write.

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.

file:flush ()

Saves any written data to file.

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.)

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.

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.

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 (see io.flush)).
• “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.

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.