fs.open

Function fs.open
Supported File Modes Opens a file so it can be read or written. mode consists of a first character which is one of the following: "r" to open the file read-only, "w" to open it for writing and remove any existing data on file open, or "a" to open for writing but keep existing data and append any writes to the end of the file optionally followed by "b" to open the file for binary access instead of text access (text mode will perform end-of-line conversions necessary to move text files between Windows, Linux, and Mac OS systems; binary files should not have these conversions applied as they will be corrupted) When you have opened a file you must always close the file handle, or else data will not be saved. Note: Depending on your version of ComputerCraft, "append" mode may or may not be capable of creating new files (and the function returns "nil" if the file doesn't already exist) - most later releases won't. Even if it works for you, if planning on distributing your code consider using fs.exists() to determine when "write" mode is appropriate.
Syntax	fs.open(string path, string mode)
Returns	table the file handle, or nil on error (e.g. if mode was "r" and the file did not exist, or if mode was "w" or "a" and the file was in a read-only location)
Part of	ComputerCraft
API	fs

Examples

Example
Creates the file "abcd" for writing, and holds a file handle to it
Code	local h = fs.open("abcd", "w")

Example
Opens "abcd" in either append or write mode, depending on whether it already exists or not.
Code	local h = fs.open("abcd", fs.exists("abcd") and "a" or "w")

File Handles

A file handle allows access to a file. A file handle is a table; the functions within the table are accessed with the dot operator (not the colon operator, as may be more intuitive!). The examples below assume a file has already been opened and the handle stored in the variable h.

Closing a file handle

When you open a file you must remember to then close the handle! With the write modes supported by ComputerCraft all the data that you are writing to a file will not be written into the file until you close the file. A file opened in any mode exposes the following close function.

Function h.close
Closes the file handle, after which it can no longer be used
Syntax	h.close()
Returns	nil
Part of	ComputerCraft
API	fs

Files opened in text read mode

A file opened in mode "r" (text read mode) exposes the following functions.

Function h.readLine
Reads the next line from the file
Syntax	h.readLine()
Returns	string the next line read from the file, with the end-of-line character stripped; or nil if there are no more lines in the file
Part of	ComputerCraft
API	fs

Function h.readAll
Reads the all the text in the file
Syntax	h.readAll()
Returns	string the entire rest of the file, with the end-of-line character on the very last line (if present) stripped
Part of	ComputerCraft
API	fs

Files opened in text write/append mode

A file opened in mode "w" (text write mode) or "a" (text append mode) exposes the following functions:

Function h.write
Writes a string of characters to the file exactly as they appear in the string data
Syntax	h.write(string data)
Returns	nil
Part of	ComputerCraft
API	fs

Function h.writeLine
Writes a string of characters to the file, then appends an end-of-line character
Syntax	h.writeLine(string data)
Returns	nil
Part of	ComputerCraft
API	fs

if you just want to save a file, without closing the handle, then you could do a flush. ( this is normally used for logging etc...)

Function h.flush
Flushes the data to the specified file. (keeps the handle available afterwards)
Syntax	h.flush()
Returns	nil
Part of	ComputerCraft
API	fs

Files opened in binary read mode

A file opened in mode "rb" (binary read mode) exposes the following functions:

Function h.read
Reads a single byte from the file and returns it
Syntax	h.read()
Returns	number the byte read from the file, or nil if there are no more bytes
Part of	ComputerCraft
API	fs

Files opened in binary write/append mode

A file opened in mode "wb" (binary write mode) or "ab" (binary append mode) exposes the following functions:

Function h.write
Writes a single byte into the file
Syntax	h.write(number byte)
Returns	nil
Part of	ComputerCraft
API	fs