File Input Output

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The following code contains a few utility file input/output functions. These raise an error if they fail. They can also be useful for writing command-line utilities and small scripts, as the error raising (rather than returning nil) avoids silently continuing on failure.

-- Find the length of a file
-- filename: file name
-- returns
-- len: length of file
-- asserts on error
function length_of_file(filename)
 local fh = assert(io.open(filename, "rb"))
 local len = assert(fh:seek("end"))
 fh:close()
 return len
end
-- Return true if file exists and is readable.
function file_exists(path)
 local file = io.open(path, "rb")
 if file then file:close() end
 return file ~= nil
end
-- Guarded seek.
-- Same as file:seek except throws string
-- on error.
-- Requires Lua 5.1.
function seek(fh, ...)
 assert(fh:seek(...))
end
-- Read an entire file.
-- Use "a" in Lua 5.3; "*a" in Lua 5.1 and 5.2
function readall(filename)
 local fh = assert(io.open(filename, "rb"))
 local contents = assert(fh:read(_VERSION <= "Lua 5.2" and "*a" or "a"))
 fh:close()
 return contents
end
-- Write a string to a file.
function write(filename, contents)
 local fh = assert(io.open(filename, "wb"))
 fh:write(contents)
 fh:flush()
 fh:close()
end
-- Read, process file contents, write.
function modify(filename, modify_func)
 local contents = readall(filename)
 contents = modify_func(contents)
 write(filename, contents)
end

Old Lua 4 Stuff

The following examples are from Lua 4, included here only for historical reasons.

-- Guarded readfrom (Lua4)
-- [f]: file name
-- Note: In Lua5 use io.input().
function readfrom(f)
 local h, err
 if f then h, err = %readfrom(f)
 else h, err = %readfrom()
 end
 affirm(h, "can't read from " .. (f or "stdin") .. ": " ..
 (err or ""))
 return h
end
-- Guarded writeto (Lua4)
-- [f]: file name
-- Note: In Lua5 use io.output().
function writeto(f)
 local h, err
 if f then h, err = %writeto(f)
 else h, err = %writeto()
 end
 affirm(h, "can't write to " .. (f or "stdout") .. ": " ..
 (err or ""))
 return h
end
-- Guarded dofile (Lua4)
-- [f]: file name
-- Note: In Lua5, dofile throws on error.
function dofile(f)
 affirm(%dofile(f), "error while executing " .. f)
end
-- Guarded seek (Lua4)
-- f: file handle
-- w: whence to seek
-- o: offset
function seek(f, w, o)
 local ok, err
 if o then ok, err = %seek(f, w, o)
 elseif w then ok, err = %seek(f, w)
 else ok, err = %seek(f)
 end
 affirm(ok, "can't seek on " .. f .. ": " .. (err or ""))
end
-- Find the length of a file (Lua4)
-- f: file name
-- returns
-- len: length of file
function length_of_file(f)
 local h, len
 h = openfile(f, "r")
 len = seek(h, "end")
 closefile(h)
 return len
end

--JayCarlson - original (Lua 4 version)

File Input Output

Old Lua 4 Stuff

See Also