Append some text to the first line of a file with Lua

Question

How to prepend some text to the first line of a file with Lua?

out = io.open('file.txt}','a')
out:write('Hello world. ')
out:write('This is different')
io.close(out)

I only know how to append content to a file using the above code.

Answer 1

You can add text to the first line of a file like so.

-- Open the file in r mode (don't modify file, just read)
local out = io.open('file.txt', 'r')

-- Fetch all lines and add them to a table
local lines = {}
for line in f:lines() do
    table.insert(lines, line)
end

-- Close the file so that we can open it in a different mode
out:close()

-- Insert what we want to write to the first line into the table
table.insert(lines, 1, "<what you want to write to the first line>\n")

-- Open temporary file in w mode (write data)
-- Iterate through the lines table and write each line to the file
local out = io.open('file.tmp.txt', 'w')
for _, line in ipairs(lines) do
    out:write(line)
end
out:close()

-- At this point, we should have successfully written the data to the temporary file

-- Delete the old file
os.remove('file.txt')

-- Rename the new file
os.rename('file.tmp.txt', 'file.txt')

I hope this proves to be useful! Let me know if it doesn't work as you want it to.

Here's a nice documentation on the IO library for further reference. http://lua-users.org/wiki/IoLibraryTutorial

Answer 2

Create a new file, insert your stuff. Then append the contents of the old file. When you are done, replace the old file with the new file.

You don't prepend (or insert) to files in general. It makes no sense within the whole concept. You save file data into clusters. They have a start and a given size. Most programming languages therefor provide no means for it.

Imagine one file is saved in buckets (clusters). Water represents the data. You fill 1 bucket from bottom to top. Then a second one and so on. When you append something to your file you just add water to your last bucket. If it is full you append another bucket and fill it from top to bottom.

Now try to prepend something to your file, half a bucket in size. Your first bucket is full so you prepend one bucket to the row. Can you fill the new bucket from top to the middle? No. You can fill it from bottom to the middle. But now you have a gap of half a bucket in your file.

It works somehow that way. I can't think of a better example atm.