Changing variables from one file to another

I am trying to make a very big stock exchange program and it runs threw multiple files. one file I call "stock-buy-1", when activated adds 1 to a variable on a file called "stock-userAmmount" this is what I have.

stock-buy-1
h = fs.open("stock-userAmmount", "a")
h.writeLine("x = 0+1")
h.close()

stock-userAmmount
x = 0
print( x )

so basicly I want stock-buy-1 to open stock-userAmmount and add one to x = 0 so when it prints it
will go up 1 number each time I run stock-buy-1, how would I fix this?

You can't inject a line partway into a file. You can add lines to the end, or overwrite the entire thing - those are the two options.

So, you've got a choice: Either overwrite the entire "stock-userAmmount" script, or write to a third file and have the "stock-userAmmount" script read from that. That latter option is the easiest, and it'd go something like this:

stock-buy-1:

local myTable
if fs.exists("stock-data") then
  -- If the data file already exists, read what's in it:
  local myFile = fs.open("stock-data","r")
  myTable = textutils.unserialize(myFile.readAll())
  myFile.close()
else
  -- Otherwise build a new table:
  myTable = {["x"] = -1}
end

-- Add one to x:
myTable.x = myTable.x + 1

-- Write the data file with the updated data:
local myFile = fs.open("stock-data","w")
myFile.write(textutils.serialize(myTable))
myFile.close()

stock-userAmmount:

local myFile = fs.open("stock-data","r")
local myTable = textutils.unserialize(myFile.readAll())
myFile.close()

print(myTable.x)

You could do much the same thing without the table, but using one makes it much, much easier to store multiple values if you later have need to do so.

I tried this, it didn't work