I just want to write a file to somewhere. Here is my code:
file = io.open("test.txt", "w")
file:write("Hello World")
file:close()
but the app crashes at the first line with:
attempt to call field 'open' (a nil value)
Even trying with the Lua online console keeps up the same error.
EDITED:
Here is my screenshot of the console
Try this code to dump the keys of io:
for k in next,io do
print(k)
end
Lua online outputs:
write
Which makes sense when you think about it:
Sandboxing works by restricting what can be done, including removal of unsafe functions.
Related
I am learning how work with files in Lua, and I came across a problem.
I know that:
io.output() allows me to specify the file that I will write output to.
The io.write() function simply gets a number of string arguments and writes them to the current output file.
io.close() then closes the file I am reading or writing on.
I first set io.output() to "myNewFile.txt" then I did io.write("Hello World!").
io.output("myNewFile.txt")
io.write("Hello World!")
This part worked well and myNewFile.txt got Hello World written to it.
Then I wrote io.close() to close the myNewFile.txt.
After that, I wrote io.write("Hello World!"), but I got an error:
C:\\Program Files\\lua\\lua54.exe: ...OneDrive\\Documents\\learning lua\\steve's teacher\\main.lua:9: default output file is closed stack traceback: \[C\]: in function 'io.write' ...OneDrive\\Documents\\learning lua\\steve's teacher\\main.lua:9: in main chunk \[C\]: in ?
I wanted io.write("Hello World!") to write Hello World in the terminal. I know can use print() but print() function adds extra characters like \n and things like that.
So my question is, how do I write Hello World in the terminal using io.write("Hello World!") in this situation?
I tried to search this error up on Google, but there weren't any results. I also tried joining many Discord servers, but I didn't get a proper response. I am a new developer to Lua so this all is really confusing to me.
After io.close() do a io.output(io.stdout) to set it back.
The clean way
local oldout = io.output() -- without argument returns actual output
io.output("file") -- set to a filename
-- do file operations here
-- use io.flush() where it is necessary
io.close() -- close file also do a flush()
io.output(oldout) -- set back to oldout ( normally io.stdout )
-- changing io.output() back should also close() and flush() the "file"
-- But better doublecheck this when using userdata without experience
I'm currently trying to make a level loading system for a game.
function love.filedropped(file)
ofile=io.open(file:getFilename(),"r")
io.input(ofile)
file:close
levelc=io.read()
for i=1,levelc do
levels[i]=io.read()
print levels[i]
end
levelc should be the first line of the file, and file:getFilename is the file to open (path included) the project gives an error message on startup, and i've used a similar structure before, but for an output. The error is at line 30, which is the levelc=io.read().
I've tried changing the name of the file pointer (it was "f" before, now "ofile") and i've tried using io.read("*l") instead of io.read() but same result.
EDITS:
-this is a love.filedropped(file)
-i need to open other files from a .txt later and i don't really understand how do do that
The parameter given by love.filedropped is a DroppedFile.
In your case helpful could be File:lines().
For example:
function love.filedropped(file)
-- Open for reading
file:open("r")
-- Iterate over the lines
local i = 0
for line in file:lines() do
i = i + 1
levels[i] = line
print(i, levels[i]) -- Notice the parentheses missing in your code
end
-- Close the file
file:close()
end
Notice that love2d usually only allows reading/writing files within the save or working directory. Dropped files are an exception.
Unrelated to this answer but things I noticed in your code:
Use locals, oFile should be local
file:close() required parentheses as its a function call
Same for the print
The filedropped callback has no end
You mentioned reading other files too, to do so, you can either:
Use love.filesystem.newFile and a similar approach as before
The recommended one-liner love.filesystem.lines
I'm attempting to constantly read and parse a log file (Minecraft log file) by using io.popen in tandem with Ubuntu's tail command so that I can send some messages upon certain events.
Now, I have mostly everything working here, except one small issue. After a while of reading, the entire program just freezes.
Here is the relevant code:
-- Open the tail command, return a file handle for it.
local pop = io.popen(config.listen_command)
-- Simply read a single line, I've pulled this into its own
-- function so that if this ever needs changing I can do so
-- easily.
local function get_line()
logger:log(4, "READ LINE")
return pop:read("*l")
end
-- For each line in the log file, check if it matches any
-- of a list of patterns, return the matches and the
-- pattern information if so.
local function match_line()
local line = get_line()
logger:log(4, "Line: %s", line)
-- This all works, and I've tested that it's not freezing
-- here. I've just included it for completion of the call
-- -stack.
for event_type, data in pairs(config.message_patterns) do
for event_name, pattern in pairs(data) do
local matches = {line:match(pattern)}
if matches[1] then
return event_type, event_name, matches
end
end
end
end
-- The main loop, simply read a line and send a message
-- if there was a match.
logger:log(4, "Main loop begin.")
while true do
local event_type, event_name, matches = match_line()
-- ...
-- The rest of the code here is not relevant.
config.listen_command = "tail -F --lines=1 latest.log"
The issue is in the get_line function. After a while of reading the log file, it completely freezes on the pop:read("*l"). It prints the READ LINE message, but never prints the Line: <whatever data here> line.
This is a really strange issue that I've been getting really confused about. I've tried swapping to different distributions of Lua (Luvit, LuaJIT, Lua) and a very large amount of debugging, changing small things, rerunning, ... But I cannot think of anything that'd be causing this.
Perhaps there's something small I've missed.
So my question here is this: Why is pop:read("*l") freezing, even though more data is being outputted to the logfile? Is there a way to fix this? Perhaps to detect if the next read will freeze indefinitely, so I can try closing the popen'ed file and re-open it (or to preferably stop it happening altogether?)
My problem is about reading a text file (which is located in my computer) in the NodeMCU development kit. I am able to read the file content in Ubuntu terminal using a Lua script. Here I am sharing the code that I have been using for reading. Both are working pretty well in the Ubuntu terminal.
First one:
local open = io.open
local function read_file(path)
local file = open(path, "rb") -- r read mode and b binary mode
if not file then return nil end
local content = file:read "*a" -- *a or *all reads the whole file
file:close()
return content
Second one:
local fileContent = read_file("output.txt");
print (fileContent);
function file_exists(file)
local f = io.open(file, "rb")
if f then f:close() end
return f ~= nil
end
-- get all lines from a file, returns an empty
-- list/table if the file does not exist
function lines_from(file)
if not file_exists(file) then return {} end
lines = {}
for line in io.lines(file) do
lines[#lines + 1] = line
end
return lines
end
-- tests the functions above
local file = 'output.txt'
local lines = lines_from(file)
-- print all line numbers and their contents
for k,v in pairs(lines) do
print('line[' .. k .. ']', v)
end
My problem occurs when I have sent the code to NodeMCU, using Esplorer to send the code in. But the error occurs like this:
attempt to index global 'io' (a nil value)
stack traceback:
applicationhuff.lua:5: in function 'file_exists'
applicationhuff.lua:13: in function 'lines_from'
applicationhuff.lua:23: in main chunk
[C]: in function 'dofile'
stdin:1: in main chunk
My general purpose is actually to read this datas and publish it to a Mosquitto Broker via MQTT protocol. I am new to these topics. If anyone can handle my problem it will be appreciated. Thanks for your help...
NodeMCU does not have an io library. Therefore you get an error for indexing io, which is a nil value.
No offense, but sometimes I wonder how you guys actually manage to find your way to StackOverflow and even write some code without knowing how to do basic web research.
https://nodemcu.readthedocs.io/en/master/en/lua-developer-faq/
The firmware has replaced some standard Lua modules that don't align
well with the SDK structure with ESP8266-specific versions. For
example, the standard io and os libraries don't work, but have been
largely replaced by the NodeMCU node and file libraries.
https://nodemcu.readthedocs.io/en/master/en/modules/file/
The file module provides access to the file system and its individual
files.
I hope that's enough help...
I need to use Lua to run a binary program that may write something in its stdout and also returns a status code (also known as "exit status").
I searched the web and couldn't find something that does what I need. However I found out that in Lua:
os.execute() returns the status code
io.popen() returns a file handler that can be used to read process output
However I need both. Writing a wrapper function that runs both functions behind the scene is not an option because of process overhead and possibly changes in result on consecutive runs. I need to write a function like this:
function run(binpath)
...
return output,exitcode
end
Does anyone has an idea how this problem can be solved?
PS. the target system rung Linux.
With Lua 5.2 I can do the following and it works
-- This will open the file
local file = io.popen('dmesg')
-- This will read all of the output, as always
local output = file:read('*all')
-- This will get a table with some return stuff
-- rc[1] will be true, false or nil
-- rc[3] will be the signal
local rc = {file:close()}
I hope this helps!
I can't use Lua 5.2, I use this helper function.
function execute_command(command)
local tmpfile = '/tmp/lua_execute_tmp_file'
local exit = os.execute(command .. ' > ' .. tmpfile .. ' 2> ' .. tmpfile .. '.err')
local stdout_file = io.open(tmpfile)
local stdout = stdout_file:read("*all")
local stderr_file = io.open(tmpfile .. '.err')
local stderr = stderr_file:read("*all")
stdout_file:close()
stderr_file:close()
return exit, stdout, stderr
end
This is how I do it.
local process = io.popen('command; echo $?') -- echo return code of last run command
local lastline
for line in process:lines() do
lastline = line
end
print(lastline) -- the return code is the last line of output
If the last line has fixed length you can read it directly using file:seek("end", -offset), offset should be the length of the last line in bytes.
This functionality is provided in C by pclose.
Upon successful return, pclose() shall return the termination status
of the command language interpreter.
The interpreter returns the termination status of its child.
But Lua doesn't do this right (io.close always returns true). I haven't dug into these threads but some people are complaining about this brain damage.
http://lua-users.org/lists/lua-l/2004-05/msg00005.html
http://lua-users.org/lists/lua-l/2011-02/msg00387.html
If you're running this code on Win32 or in a POSIX environment, you could try this Lua extension: http://code.google.com/p/lua-ex-api/
Alternatively, you could write a small shell script (assuming bash or similar is available) that:
executes the correct executable, capturing the exit code into a shell variable,
prints a newline and terminal character/string onto standard out
prints the shell variables value (the exit code) onto standard out
Then, capture all the output of io.popen and parse backward.
Full disclosure: I'm not a Lua developer.
yes , your are right that os.execute() has returns and it's very simple if you understand how to run your command with and with out lua
you also may want to know how many variables it returns , and it might take a while , but i think you can try
local a, b, c, d, e=os.execute(-what ever your command is-)
for my example a is an first returned argument , b is the second returned argument , and etc.. i think i answered your question right, based off of what you are asking.