Lua - Checking if File is Readable

It is always good to check if a file is readable before reading a file and then handle the situation gracefully. To check if file is readable or not, we can check the io.open() status.

Syntax - io.open()

f = io.open(filename, [mode])

Where−

• filename− path of the file along with file name to be opened.

• mode− optional flag like r for read, w for write, a for append and so on.

f represents the file handle returned by io.open() method. In case of successful open operation, f is not nil. Following examples show the use cases of checking a file is readable or not.

We're making a check on example.txt which is present in the current directory and is readable where lua program is running.

main.lua

-- check if file is readable
function readable(filename)

   local isReadable = true
   -- Opens a file in read mode
   f = io.open(filename, "r")

   -- if file is not readable, f will be nil
   if not f then
      isReadable = false
   else
      -- close the file  
      f:close()
   end

   -- return status
   return isReadable
end

-- check if file is readable
if readable("example.txt") then
   print("example.txt is readable.")
else
   print("example.txt is not readable.")
end

Output

When we run the above program, we will get the following output−

example.txt is readable.

Example - Checking if file is not readable

We're making a check on system file which is not readable where lua program is running.

main.lua

-- check if file is readable
function readable(filename)

   local isReadable = true
   -- Opens a file in read mode
   f = io.open(filename, "r")

   -- if file is not readable, f will be nil
   if not f then
      isReadable = false
   else
      -- close the file  
      f:close()
   end

   -- return status
   return isReadable
end

-- check if file is not readable
if not readable("C:\Windows\System32\dnsrslvr.dll") then
   print("dnsrslvr.dll is not readable.")
else
   print("dnsrslvr.dll is readable.")
end

Output

When we run the above program, we will get the following output−

example1.txt does not exist.