It is a curious 'feature' that Windows does not set the %ERRORLEVEL% dynamic variable to an error upon redirection failure unless the || operator is used. Most likely it fails because of a locking issue, though technically there could be other reasons for failure. Presumably echo off will never fail, so the only possible reason for failure would be the redirection failed. The standard Windows & and || operators are used to detect whether the command inside the parens was successful or failed. So the error message is effectively hidden, but the error code is still propagated outside of the parens.

That is why I enclose the command in parentheses and then redirect the error output to nul outside of the parens. Simply adding 2>nul to the command will not prevent the error message because it redirects the error output for the command, not the redirection. If the command and the redirection succeed, then a success code is returned. Also an error code will be generated upon redirection failure. So if another process already has 'test.txt' locked for writing, then the redirection will fail with the following error message sent to stderr - 'The process cannot access the file because it is being used by another process.'. (There are OS system calls that allow opening a file for writing in a shared mode, but that is not the default). Most processes lock a file whenever they open a file for write access. So >test.txt echo off will attempt to open the file, it writes nothing to the file (assuming echo is already off), and then it closes the file. The > redirection operator opens the file in append mode. The Windows CMD session will attempt to open the file, even if the command does not produce any output. Whenever the output of a command is redirected to a file, the file of course must be opened for write access.

Once I press in the 1st window, I get the 'unlocked' message if I rerun the 1st script. When I run the 2nd script from one window and then run the 1st script from a second window, I get my 'locked' message. I use the following test script from another window to place a lock on the file. ) & (echo file is not locked) || (echo file is locked) Not sure about locked directories (does Windows have that?)īut detecting if a file is being written to by another process is not difficult.

How to lock a folder using cmd