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Node.js – fs.open / fs.writeFile, mode, file permissions, umask, and how they interact

Girl Reading Book
While working with Node.js recently, I ran into a situation with the mode parameter used in several functions; fs.open, fs.appendFile, fs.writeFile, etc. When trying to create a new file and specifying a specific mode parameter to allow user and group writes, I found that the file would get created, however would have the wrong permissions.  After much experimenting, I realized that the Linux server’s umask was getting applied, and preventing the creation of a file with the permissions I wanted.

The work around turned out to be fairly simple, although it does create a slight security hole, and should only be used in cases where you can control access to the application.

I simply wrapped the file create calls with the following code :

oldmask = process.umask(newmask);
fs.open(path, flags, [mode], callback)
process.umask(oldmask);

This only needs be performed when the open (or other method) will create a new file.  Let me know if you have experienced a similar issue, and if you have found other solutions to resolve.

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