As @Abdelkrim Hadjidj mentioned, this is not currently possible in NiFi out of the box. You have a few options. Change the file permissions to be accessible by the OS user that is running NiFi. Assuming that user is "nifi", this could be done by adding the "nifi" account to a group with R/W access to the file, or by changing the owner to the "nifi" user (assuming the owner has R/W). This is the recommended solution. You could use an ExecuteStreamCommand or ExecuteProcess processor to run a shell command which reads the contents of the file into a flowfile. Because you are running a shell command, you can use something like "$ echo <other_account_password> | sudo -S -u OTHER_USER more ${path/to/file}". It would be more secure to provide the password in a separate password file (secured via OS permissions) to avoid history leakage, but you can also prepend the command with a space to avoid it showing up in the history. You might be able to use the GetHDFS processor with a configuration file that mapped it to the local file system and allowed user impersonation. This is not recommended and definitely not supported, but potentially technically possible. You could write a custom processor, but Java does not have any API for reading a file as an OS user other than the one that the JVM is running under. You could possibly use the Java FileSystem API to change the OS ownership or permissions on the file, but in that case, #1 is a better solution.
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