how to convert NSNumber into NSString rwx type

i'm new in unix permission and chmod.

In my IOS app, i try to get a file permission开发者_运维百科 value from SFTP server. The value i've got is in a NSNumber value and the value is 19455. I want to convert it into NSString in rwx (read, write, execute) value, how can i do that?

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objective-c NSFilePosixPermissions to human readable NSString