chmod feature request: setting different modes for files and directories

[Tobia Conforto]

Dear coreutils maintainers,

I'd like to ask for an often needed feature of chmod: the ability to  
set different modes for files and directories. I will briefly explain  
the need, and then propose a possible syntax.

For example, to make a directory setgid, one needs to give the setgid  
permission to all sub-directories already present, but not to the  
files. Currently one has to write something like this:

        find some/path -type d -exec chmod g+s {} +

Or some variation with -print0 and xargs. The same is true when  
someone extracts some files from an archive or copies them over a  
removable media, where permissions need to be reset to something sane,  
like 755/644:

        chmod -R 755 another/path
        find another/path -type f -exec chmod 644 {} +

These could all be written more concisely if chmod supported selection  
of permissions by inode type. Here is the syntax I'm thinking about,  
for the two examples above:

        chmod -R d:g+s some/path

        chmod -R d:755,f:644 another/path

This is backwards-compatible, intuitive, and concise. The character  
before the colon could be made to mimic find's -type option.

Let me know if you'd like me to come up with an implementation, or if  
there is some other list where this topic would be more appropriate.

-Tobia