Delete all files except in a certain subdirectory with find

one way is to use -exec rm instead of -delete.

find a \( -name b -prune \) -o -type f -exec rm {} +

alternatively use -not -path instead of -prune:

find a -not -path "*/b*" -type f -delete

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Explanation why -prune collides with -delete:

find complains when you try to use -delete with -prune because -delete implies -depth and -depth makes -prune ineffective.

observe the behaviour of find with and without -depth:

$ find foo/
foo/
foo/f1
foo/bar
foo/bar/b2
foo/bar/b1
foo/f2

There is no guarantee about the order in a single directory. But there is a guarantee that a directory is processed before its contents. Note foo/ before any foo/* and foo/bar before any foo/bar/*.

This can be reversed with -depth.

$ find foo/ -depth
foo/f2
foo/bar/b2
foo/bar/b1
foo/bar
foo/f1
foo/

Note that now all foo/* appear before foo/. Same with foo/bar.

more explanation:

• -prune prevents find from descending into a directory. In other words -prune skips the contents of the directory. In your case -name b -prune means that when find reaches a directory with the name b it will skip the directory including all subdirectories.
• -depth makes find to process the contents of a directory before the directory itself. That means by the time find gets to process the directory entry b its contents has already been processed. Thus -prune is ineffective with -depth in effect.
• -delete implies -depth so it can delete the contents first and then the empty directory. -delete refuses to delete non-empty directories.

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Explanation of alternative method:

find a -not -path "*/b*" -type f -delete

This may or may not be easier to remember.

This command still descends into the directory b and proceses every single file in it only for -not to reject them. This can be a performance issue if the directory b is huge.

-path works differently than -name. -name only matches against the name (of the file or directory) while -path is matching against the entire path. For example observe the path /home/lesmana/foo/bar. -name bar will match because the name is bar. -path "*/foo*" will match because the string /foo is in the path. -path has some intricacies you should understand before using it. Read the man page of find for more details.

Beware that this is not 100% foolproof. There are chances of "false positives". The way the command is written above it will skip any file which has any parent directory which name is starting with b (positive). But it will also skip any file which name is starting with b regardless of position in the tree (false positive). This can be fixed by writing a better expression than "*/b*". That is left as an exercise for the reader.

I assume that you used a and b as placeholders and the real names are more like allosaurus and brachiosaurus. If you put brachiosaurus in place of b then the amount of false positives will be drastically reduced.

At least the false positives will be not deleted, so it will be not as tragic. Furthermore, you can check for false positives by first running the command without -delete (but remember to place the implied -depth) and examine the output.

find a -not -path "*/b*" -type f -depth

Just use rm instead of -delete:

find a -name b -prune -o -type f -exec rm -f {} +