How to use ‘-prune’ option of ‘find’ in sh?

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The thing I’d found confusing about -prune is that it’s an action (like -print), not a test (like -name). It alters the “to-do” list, but always returns true.

The general pattern for using -prune is this:

find [path] [conditions to prune] -prune -o \
            [your usual conditions] [actions to perform]

You pretty much always want the -o (logical OR) immediately after -prune, because that first part of the test (up to and including -prune) will return false for the stuff you actually want (ie: the stuff you don’t want to prune out).

Here’s an example:

find . -name .snapshot -prune -o -name '*.foo' -print

This will find the “*.foo” files that aren’t under “.snapshot” directories. In this example, -name .snapshot makes up the [conditions to prune], and -name '*.foo' -print is [your usual conditions] and [actions to perform].

Important notes:

  1. If all you want to do is print the results you might be used to leaving out the -print action. You generally don’t want to do that when using -prune.

    The default behavior of find is to “and” the entire expression with the -print action if there are no actions other than -prune (ironically) at the end. That means that writing this:

     find . -name .snapshot -prune -o -name '*.foo'              # DON'T DO THIS

    is equivalent to writing this:

     find . \( -name .snapshot -prune -o -name '*.foo' \) -print # DON'T DO THIS

    which means that it’ll also print out the name of the directory you’re pruning, which usually isn’t what you want. Instead it’s better to explicitly specify the -print action if that’s what you want:

     find . -name .snapshot -prune -o -name '*.foo' -print       # DO THIS

  2. If your “usual condition” happens to match files that also match your prune condition, those files will not be included in the output. The way to fix this is to add a -type d predicate to your prune condition.

    For example, suppose we wanted to prune out any directory that started with .git (this is admittedly somewhat contrived — normally you only need to remove the thing named exactly .git), but other than that wanted to see all files, including files like .gitignore. You might try this:

    find . -name '.git*' -prune -o -type f -print               # DON'T DO THIS

    This would not include .gitignore in the output. Here’s the fixed version:

    find . -name '.git*' -type d -prune -o -type f -print       # DO THIS

Extra tip: if you’re using the GNU version of find, the texinfo page for find has a more detailed explanation than its manpage (as is true for most GNU utilities).

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