17.11 Using -exec to Create Custom Tests

Here's something that will really make your head spin. Remember that -exec doesn't necessarily evaluate to "true"; it only evaluates to true if the command it executes returns a zero exit status (44.7). You can use this to construct custom find tests.

Assume that you want to list files that are "beautiful." You have written a program called beauty that returns zero if a file is beautiful, and non-zero otherwise. (This program can be a shell script (44.11), a perl (37.1) script, an executable from a C program, or anything you like.)

Here's an example:

% find . -exec beauty {} \; -print

In this command, -exec is just another find operator. The only difference is that we care about its value; we're not assuming that it will always be "true." find executes the beauty command for every file. Then -exec evaluates to true when find is looking at a "beautiful" program, causing find to print the filename. (Excuse me, causing find to evaluate the -print. :-))

Of course, this ability is capable of infinite variation. If you're interested in finding beautiful C code, you could use the command:

% find . -name "*.[ch]" -exec beauty {} \; -print

And so on. For performance reasons, it's a good idea to put the -exec operator as close to the end as possible. This avoids starting processes unnecessarily; the -exec command will execute only when the previous operators evaluate to true.