How to detect whether “find” found any matches

[ and test are synonyms (except [ requires ]), so you don't want to use [ test:

[ -x /bin/cat ] && echo 'cat is executable'
test -x /bin/cat && echo 'cat is executable'

test returns a zero exit status if the condition is true, otherwise nonzero. This can actually be replaced by any program to check its exit status, where 0 indicates success and non-zero indicates failure:

# echoes "command succeeded" because echo rarely fails
if /bin/echo hi; then echo 'command succeeded'; else echo 'command failed'; fi

# echoes "command failed" because rmdir requires an argument
if /bin/rmdir; then echo 'command succeeded'; else echo 'command failed'; fi

However, all of the above examples only test against the program's exit status, and ignore the program's output.

For find, you will need to test if any output was generated. -n tests for a non-empty string:

if [[ -n $(find /var/log/crashes -name "app-*.log" -mmin -5) ]]
then
    service myapp restart
fi

A full list of test arguments is available by invoking help test at the bash commandline.

_{If you are using bash (and not sh), you can use [[ condition ]], which behaves more predictably when there are spaces or other special cases in your condition. Otherwise it is generally the same as using [ condition ]. I've used [[ condition ]] in this example, as I do whenever possible.}

_{I also changed `command` to $(command), which also generally behaves similarly, but is nicer with nested commands.}

You can do:

find . -exec ./is_dir.py {} \; -o -print

It will list everything that is not a directory. Assuming that is_dir.py is executable (chmod +x is_dir.py) and contains something like:

#!/usr/bin/env python

import sys
import os

if os.path.isdir(sys.argv[1]):
    sys.exit(0)
sys.exit(1)

And if is_dir.py generates output of its own, you can do:

 rm -f nondir.lst
 find . -exec ./is_dir.py {} \; -o -exec echo  {} >> nondir.lst \;

so that its outoput doesn't get mixed with the list of names for which is_dir.py fails.