Unix tricks - Hackdot

Re:Unix tricks by jcr on Thursday March 07, 2013 @07:07PM (#5341285)

Sorry, I didn't see your 'EDIT' caveat when I responded --now that will teach me to reply too soon. ;-)

Also, it seems I failed to be clear; I'm probably too tired I suppose.

My point was there is plenty of ancient and buggy code out there. It could be "most", or even "many" modern unix variants have fixed a lot of the old bugs in find(1), but if you don't have the luxury of working on a current system, and your not allowed to upgrade it, then plenty bad things can happen due to invoking a shell, handling space, quote, and delimiter characters, and so forth.

reference:

  $ uname -a
  OpenBSD alien.foo.test 5.1 GENERIC.MP#207 amd64

setup:

  $ mkdir test
  $ cd test
  $ touch file1
  $ touch file2
  $ touch file3
  $ mkdir ';ls'

bad:

  $ find . -type d -exec sh -c {} \; 
  sh: ./: cannot execute - Is a directory
  ;ls     file1   file2   file3   test.sh

better:

  $ find . -type d -exec sh -ec {} \;
  sh: ./: cannot execute - Is a directory

also bad:

  $ find . -type d -print0 | xargs -0 -r -n 1 -J % sh -c "%"
  sh: ./: cannot execute - Is a directory
  ;ls     file1   file2   file3   test.sh

better:

  $ find . -type d -print0 | xargs -0 -r -n 1 -x -J % sh -ec "%"
  sh: ./: cannot execute - Is a directory

better;

  $ find . -type d -print0 | xargs -0 -r -J % sh -c "%"

best:

  $ find . -type d -print0 | xargs -0 -r -J % sh -ec "%"

POSIX is all great and wonderful in theory, but in practice it's no different than the bogus Java "write once, run anywhere" claim. If a system or utility claims to be POSIX compliant, then you're probably close, but you'll still need to do testing and debugging.

At least some of the issues with find/xargs are mentioned in the following wikipedia article. It's probably more clear than I am right now.

http://en.wikipedia.org/wiki/Xargs

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Re:Unix tricks by neoteric on Thursday March 07, 2013 @09:19PM (#5341756)

The contrived examples you've shown aren't examples of POSIX-incompatibility, or bugs in `find` at all. You've explicitly involved the shell. Of course trying to run every directory name as a shell command string is going to result in executed code!

Your original argument was that given:

    find . -type d -exec chmod g+x {} ';'

It is possible to force code execution of arbitrary commands given a carefully crafted directory name. The key difference in this case is that the shell is not involved _at all_. I challenge you to find an implementation of `find` that is broken in this way.

As a side note, it is even possible to involve the shell in the picture in a safe way with `find`, without the use of `xargs` (and thus avoid the overhead of setting up a pipeline):

    find . -type d -exec sh -c 'chmod g+x "$1"' _ {} ';'

(my contrived example is quite poor, though, since it does nothing but introduce unnecessary shell overhead)

Modern (POSIX > 2001?) `find`'s support `-exec {} +` which further reduces the number of reasons to invoke `xargs`:

    find . -type d -exec sh -c '
        for x; do
            do_foo "$x"
            do_bar "$x"
            do_baz "$x"
        done
    ' _ {} +

(example above shows how to make proper use of this feature with an explicit shell invocation)

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