I want to watch a folder on my Mac and then execute a bash script, passing it the name of whatever file/folder was just moved into or created in the watched directory.

fswatch

fswatch is a small program using the Mac OS X FSEvents API to monitor a directory. When an event about any change to that directory is received, the specified shell command is executed by /bin/bash

If you're on GNU/Linux, inotifywatch (part of the inotify-tools package on most distributions) provides similar functionality.

Update: fswatch can now be used across many platforms including BSD, Debian, and Windows.

Syntax / A Simple Example

The new way that can watch multiple paths - for versions 1.x and higher:

fswatch -o ~/path/to/watch | xargs -n1 -I{} ~/script/to/run/when/files/change.sh

Note: The number output by -o will get added to the end of the xargs command if not for the -I{}. If you do choose to use that number, place {} anywhere in your command.

The older way for versions 0.x:

fswatch ~/path/to/watch ~/script/to/run/when/files/change.sh

Installation with Homebrew

As of 9/12/13 it was added back in to homebrew - yay! So, update your formula list (brew update) and then all you need to do is:

brew install fswatch

Installation without Homebrew

Type these commands in Terminal.app

cd /tmp
git clone https://github.com/alandipert/fswatch
cd fswatch/
make
cp fswatch /usr/local/bin/fswatch

If you don't have a c compiler on your system you may need to install Xcode or Xcode command line tools - both free. However, if that is the case, you should probably just check out homebrew.

Additional Options for fswatch version 1.x

Usage:
fswatch [OPTION] ... path ...

Options:
 -0, --print0          Use the ASCII NUL character (0) as line separator.
 -1, --one-event       Exit fsw after the first set of events is received.
 -e, --exclude=REGEX   Exclude paths matching REGEX.
 -E, --extended        Use exended regular expressions.
 -f, --format-time     Print the event time using the specified format.
 -h, --help            Show this message.
 -i, --insensitive     Use case insensitive regular expressions.
 -k, --kqueue          Use the kqueue monitor.
 -l, --latency=DOUBLE  Set the latency.
 -L, --follow-links    Follow symbolic links.
 -n, --numeric         Print a numeric event mask.
 -o, --one-per-batch   Print a single message with the number of change events.
                       in the current batch.
 -p, --poll            Use the poll monitor.
 -r, --recursive       Recurse subdirectories.
 -t, --timestamp       Print the event timestamp.
 -u, --utc-time        Print the event time as UTC time.
 -v, --verbose         Print verbose output.
 -x, --event-flags     Print the event flags.

See the man page for more information.