Here are some of the POSIX shell scripts that can flatten directories by copying all files to a new directory:

```bash
LC_ALL=C find /dir1 -path '/dir1/*/*' -type f -exec sh -c '
  exec mv "$@" /dir1' sh {} +
```

```bash
find dir1 -type f -exec mv {} dir1 \;
find dir1 -depth -exec rmdir {} \;
```

```bash
find dir1 -type f -exec sh -c 'mv "$@" dir1' _ {} +
find dir1 -depth -exec rmdir {} +
```

And here are the Bash shell equivalents:

```bash
find /foo -iname '*.txt' -exec cp \{\} /dest/ \;
```

Note that with the above script, it forks off a new "cp" process for every file, which is not terribly efficient especially if there is a large number of files.

For performance, I have another solution:

```bash
find . -name "*.xml" -print0 | xargs -0 echo cp -t a
```
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