The first line of a script is a magic line (often called the shebang line) that tells the kernel what interpreter to use to process the rest of the file. The kernel will also look for a single option to the specified interpreter. There are some attacks that take advantage of this fact, but if you pass an argument along, they are avoided. See http://www.faqs.org/faqs/unix-faq/faq/part4/section-7.html for details.
However, hard-coding the path to bash may present a portability issue. See Finding bash Portably for #! for details.
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