Keep in mind that I know absolutely no PHP, nor do I even know if this function exists in that language...
C(++):
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//This would need main()/etc
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system("/usr/bin/bash");
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Python:
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import os
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os.system('/usr/bin/bash')
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Actually...you should probably use exec() or popen() (Google for how to use them, I'm not entirely certain), as system() under *nix passes the parameters passed to it to the shell as commands to be run rather than starting a new process.