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Bash Shell Find Out If A Command Exists On UNIX / Linux System ($PATH) OR Not

I want find out if a command exists on UNIX from a bash shell script. If command does not exists in $PATH, then I would like to display an error message on screen. How do I find out if a command exists on Posix system or not?
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Sysadmin because even developers need heroes!!!

How do I determine if a function called foo() is already defined in a bash script or not? If a function called foo() is defined just call it or show an error message?
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