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 exists 
string
      
exists is NOT undoable, NOT queryable, and NOT editable.
This command is designed to allow the user to query the existence of a command or Mel procedure. It takes one argument of type string. An integer value of 1 is returned if the argument represents the name of a Maya command, a built-in Mel command, or a user written Mel procedure or script. This command returns 0 if none of these can be found.| int | 0 or 1 | 
if (`exists sphere`) {
    sphere;  // make a sphere
}
string $myCommand="cone";
if (`exists $myCommand`) {
    eval $myCommand;
}