Answer: After the command line is terminated by the <enter> key, the shel goes ahead with processing the command line in one or more passes. The sequence is well defined and assumes the following order.Parsing: The shell first breaks up the command line into words, using spaces and the delimiters, unless quoted. All consecutive occurrences of a space or tab are replaced here with a single space.Variable evaluation: All words preceded by a $ are avaluated as variables, unless quoted or escaped.Command substitution: Any command surrounded by backquotes is executed by the shell which then replaces the standard output of the command into the command line.Wild-card interpretation: The shell finally scans the command line for wild-cards (the characters *, ?, [, ]). Any word containing a wild-card is replaced by a sorted list of filenames that match the pattern. The list of these filenames then forms the arguments to the command.PATH evaluation: It finally looks for the PATH variable to determine the sequence of directories it has to search in order to hunt for the command.
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