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After the program name as, the command line may contain options and file names. Options may appear in any order, and may be before, after, or between file names. The order of file names is significant.
-- (two hyphens) by itself names the standard input file explicitly, as one of the files for as to assemble.
Except for ‘--’ any command line argument that begins with a hyphen (‘-’) is an option. Each option changes the behavior of as. No option changes the way another option works. An option is a ‘-’ followed by one or more letters; the case of the letter is important. All options are optional.
Some options expect exactly one file name to follow them. The file name may either immediately follow the option's letter (compatible with older assemblers) or it may be the next command argument (gnu standard). These two command lines are equivalent:
as -o my-object-file.o mumble.s
as -omy-object-file.o mumble.s