The source program is a concatenation of the text in all the files, in the order specified.
Each time you run as, it assembles exactly one source program. The source program is made up of one or more files. (The standard input is also a file.)
You give as a command line that has zero or more input file names. The input files are read (from left file name to right). A command line argument (in any position) that has no special meaning is taken to be an input file name.
If you give as no file names it attempts to read one input file from the as standard input, which is normally your terminal. You may have to type Ctrl-D to tell as there is no more program to assemble.
Use ‘--’ if you need to explicitly name the standard input file in your command line.
If the source is empty, as produces a small, empty object file.
Filenames and line-numbers
There are two ways of locating a line in the input file (or files) and either may be used in reporting error messages. One way refers to a line number in a physical file; the other refers to a line number in a logical file. See also Error and warning messages.