What is a “non-option argument”?

Question

I am trying to understand info who but completly fail at the term »non-option argument«. Can someone please explain this term to me in simple words or an example?

UPDATE: from ' info who' :

If given no non-option arguments, `who' prints the following information for each user currently logged on: login name, terminal line, login time, and remote hostname or X display.

If given one non-option argument, who' uses that instead of a default system-maintained file (often/var/run/utmp' or /etc/utmp') as the name of the file containing the record of users logged on. /var/log/wtmp' is commonly given as an argument to `who' to look at who has previously logged on.

If given two non-option arguments, who' prints only the entry for the user running it (determined from its standard input), preceded by the hostname. Traditionally, the two arguments given aream i', as in `who am i'.

I [thought to] know the difference between an argument and an option, but this [again] nixes a lot.

Answer 1

The terminology is not completely fixed, so different documentation uses different terms, or worse, the same terms with different meanings. The terminology in the man page you're reading is a common one. It is the one used in the POSIX standard. In a nutshell, each word after the command is an argument, and the arguments that start with - are options.

Argument

In the shell command language, a parameter passed to a utility as the equivalent of a single string in the argv array created by one of the exec functions. An argument is one of the options, option-arguments, or operands following the command name.

Operand

An argument to a command that is generally used as an object supplying information to a utility necessary to complete its processing. Operands generally follow the options in a command line.

Option

An argument to a command that is generally used to specify changes in the utility's default behavior.

“Utility” is what is generally called “command” (the standard uses the word utility to avoid ambiguity with the meaning of “command” that includes the arguments or even compound shell commands).

Most commands follow the standard utility argument syntax, where options start with a - (dash a.k.a. minus). So an option is something like -a (short option, follows the POSIX guidelines) or --all (long option, an extension from GNU). A non-option argument is an argument that doesn't begin with -, or that consists solely of - (which who treats as a literal file name but many commands treat as meaning either standard input or standard output).

In addition, some options themselves have an argument. This argument can be passed in several ways:

• For a single-letter option, in the same argument to the utility: foo -obar: bar is the argument to the single-letter option -o.
• In the GNU long argument syntax, in the same argument, separated by an equal sign: foo --option=bar.
• In a separate argument: foo -o bar. If the option -o takes an argument, then bar is the argument of the option -o. If -o does not take an argument then -o is an operand.

Here's a longer example:

tail -n 3 myfile

-n is an option, 3 is an argument to the option -n, and myfile is an operand.

Terminology differs, so you may find documents that use argument in the sense where POSIX uses operand. But “non-option argument” is more common than either term for this meaning.

Answer 2

The thing is, options (or switches or flags, whatever you like to call them) count as arguments too. In fact, anything you supply after the command name itself makes up the command's arguments (except for constructs used by the shell like redirection, for instance).

Your program/script receives everything together as arguments and needs to separate the option arguments (arguments that are options) from other... you guessed it!... non-option arguments.

So what the info page is saying is that if who receives an argument that is not an option, it will consider that as an alternative to the default file it consults for log in information.

Answer 3

I still think the phrasing is poor on this and would try not to get hung up. What they're trying to say is that in the prototype statement they included like this:

 `who' [OPTION] [FILE] [am i]

So when they say "If given no non-option arguments" they're referring to [FILE] and [am i].

Following that logic when they say "If given one non-option argument", i.e. [FILE], then you're overriding the [FILE] location, /var/run/utmp.

Lastly for this comment, "If given two non-option arguments", i.e. [am i] then you're asking for info about the user that just ran the command:

$ who am i
saml     pts/5        2013-10-18 16:29 (:0.0)

-or-

$ who mom likes
saml     pts/5        2013-10-18 16:29 (:0.0)

The description is apt, but they've definitely confused it by using the term "no non-option arguments".

In the authors mind these are the options:

  -a, --all         same as -b -d --login -p -r -t -T -u
  -b, --boot        time of last system boot
  -d, --dead        print dead processes
  -H, --heading     print line of column headings
  -l, --login       print system login processes
      --lookup      attempt to canonicalize hostnames via DNS

  ....

And [FILE] and [am i] and [mom likes] are the non-options. But these are ALL options, so the difference is limited to just this tool!