Command Line – What is a Non-Option Argument?

argumentscommand lineoptions

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.

Best Answer

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 or foo --option bar. If the option -o (or --option) takes an argument, then bar is the argument of the option -o (or --option). If -o (or --option) does not take an argument then bar 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.

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