Difference between terms: "option", "argument", and "parameter"?

Question

What are the differences between these terms: "option", "argument", and "parameter"? In man pages these terms often seem to be used interchangeably.

Answer 1

A command is split into an array of strings named arguments. Argument 0 is (normally) the command name, argument 1, the first element following the command, and so on. These arguments are sometimes called positional parameters.

$ ls -la /tmp /var/tmp
arg0 = ls
arg1 = -la
arg2 = /tmp
arg3 = /var/tmp

An option is a documented¹ type of argument modifying the behavior of a command, e.g. -l commonly means "long", -v verbose. -lv are two options combined in a single argument. There are also long options like --verbose (see also Using getopts to process long and short command line options). As their name suggests, options are usually optional. There are however some commands with paradoxical "mandatory options".

$ ls -la /tmp /var/tmp
option1= -l
option2= -a

A parameter is an argument that provides information to either the command or one of its options, e.g. in -o file, file is the parameter of the -o option. Unlike options, whose possible values are hard coded in programs, parameters are usually not, so the user is free to use whatever string suits his/her needs. Should you need to pass a parameter that looks like an option but shouldn't be interpreted as such, you can separate it from the beginning of the command line with a double dash: --².

$ ls -la /tmp /var/tmp
parameter1= /tmp
parameter2= /var/tmp

$ ls -l -- -a
option1    = -l
parameter1 = -a

A shell parameter is anything that store a value in the context of the shell. This includes positional parameters (e.g. $1, $2...), variables (e.g. $foo, $bar...) and special character ones (e.g. $@)

Finally, there are subcommands, also known as functions / (low-level) commands, which are used with "metacommands" that embed multiple separate commands, like busybox, git, apt-get, openssl, and the likes. With them, you might have global options preceeding the subcommand, and subcommand specific options that follow the subcommand. Unlike parameters, the list of possible subcommands is hardcoded in the command itself. e.g.:

$ busybox ls -l
command            = busybox
subcommand         = ls
subcommand option1 = -l

$ git --git-dir=a.git --work-tree=b -C c status -s
command            = git
command option1    = --git-dir=a.git
command option2    = --work-tree=b
command option3    = -C c
subcommand         = status
subcommand option1 = -s

Note that some commands like test, tar, dd and find have more complex argument parsing syntax than the ones described previously and can have some or all of their arguments parsed as expressions, operands, keys and similar command specific components.

Note also that optional variable assignments and redirections, despite being processed by the shell for tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal like other command line parameters are not taken into account in my reply because they have disappeared when the command is actually called and passed its arguments.

¹_{I should have written usually documented because of course, undocumented options are still options.}
²_{The double dash feature need to be implemented by the program though.}

Answer 2

The man page for a typical Unix command often uses the terms argument, option and parameter. At the lowest level, we have argument and everything is an argument, including the (filesystem path to the) command itself.

In a shell script you access arguments using the special variables $0 .. $n. Other languages have similar ways to access them (commonly through an array with a name like argv).

Arguments may be interpreted as options if you wish. How this is done is implementation-specific. You can either roll your own, for exampe a shell (such as bash) script can use provided getopts or getopt commands.

These typically define an option as an argument beginning with a hyphen (-) and some options may use proceeding arguments as its parameters. More capable parsers (e.g getopt) support mixing short-form (-h) and long-form (--help) options.

Typically, most options take zero or one parameter. Such parameters are also sometimes called values.

The supported options are coded in the program code (e.g in the invocation of getopts within a shell script). Any remaining arguments after the options have been consumed are commonly called positional parameters when the order in which they are given is significant (this is in contrast to options which usually can be given in any order).

Again, the script defines what the positional parameters are by how it consumes and uses them.

So a typical command

$ ls -I README -l foo 'bar car' baz

has seven arguments: /usr/bin/ls, -I, README, -l, foo, bar car, and baz accessible as $0 thru $6. The -l and -I are interpreted as options, the latter having a parameter (or value) of README. What remains are positional parameters (foo, bar car and baz).

Option parsing may alter the argument list by removing those it consumes (e.g using shift or set) so that only the positional parameters remain and are thereafter accessible as $1 .. $n.

Answer 3

Since the question is tagged "bash", I looked for relevant sections in the Bash manual. I list these as quoted passages below together with my own one sentence summaries.

Arguments

Everything following the command is an argument.

A simple shell command such as echo a b c consists of the command itself followed by arguments, separated by spaces.

A simple command is the kind of command encountered most often. It’s just a sequence of words separated by blanks, terminated by one of the shell’s control operators (see Definitions). The first word generally specifies a command to be executed, with the rest of the words being that command’s arguments.

Parameters

Arguments are referred to as parameters during function execution.

When a function is executed, the arguments to the function become the positional parameters during its execution

A parameter is an entity that stores values. It can be a name, a number, or one of the special characters listed below. A variable is a parameter denoted by a name.

A positional parameter is a parameter denoted by one or more digits, other than the single digit 0. Positional parameters are assigned from the shell’s arguments when it is invoked, and may be reassigned using the set builtin command. Positional parameter N may be referenced as ${N}, or as $N when N consists of a single digit.

Options

There is no dedicated section to defining what an option is, but they are referred to as hyphen-prefixed characters throughout the manual.

The -p option changes the output format to that specified by POSIX