Passing options to a program: what's the convention for one dash vs two?

Question

Some programs will take options like this:

$ someprogram -orange apple

And other programs will use something like this:

$ otherprogram --orange apple

Is there a "rule" or convention for this in Linux/Unix/OSX?

Answer 1

The most common is GNU getopt style, with one dash for short options and two dashes for long options.

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• Initially, Unix programs took single-letter options preceded by a single dash and optionally bundled:

  ls -laF
  
  ls -l -a -F
  

  The two commands above are equal.

  When an option takes a value, it overrides bundling: In gpg -aofoo.gpg, -a and -o are options and foo.gpg is the value given to -o.

• Most of them did, anyway.

  tar cvzf is a common sight (although current versions accept tar -cvzf too).

  BSD ps always uses -; SysV ps never does. The Linux version accepts both versions and changes its behavior depending on whether an option was prefixed with a dash.

• X11 programs used long options preceded by a single dash or sometimes a plus sign:

  xterm -class FooTerm +vb -u8
  

  This sets options class and u8, and unsets the vb option.

  X11 style is incompatible with option bundling.

• Later, long options were added to GNU getopt() in a way that's compatible with one-letter options.

  gpg -se --no-armor --output=signed.gpg
  

  This sets -s, -e, and --no-armor (which is opposite to --armor).

  Usually, --output=signed.gpg and --output signed.gpg are equivalent.

  (If I recall correctly, long options used + as the prefix before it was changed to --.)

• The POSIX specification has a section Utility Argument Syntax, which describes the one-character options.

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Most Windows programs use their own parsers, driving users mad.

• Some require /a /b /c, others allow VMS-style /a/b/c, yet others prefer Unix-style /abc.
• Most use / as prefix, some also accept -, others accept only -.
• Values can be given as /foo bar, /foo=bar, /foo:bar.
• Usually spaces can be /quoted "like this", but some programs take the " as a literal character. (This is a downside of letting the program do its own word-splitting; in Unix this is handled by the shell.)
• Cross-platform programs may use an implementation of getopt.

Answer 2

Generally the first is found in older programs which are too well entrenched to change. Those kind of long options are incompatible with the standard getopt() function.

The second style was introduced by GNU getopt_long(), and is compliant with existing standards which expect the first style to be bundled short options (that is, -orange is expected to mean -o -r -a -n -g -e). This style of long option is strongly preferred.