Unix tradition encourages the use of command-line switches to control programs, so that options can be specified from scripts. This is especially important for programs that function as pipes or filters. Three conventions for how to distinguish command-line options from ordinary arguments exist; the original Unix style, the GNU style, and the X toolkit style.
In the original Unix tradition, command-line options are single letters preceded by a single hyphen. Mode-flag options that do not take following arguments can be ganged together; thus, if -a and -b are mode options, -ab or -ba is also correct and enables both. The argument to an option, if any, follows it (optionally separated by whitespace). In this style, lowercase options are preferred to uppercase. When you use uppercase options, it's good form for them to be special variants of the lowercase option.
The original Unix style evolved on slow ASR-33 teletypes that made terseness a virtue; thus the single-letter options. Holding down the shift key required actual effort; thus the preference for lower case, and the use of “-” (rather than the perhaps more logical “+”) to enable options.
The GNU style uses option keywords (rather than keyword letters) preceded by two hyphens. It evolved years later when some of the rather elaborate GNU utilities began to run out of single-letter option keys (this constituted a patch for the symptom, not a cure for the underlying disease). It remains popular because GNU options are easier to read than the alphabet soup of older styles. GNU-style options cannot be ganged together without separating whitespace. An option argument (if any) can be separated by either whitespace or a single “=” (equal sign) character.
The GNU double-hyphen option leader was chosen so that traditional single-letter options and GNU-style keyword options could be unambiguously mixed on the same command line. Thus, if your initial design has few and simple options, you can use the Unix style without worrying about causing an incompatible ‘flag day’ if you need to switch to GNU style later on. On the other hand, if you are using the GNU style, it is good practice to support single-letter equivalents for at least the most common options.
The X toolkit style, confusingly, uses a single hyphen and keyword options. It is interpreted by X toolkits that filter out and process certain options (such as -geometry and -display) before handing the filtered command line to the application logic for interpretation. The X toolkit style is not properly compatible with either the classic Unix or GNU styles, and should not be used in new programs unless the value of being compatible with older X conventions seems very high.
Many tools accept a bare hyphen, not associated with any option letter, as a pseudo-filename directing the application to read from standard input. It is also conventional to recognize a double hyphen as a signal to stop option interpretation and treat all following arguments literally.
Most Unix programming languages offer libraries that will parse a command line for you in either classic-Unix or GNU style (interpreting the double-hyphen convention as well).
Over time, frequently-used options in well-known Unix programs have established a loose sort of semantic standard for what various flags might be expected to mean. The following is a list of options and meanings that should prove usefully unsurprising to an experienced Unix user:
All (without argument). If there is a GNU-style --all option, for -a to be anything but a synonym for it would be quite surprising. Examples: fuser(1), fetchmail(1).
Append, as in tar(1). This is often paired with -d for delete.
Buffer or block size (with argument). Set a critical buffer size, or (in a program having to do with archiving or managing storage media) set a block size. Examples: du(1), df(1), tar(1).
Batch. If the program is naturally interactive, -b may be used to suppress prompts or set other options appropriate to accepting input from a file rather than a human operator. Example: flex(1).
Command (with argument). If the program is an interpreter that normally takes commands from standard input, it is expected that the option of a -c argument will be passed to it as a single line of input. This convention is particularly strong for shells and shell-like interpreters. Examples: sh(1), ash(1), bsh(1), ksh(1), python(1). Compare -e below.
Check (without argument). Check the correctness of the file argument(s) to the command, but don't actually perform normal processing. Frequently used as a syntax-check option by programs that do interpretation of command files. Examples: getty(1), perl(1).
Debug (with or without argument). Set the level of debugging messages. This one is very common.
Occasionally -d has the sense of ‘delete’ or ‘directory’.
Define (with argument). Set the value of some symbol in an interpreter, compiler, or (especially) macro-processor-like application. The model is the use of -D by the C compiler's macro preprocessor. This is a strong association for most Unix programmers; don't try to fight it.
Execute (with argument). Programs that are wrappers, or that can be used as wrappers, often allow -e to set the program they hand off control to. Examples: xterm(1), perl(1).
Edit. A program that can open a resource in either a read-only or editable mode may allow -e to specify opening in the editable mode. Examples: crontab(1), and the get(1) utility of the SCCS version-control system.
Occasionally -e has the sense of ‘exclude’ or ‘expression’.
File (with argument). Very often used with an argument to specify an input (or, less frequently, output) file for programs that need to randomly access their input or output (so that redirection via < or > won't suffice). The classic example is tar(1); others abound. It is also used to indicate that arguments normally taken from the command line should be taken from a file instead; see awk(1) and egrep(1) for classic examples. Compare -o below; often, -f is the input-side analog of -o.
Force (typically without argument). Force some operation (such as a file lock or unlock) that is normally performed conditionally. This is less common.
Daemons often use -f in a way that combines these two meanings, to force processing of a configuration file from a nondefault location. Examples: ssh(1), httpd(1), and many other daemons.
Headers (typically without argument). Enable, suppress, or modify headers on a tabular report generated by the program. Examples: pr(1), ps(1).
Help. This is actually less common than one might expect offhand — for much of Unix's early history developers tended to think of on-line help as memory-footprint overhead they couldn't afford. Instead they wrote manual pages (this shaped the man-page style in ways we'll discuss in Chapter 18).
Initialize (usually without argument). Set some critical resource or database associated with the program to an initial or empty state. Example: ci(1) in RCS.
Interactive (usually without argument). Force a program that does not normally query for confirmation to do so. There are classical examples (rm(1), mv(1)) but this use is not common.
Include (with argument). Add a file or directory name to those searched for resources by the application. All Unix compilers with any equivalent of source-file inclusion in their languages use -I in this sense. It would be extremely surprising to see this option letter used in any other way.
Keep (without argument). Suppress the normal deletion of some file, message, or resource. Examples: passwd(1), bzip(1), and fetchmail(1).
Occasionally -k has the sense of ‘kill’.
List (without argument). If the program is an archiver or interpreter/player for some kind of directory or archive format, it would be quite surprising for -l to do anything but request an item listing. Examples: arc(1), binhex(1), unzip(1). (However, tar(1) and cpio(1) are exceptions.)
In programs that are already report generators, -l almost invariably means “long” and triggers some kind of long-format display revealing more detail than the default mode. Examples: ls(1), ps(1).
Load (with argument). If the program is a linker or a language interpreter, -l invariably loads a library, in some appropriate sense. Examples: gcc(1), f77(1), emacs(1).
Login. In programs such as rlogin(1) and ssh(1) that need to specify a network identity, -l is how you do it.
Occasionally -l has the sense of ‘length’ or ‘lock’.
Message (with argument). Used with an argument, -m passes it in as a message string for some logging or announcement purpose. Examples: ci(1), cvs(1).
Occasionally -m has the sense of ‘mail’, ‘mode’, or ‘modification-time’.
Number (with argument). Used, for example, for page number ranges in programs such as head(1), tail(1), nroff(1), and troff(1). Some networking tools that normally display DNS names accept -n as an option that causes them to display the raw IP addresses instead; ifconfig(1) and tcpdump(1) are the archetypal examples.
Not (without argument). Used to suppress normal actions in programs such as make(1).
Output (with argument). When a program needs to specify an output file or device by name on the command line, the -o option does it. Examples: as(1), cc(1), sort(1). On anything with a compiler-like interface, it would be extremely surprising to see this option used in any other way. Programs that support -o often (like gcc) have logic that allows it to be recognized after ordinary arguments as well as before.
Port (with argument). Especially used for options that specify TCP/IP port numbers. Examples: cvs(1), the PostgreSQL tools, the smbclient(1), snmpd(1), ssh(1).
Protocol (with argument). Examples: fetchmail(1), snmpnetstat(1).
Quiet (usually without argument). Suppress normal result or diagnostic output. This is very common. Examples: ci(1), co(1), make(1). See also the ‘silent’ sense of -s.
Recurse (without argument). If the program operates on a directory, then this option might tell it to recurse on all subdirectories. Any other use in a utility that operated on directories would be quite surprising. The classic example is, of course, cp(1).
Reverse (without argument). Examples: ls(1), sort(1). A filter might use this to reverse its normal translation action (compare -d).
Silent (without argument). Suppress normal diagnostic or result output (similar to -q; when both are supported, q means ‘quiet’ but -s means ‘utterly silent’). Examples: csplit(1), ex(1), fetchmail(1).
Subject (with argument). Always used with this meaning on commands that send or manipulate mail or news messages. It is extremely important to support this, as programs that send mail expect it. Examples: mail(1), elm(1), mutt(1).
Occasionally -s has the sense of ‘size’.
Tag (with argument). Name a location or give a string for a program to use as a retrieval key. Especially used with text editors and viewers. Examples: cvs(1), ex(1), less(1), vi(1).
User (with argument). Specify a user, by name or numeric UID. Examples: crontab(1), emacs(1), fetchmail(1), fuser(1), ps(1).
Verbose (with or without argument). Used to enable transaction-monitoring, more voluminous listings, or debugging output. Examples: cat(1), cp(1), flex(1), tar(1), many others.
Version (without argument). Display program's version on standard output and exit. Examples: cvs(1), chattr(1), patch(1), uucp(1). More usually this action is invoked by -V.
Version (without argument). Display program's version on standard output and exit (often also prints compiled-in configuration details as well). Examples: gcc(1), flex(1), hostname(1), many others. It would be quite surprising for this switch to be used in any other way.
Width (with argument). Especially used for specifying widths in output formats. Examples: faces(1), grops(1), od(1), pr(1), shar(1).
Warning (without argument). Enable warning diagnostics, or suppress them. Examples: fetchmail(1), flex(1), nsgmls(1).
Enable debugging (with or without argument). Like -d. Examples: sh(1), uucp(1).
Extract (with argument). List files to be extracted from an archive or working set. Examples: tar(1), zip(1).
Yes (without argument). Authorize potentially destructive actions for which the program would normally require confirmation. Examples: fsck(1), rz(1).
Enable compression (without argument). Archiving and backup programs often use this. Examples: bzip(1), GNU tar(1), zcat(1), zip(1), cvs(1).
The preceding examples are taken from the Linux toolset, but should be good on most modern Unixes.
When you're choosing command-line option letters for your program, look at the manual pages for similar tools. Try to use the same option letters they use for the analogous functions of your program. Note that some particular application areas that have particularly strong conventions about command-line switches which you violate at your peril — compilers, mailers, text filters, network utilities and X software are all notable for this. Anybody who wrote a mail agent that used -s as anything but a Subject switch, for example, would have scorn rightly heaped upon the choice.
The GNU project recommends conventional meanings for a few double-dash options in the GNU coding standards.[104] It also lists long options which, though not standardized, are used in many GNU programs. If you are using GNU-style options, and some option you need has a function similar to one of those listed, by all means obey the Rule of Least Surprise and reuse the name.
To have command-line options, you have to have a command line. The MS-DOS family does, of course, though in Windows it's hidden by a GUI and its use is discouraged; the fact that the option character is normally ‘/’ rather than ‘-’ is merely a detail. MacOS classic and other pure GUI environments have no close equivalent of command-line options.