Take the 2-minute tour ×
Server Fault is a question and answer site for professional system and network administrators. It's 100% free, no registration required.

I'm new to working in the shell and the usage of these commands seems arbitrary. Is there a reason one flag has a single dash and another might have a double dash?

share|improve this question
    
It's supposed to be governed by the POSIX standard: iam.ubc.ca/guides/javatut99/essential/attributes/_posix.html –  cjc May 10 '12 at 14:44

3 Answers 3

up vote 39 down vote accepted

A single hyphen can be followed by multiple single-character flags. A double hyphen prefixes a single, multicharacter option.

Consider this example:

tar -czf

In this example, -czf specifies three single-character flags: c, z, and f.

Now consider another example:

tar --exclude

In this case, --exclude specifies a single, multicharacter option named exclude. The double hyphen disambiguates the command-line argument, ensuring that the shell interprets it as exclude rather than a combination of e, x, c, l, u, d, and e.

share|improve this answer
    
Ohh... totally makes sense. Would tar --c also be recognized? –  kylex May 10 '12 at 14:35
    
@kylex, no, since there is no long option named just "c" and the -- means a long option, not a single character option follows. –  psusi May 10 '12 at 14:37
5  
Sometimes even long commands can be single-dashed. For example 'cdrecord' uses all single-dashed commands (-eject -dao ...). It all depends on the program, but most(!) of them use - for single and -- for multiple-character (long) commands –  mulaz May 10 '12 at 14:42
3  
@mulaz, yes, cdrecord does quite a few goofy things. –  psusi May 10 '12 at 14:46
5  
also bear in mind -- used on its own usually signifys the end of options. see here for more info: unix.stackexchange.com/questions/11376/… –  Sirex May 10 '12 at 19:51

It all depends on the program. Usually "-" is used for 'short' options (one-letter, -h), and "--" is used for "long"(er) options (--help).

Short options can usually be combined (so "-h -a" is same as "-ha")

In Unix-like systems, the ASCII hyphen–minus is commonly used to specify options. The character is usually followed by one or more letters. An argument that is a single hyphen–minus by itself without any letters usually specifies that a program should handle data coming from the standard input or send data to the standard output. Two hyphen–minus characters ( -- ) are used on some programs to specify "long options" where more descriptive option names are used. This is a common feature of GNU software.

source

share|improve this answer

It's really a convention. However, it can aid parsers to know more efficiently about options passed to the program. Besides, there are neat utilities that can help parsing these commands, such as getopt(3) or the non-standard getopt_long(3) to help parse the arguments of a program.

It is nice, for we can have multiple short options combined, as other answers say, like tar -xzf myfile.tar.gz.

If there was a "lisa" argument for ls, there would probably have a different meaning to type ls -lisa than ls --lisa. The former are the l, i, s, and a parameters, not the word.

In fact, you could write ls -l -i -s -a, meaning exactly the same as ls -lisa, but that would depend on the program.

There are also programs that don't obey this convention. Most notably for my sight, dd and gcc.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.