command line - Why do some Linux programs specify long form and shorthand arguments? -

whenever through documentation , man-pages, notice linux programs have 2 types of arguments, , genuine interest have in finding out...

• one seems full argument name (long form).
• and 1 seems shorthand.

take grep example. if run grep --help disclaimer: not question grep in particular. i've seen in many other tools.

regexp selection , interpretation:   -e, --extended-regexp     pattern extended regular expression   -f, --fixed-strings       pattern set of newline-separated strings   -g, --basic-regexp        pattern basic regular expression   -p, --perl-regexp         pattern perl regular expression   -e, --regexp=pattern      use pattern regular expression   -f, --file=file           obtain pattern file   -i, --ignore-case         ignore case distinctions   -w, --word-regexp         force pattern match whole words   -x, --line-regexp         force pattern match whole lines   -z, --null-data           data line ends in 0 byte, not newline 

one can see there 2 types of arguments. shorthand, , full argument name.

another thing note, if specify shorthand of these arguments, separated space, when using full argument, seems separated = sign. e.g: grep --file=file vs grep -f file

1. what purpose of having 2 different types of arguments same thing?
2. are able use grep --file file instead of grep --file=file?
3. to question 2, if answer "because how programmed it," question why program way? there reason can't use grep --file file?
4. is there convention should follow?
5. does leading - in grep *-*-file play important part in something?

i'd appreciate solid answer on can understand why programmers way!

thanks!

in addition other answers, long , short options handled argp & getopt_long (from glibc), , several other options handling functions (e.g. in qt or gtk).

and gnu coding standards recommend them, , --help , --version.

some (rarely useful) command options have long form (e.g. --time-style gnu ls).

^{btw, if write command line software linux either free software or distributed (perhaps sold), please accept --help option: becoming common convention. hate few programs not following it!}