From its earliest days, Unix has used a single hyphen to introduce command options or flags, like 'ls -l', which produces the long form directory listing, and '

From its earliest days, Unix has used a single hyphen to introduce command options or flags, like 'ls -l', which produces the long form directory listing, and 'cat -n', which numbers the lines that it outputs. Every author of a command makes up the conventions for these flags. The shell processes the command line, invokes the command and passes the processed words to the command as arguments. While there are standard library routines for parsing these arguments, they don't have to be used and many original Unix commands would have been written with their own parsing code. The oldest Unix commands like 'ls', 'cp', 'rm', and 'ps' mostly take single letter arguments. Authors often allowed such single letter arguments to be stacked up: 'ls -l' means list directory contents in long form while 'ls -r' means list directory in reverse alphabetic order and 'ls -lr', 'ls -l -r', and 'ls -r -l' mean all do the same thing, list the directory contents in long form in reverse alphabetic order. It wasn't always the case, but by now, 'ls' has over 40 or 50 single character arguments so I find myself needing to check the man page for the less common ones. There are other quirks too. Both 'tar' and 'ps' accept arguments that don't have the dash. 'ps -a' does (almost) the same thing as 'ps a'. 'tar -xv' will extract an archive, verbosely, but so will 'tar xv'. The author of these ancient Unix commands just decided to parse the command line arguments this way. What always bothered me was the use of full words instead of single character options introduced by a single dash. The 'find' command is one of these early Unix commands (introduced 44 years ago) that does this. "find /home/joe -name 'x*y' -print" outputs every file under directory /home/joe with a name starting with x and ending in y. This very useful command has a dizzying list of uses so the author made it easier to understand by using full words after the single dashes for elements on the command line. The recent versions of 'find' also take some single character flags: "find -d /home/joe -name 'x*y' -print" does almost the same thing as the previous example in this paragraph, but it descends into directories in a depth-first order. Compilers, with there own complex set of command line flags, options, and arguments have long used their own, offbeat, interpretations of their command lines arguments. Most Unix commands attempt to parse their command line arguments and if they couldn't, they would print out a few lines with a compact usage description. For example 'ps -Q' is not something that the 'ps' command understands so it produces: ps: illegal option -- Q usage: ps [-AaCcEefhjlMmrSTvwXx] [-O fmt | -o fmt] [-G gid[,gid...]] [-g grp[,grp...]] [-u [uid,uid...]] [-p pid[,pid...]] [-t tty[,tty...]] [-U user[,user...]] ps [-L] If this isn't enough help, one would check the man pages (with 'man ps'). I know that I saw commands written to use help when passed '-h' long ago, but around 1986 or so it wasn't common and often '-h' simply triggered the standard usage output because there was no option '-h'.