GAWK: Effective AWK Programming

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204 GAWK: Effective AWK Programmingif (_opti == 0)_opti = 2thisopt = substr(argv[Optind], _opti, 1)Optopt = thisopti = index(options, thisopt)if (i == 0) {if (Opterr)printf("%c -- invalid option\n",thisopt) > "/dev/stderr"if (_opti >= length(argv[Optind])) {Optind++_opti = 0} else_opti++return "?"}The _opti variable tracks the position in the current command-line argument(argv[Optind]). If multiple options are grouped together with one ‘-’ (e.g., ‘-abx’), it isnecessary to return them to the user one at a time.If _opti is equal to zero, it is set to two, which is the index in the string of the nextcharacter to look at (we skip the ‘-’, which is at position one). The variable thisopt holdsthe character, obtained with substr. It is saved in Optopt for the main program to use.If thisopt is not in the options string, then it is an invalid option. If Opterr is nonzero,getopt prints an error message on the standard error that is similar to the message fromthe C version of getopt.Because the option is invalid, it is necessary to skip it and move on to the next optioncharacter. If _opti is greater than or equal to the length of the current command-lineargument, it is necessary to move on to the next argument, so Optind is incremented and_opti is reset to zero. Otherwise, Optind is left alone and _opti is merely incremented.In any case, because the option is invalid, getopt returns ‘?’. The main program canexamine Optopt if it needs to know what the invalid option letter actually is. Continuingon:if (substr(options, i + 1, 1) == ":") {# get option argumentif (length(substr(argv[Optind], _opti + 1)) > 0)Optarg = substr(argv[Optind], _opti + 1)elseOptarg = argv[++Optind]_opti = 0} elseOptarg = ""If the option requires an argument, the option letter is followed by a colon in theoptions string. If there are remaining characters in the current command-line argument(argv[Optind]), then the rest of that string is assigned to Optarg. Otherwise, the nextcommand-line argument is used (‘-xFOO’ versus ‘-x FOO’). In either case, _opti is reset
Chapter 12: A Library of awk Functions 205to zero, because there are no more characters left to examine in the current command-lineargument. Continuing:}if (_opti == 0 || _opti >= length(argv[Optind])) {Optind++_opti = 0} else_opti++return thisoptFinally, if _opti is either zero or greater than the length of the current commandlineargument, it means this element in argv is through being processed, so Optind isincremented to point to the next element in argv. If neither condition is true, then only_opti is incremented, so that the next option letter can be processed on the next call togetopt.The BEGIN rule initializes both Opterr and Optind to one. Opterr is set to one, sincethe default behavior is for getopt to print a diagnostic message upon seeing an invalidoption. Optind is set to one, since there’s no reason to look at the program name, which isin ARGV[0]:BEGIN {Opterr = 1Optind = 1}# default is to diagnose# skip ARGV[0]# test programif (_getopt_test) {while ((_go_c = getopt(ARGC, ARGV, "ab:cd")) != -1)printf("c = , optarg = \n",_go_c, Optarg)printf("non-option arguments:\n")for (; Optind < ARGC; Optind++)printf("\tARGV[%d] = \n",Optind, ARGV[Optind])}The rest of the BEGIN rule is a simple test program. Here is the result of two sampleruns of the test program:$ awk -f getopt.awk -v _getopt_test=1 -- -a -cbARG bax -x⊣ c = , optarg = ⊣ c = , optarg = ⊣ c = , optarg = ⊣ non-option arguments:⊣ ARGV[3] = ⊣ ARGV[4] = $ awk -f getopt.awk -v _getopt_test=1 -- -a -x -- xyz abc⊣ c = , optarg =
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