GAWK: Effective AWK Programming

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202 GAWK: Effective AWK ProgrammingoptindoptargopterroptoptThe index in the argument value array (argv) where the first nonoptioncommand-line argument can be found.The string value of the argument to an option.Usually getopt prints an error message when it finds an invalid option. Settingopterr to zero disables this feature. (An application might want to print itsown error message.)The letter representing the command-line option.The following C fragment shows how getopt might process command-line arguments forawk:intmain(int argc, char *argv[]){.../* print our own message */opterr = 0;while ((c = getopt(argc, argv, "v:f:F:W:")) != -1) {switch (c) {case ’f’: /* file */...break;case ’F’: /* field separator */...break;case ’v’: /* variable assignment */...break;case ’W’: /* extension */...break;case ’?’:default:usage();break;}}...}As a side point, gawk actually uses the GNU getopt_long function to process bothnormal and GNU-style long options (see Section 11.2 [Command-Line Options], page 177).The abstraction provided by getopt is very useful and is quite handy in awk programsas well. Following is an awk version of getopt. This function highlights one of the greatestweaknesses in awk, which is that it is very poor at manipulating single characters. Repeated
Chapter 12: A Library of awk Functions 203calls to substr are necessary for accessing individual characters (see Section 8.1.3 [String-Manipulation Functions], page 132). 6The discussion that follows walks through the code a bit at a time:# getopt.awk --- do C library getopt(3) function in awk# External variables:# Optind -- index in ARGV of first nonoption argument# Optarg -- string value of argument to current option# Opterr -- if nonzero, print our own diagnostic# Optopt -- current option letter# Returns:# -1 at end of options# ? for unrecognized option# a character representing the current option# Private Data:# _opti -- index in multi-flag option, e.g., -abcThe function starts out with a list of the global variables it uses, what the return valuesare, what they mean, and any global variables that are “private” to this library function.Such documentation is essential for any program, and particularly for library functions.The getopt function first checks that it was indeed called with a string of options (theoptions parameter). If options has a zero length, getopt immediately returns −1:function getopt(argc, argv, options, thisopt, i){if (length(options) == 0) # no options givenreturn -1if (argv[Optind] == "--") { # all doneOptind++_opti = 0return -1} else if (argv[Optind] !~ /^-[^: \t\n\f\r\v\b]/) {_opti = 0return -1}The next thing to check for is the end of the options. A ‘--’ ends the command-lineoptions, as does any command-line argument that does not begin with a ‘-’. Optind is usedto step through the array of command-line arguments; it retains its value across calls togetopt, because it is a global variable.The regular expression that is used, /^-[^: \t\n\f\r\v\b]/, is perhaps a bit of overkill;it checks for a ‘-’ followed by anything that is not whitespace and not a colon. If the currentcommand-line argument does not match this pattern, it is not an option, and it ends optionprocessing:6 This function was written before gawk acquired the ability to split strings into single characters using ""as the separator. We have left it alone, since using substr is more portable.
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GAWK: Effective AWK Programming

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