GAWK: Effective AWK Programming

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230 GAWK: Effective AWK ProgrammingThe program begins with a usage function and then a brief outline of the options andtheir meanings in a comment. The BEGIN rule deals with the command-line arguments andoptions. It uses a trick to get getopt to handle options of the form ‘-25’, treating suchan option as the option letter ‘2’ with an argument of ‘5’. If indeed two or more digitsare supplied (Optarg looks like a number), Optarg is concatenated with the option digitand then the result is added to zero to make it into a number. If there is only one digit inthe option, then Optarg is not needed. In this case, Optind must be decremented so thatgetopt processes it next time. This code is admittedly a bit tricky.If no options are supplied, then the default is taken, to print both repeated and nonrepeatedlines. The output file, if provided, is assigned to outputfile. Early on, outputfileis initialized to the standard output, ‘/dev/stdout’:# uniq.awk --- do uniq in awk## Requires getopt and join library functionsfunction usage( e){e = "Usage: uniq [-udc [-n]] [+n] [ in [ out ]]"print e > "/dev/stderr"exit 1}# -c count lines. overrides -d and -u# -d only repeated lines# -u only non-repeated lines# -n skip n fields# +n skip n characters, skip fields firstBEGIN \{count = 1outputfile = "/dev/stdout"opts = "udc0:1:2:3:4:5:6:7:8:9:"while ((c = getopt(ARGC, ARGV, opts)) != -1) {if (c == "u")non_repeated_only++else if (c == "d")repeated_only++else if (c == "c")do_count++else if (index("0123456789", c) != 0) {# getopt requires args to options# this messes us up for things like -5if (Optarg ~ /^[0-9]+$/)fcount = (c Optarg) + 0else {fcount = c + 0
Chapter 13: Practical awk Programs 231}Optind--}} elseusage()}if (ARGV[Optind] ~ /^\+[0-9]+$/) {charcount = substr(ARGV[Optind], 2) + 0Optind++}for (i = 1; i < Optind; i++)ARGV[i] = ""if (repeated_only == 0 && non_repeated_only == 0)repeated_only = non_repeated_only = 1if (ARGC - Optind == 2) {outputfile = ARGV[ARGC - 1]ARGV[ARGC - 1] = ""}The following function, are_equal, compares the current line, $0, to the previous line,last. It handles skipping fields and characters. If no field count and no character countare specified, are_equal simply returns one or zero depending upon the result of a simplestring comparison of last and $0. Otherwise, things get more complicated. If fields haveto be skipped, each line is broken into an array using split (see Section 8.1.3 [String-Manipulation Functions], page 132); the desired fields are then joined back into a line usingjoin. The joined lines are stored in clast and cline. If no fields are skipped, clast andcline are set to last and $0, respectively. Finally, if characters are skipped, substr isused to strip off the leading charcount characters in clast and cline. The two strings arethen compared and are_equal returns the result:function are_equal( n, m, clast, cline, alast, aline){if (fcount == 0 && charcount == 0)return (last == $0)if (fcount > 0) {n = split(last, alast)m = split($0, aline)clast = join(alast, fcount+1, n)cline = join(aline, fcount+1, m)} else {clast = lastcline = $0}
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