GAWK: Effective AWK Programming

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216 GAWK: Effective AWK ProgrammingA common use of cut might be to pull out just the login name of logged-on users fromthe output of who. For example, the following pipeline generates a sorted, unique list of thelogged-on users:who | cut -c1-8 | sort | uniqThe options for cut are:-c list Use list as the list of characters to cut out. Items within the list may beseparated by commas, and ranges of characters can be separated with dashes.The list ‘1-8,15,22-35’ specifies characters 1 through 8, 15, and 22 through35.-f list Use list as the list of fields to cut out.-d delim Use delim as the field-separator character instead of the tab character.-s Suppress printing of lines that do not contain the field delimiter.The awk implementation of cut uses the getopt library function (see Section 12.4 [ProcessingCommand-Line Options], page 201) and the join library function (see Section 12.2.7[Merging an Array into a String], page 195).The program begins with a comment describing the options, the library functions needed,and a usage function that prints out a usage message and exits. usage is called if invalidarguments are supplied:# cut.awk --- implement cut in awk# Options:# -f list Cut fields# -d c Field delimiter character# -c list Cut characters## -s Suppress lines without the delimiter## Requires getopt and join library functionsfunction usage( e1, e2){e1 = "usage: cut [-f list] [-d c] [-s] [files...]"e2 = "usage: cut [-c list] [files...]"print e1 > "/dev/stderr"print e2 > "/dev/stderr"exit 1}The variables e1 and e2 are used so that the function fits nicely on the page.Next comes a BEGIN rule that parses the command-line options. It sets FS to a singleTAB character, because that is cut’s default field separator. The output field separator isalso set to be the same as the input field separator. Then getopt is used to step throughthe command-line options. Exactly one of the variables by_fields or by_chars is set totrue, to indicate that processing should be done by fields or by characters, respectively.When cutting by characters, the output field separator is set to the null string:
Chapter 13: Practical awk Programs 217BEGIN \{FS = "\t" # defaultOFS = FSwhile ((c = getopt(ARGC, ARGV, "sf:c:d:")) != -1) {if (c == "f") {by_fields = 1fieldlist = Optarg} else if (c == "c") {by_chars = 1fieldlist = OptargOFS = ""} else if (c == "d") {if (length(Optarg) > 1) {printf("Using first character of %s" \" for delimiter\n", Optarg) > "/dev/stderr"Optarg = substr(Optarg, 1, 1)}FS = OptargOFS = FSif (FS == " ") # defeat awk semanticsFS = "[ ]"} else if (c == "s")suppress++elseusage()}for (i = 1; i < Optind; i++)ARGV[i] = ""Special care is taken when the field delimiter is a space. Using a single space (" ") forthe value of FS is incorrect—awk would separate fields with runs of spaces, tabs, and/ornewlines, and we want them to be separated with individual spaces. Also remember thatafter getopt is through (as described in Section 12.4 [Processing Command-Line Options],page 201), we have to clear out all the elements of ARGV from 1 to Optind, so that awk doesnot try to process the command-line options as file names.After dealing with the command-line options, the program verifies that the options makesense. Only one or the other of ‘-c’ and ‘-f’ should be used, and both require a field list.Then the program calls either set_fieldlist or set_charlist to pull apart the list offields or characters:if (by_fields && by_chars)usage()if (by_fields == 0 && by_chars == 0)by_fields = 1 # default
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