GAWK: Effective AWK Programming

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244 GAWK: Effective AWK Programmingone field), that the sort keys should be treated as numeric quantities (otherwise ‘15’ wouldcome before ‘5’), and that the sorting should be done in descending (reverse) order.The sort could even be done from within the program, by changing the END action to:END {sort = "sort -k 2nr"for (word in freq)printf "%s\t%d\n", word, freq[word] | sortclose(sort)}This way of sorting must be used on systems that do not have true pipes at the commandline(or batch-file) level. See the general operating system documentation for more informationon how to use the sort program.13.3.6 Removing Duplicates from Unsorted TextThe uniq program (see Section 13.2.6 [Printing Nonduplicated Lines of Text], page 229),removes duplicate lines from sorted data.Suppose, however, you need to remove duplicate lines from a data file but that you wantto preserve the order the lines are in. A good example of this might be a shell history file.The history file keeps a copy of all the commands you have entered, and it is not unusualto repeat a command several times in a row. Occasionally you might want to compactthe history by removing duplicate entries. Yet it is desirable to maintain the order of theoriginal commands.This simple program does the job. It uses two arrays. The data array is indexed bythe text of each line. For each line, data[$0] is incremented. If a particular line has notbeen seen before, then data[$0] is zero. In this case, the text of the line is stored inlines[count]. Each element of lines is a unique command, and the indices of linesindicate the order in which those lines are encountered. The END rule simply prints out thelines, in order:# histsort.awk --- compact a shell history file# Thanks to Byron Rakitzis for the general idea{if (data[$0]++ == 0)lines[++count] = $0}END {for (i = 1; i
Chapter 13: Practical awk Programs 24513.3.7 Extracting Programs from Texinfo Source FilesBoth this chapter and the previous chapter (Chapter 12 [A Library of awk Functions],page 186) present a large number of awk programs. If you want to experiment with theseprograms, it is tedious to have to type them in by hand. Here we present a program thatcan extract parts of a Texinfo input file into separate files.This book is written in Texinfo, the GNU project’s document formatting language. Asingle Texinfo source file can be used to produce both printed and online documentation.Texinfo is fully documented in the book Texinfo—The GNU Documentation Format, availablefrom the Free Software Foundation.For our purposes, it is enough to know three things about Texinfo input files:• The “at” symbol (‘@’) is special in Texinfo, much as the backslash (‘\’) is in C or awk.Literal ‘@’ symbols are represented in Texinfo source files as ‘@@’.• Comments start with either ‘@c’ or ‘@comment’. The file-extraction program works byusing special comments that start at the beginning of a line.• Lines containing ‘@group’ and ‘@end group’ commands bracket example text thatshould not be split across a page boundary. (Unfortunately, TEX isn’t always smartenough to do things exactly right, and we have to give it some help.)The following program, ‘extract.awk’, reads through a Texinfo source file and does twothings, based on the special comments. Upon seeing ‘@c system ...’, it runs a command, byextracting the command text from the control line and passing it on to the system function(see Section 8.1.4 [Input/Output Functions], page 143). Upon seeing ‘@c file filename’,each subsequent line is sent to the file filename, until ‘@c endfile’ is encountered. The rulesin ‘extract.awk’ match either ‘@c’ or ‘@comment’ by letting the ‘omment’ part be optional.Lines containing ‘@group’ and ‘@end group’ are simply removed. ‘extract.awk’ uses thejoin library function (see Section 12.2.7 [Merging an Array into a String], page 195).The example programs in the online Texinfo source for GAWK: Effective AWK Programming(‘gawk.texi’) have all been bracketed inside ‘file’ and ‘endfile’ lines. Thegawk distribution uses a copy of ‘extract.awk’ to extract the sample programs and installmany of them in a standard directory where gawk can find them. The Texinfo file lookssomething like this:...This program has a @code{BEGIN} rule,that prints a nice message:@example@c file examples/messages.awkBEGIN @{ print "Don’t panic!" @}@c end file@end exampleIt also prints some final advice:@example@c file examples/messages.awk
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