GAWK: Effective AWK Programming

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226 GAWK: Effective AWK ProgrammingThe test in the for loop is worth noting. Any supplementary groups in the PROCINFOarray have the indices "group1" through "groupN" for some N, i.e., the total number ofsupplementary groups. However, we don’t know in advance how many of these groups thereare.This loop works by starting at one, concatenating the value with "group", and thenusing in to see if that value is in the array. Eventually, i is incremented past the last groupin the array and the loop exits.The loop is also correct if there are no supplementary groups; then the condition is falsethe first time it’s tested, and the loop body never executes.13.2.4 Splitting a Large File into PiecesThe split program splits large text files into smaller pieces. Usage is as follows:split [-count] file [ prefix ]By default, the output files are named ‘xaa’, ‘xab’, and so on. Each file has 1000 linesin it, with the likely exception of the last file. To change the number of lines in each file,supply a number on the command line preceded with a minus; e.g., ‘-500’ for files with500 lines in them instead of 1000. To change the name of the output files to somethinglike ‘myfileaa’, ‘myfileab’, and so on, supply an additional argument that specifies the filename prefix.Here is a version of split in awk. It uses the ord and chr functions presented inSection 12.2.6 [Translating Between Characters and Numbers], page 193.The program first sets its defaults, and then tests to make sure there are not too manyarguments. It then looks at each argument in turn. The first argument could be a minussign followed by a number. If it is, this happens to look like a negative number, so it ismade positive, and that is the count of lines. The data file name is skipped over and thefinal argument is used as the prefix for the output file names:# split.awk --- do split in awk## Requires ord and chr library functions# usage: split [-num] [file] [outname]BEGIN {outfile = "x"count = 1000if (ARGC > 4)usage()# defaulti = 1if (ARGV[i] ~ /^-[0-9]+$/) {count = -ARGV[i]ARGV[i] = ""i++}# test argv in case reading from stdin instead of fileif (i in ARGV)
Chapter 13: Practical awk Programs 227}i++ # skip data file nameif (i in ARGV) {outfile = ARGV[i]ARGV[i] = ""}s1 = s2 = "a"out = (outfile s1 s2)The next rule does most of the work. tcount (temporary count) tracks how many lineshave been printed to the output file so far. If it is greater than count, it is time to closethe current file and start a new one. s1 and s2 track the current suffixes for the file name.If they are both ‘z’, the file is just too big. Otherwise, s1 moves to the next letter in thealphabet and s2 starts over again at ‘a’:{}if (++tcount > count) {close(out)if (s2 == "z") {if (s1 == "z") {printf("split: %s is too large to split\n",FILENAME) > "/dev/stderr"exit 1}s1 = chr(ord(s1) + 1)s2 = "a"}elses2 = chr(ord(s2) + 1)out = (outfile s1 s2)tcount = 1}print > outThe usage function simply prints an error message and exits:function usage( e){e = "usage: split [-num] [file] [outname]"print e > "/dev/stderr"exit 1}The variable e is used so that the function fits nicely on the page.This program is a bit sloppy; it relies on awk to automatically close the last file insteadof doing it in an END rule. It also assumes that letters are contiguous in the character set,which isn’t true for EBCDIC systems.
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GAWK: Effective AWK Programming

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