GAWK: Effective AWK Programming

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198 GAWK: Effective AWK ProgrammingEND { endfile(FILENAME) }This file must be loaded before the user’s “main” program, so that the rule it suppliesis executed first.This rule relies on awk’s FILENAME variable that automatically changes for each newdata file. The current file name is saved in a private variable, _oldfilename. If FILENAMEdoes not equal _oldfilename, then a new data file is being processed and it is necessaryto call endfile for the old file. Because endfile should only be called if a file has beenprocessed, the program first checks to make sure that _oldfilename is not the null string.The program then assigns the current file name to _oldfilename and calls beginfile forthe file. Because, like all awk variables, _oldfilename is initialized to the null string, thisrule executes correctly even for the first data file.The program also supplies an END rule to do the final processing for the last file. Becausethis END rule comes before any END rules supplied in the “main” program, endfile is calledfirst. Once again the value of multiple BEGIN and END rules should be clear.This version has same problem as the first version of nextfile (see Section 12.2.1 [Implementingnextfile as a Function], page 188). If the same data file occurs twice in a rowon the command line, then endfile and beginfile are not executed at the end of the firstpass and at the beginning of the second pass. The following version solves the problem:# ftrans.awk --- handle data file transitions## user supplies beginfile() and endfile() functionsFNR == 1 {if (_filename_ != "")endfile(_filename_)_filename_ = FILENAMEbeginfile(FILENAME)}END { endfile(_filename_) }Section 13.2.7 [Counting Things], page 233, shows how this library function can be usedand how it simplifies writing the main program.12.3.2 Rereading the Current FileAnother request for a new built-in function was for a rewind function that would make itpossible to reread the current file. The requesting user didn’t want to have to use getline(see Section 3.8 [Explicit Input with getline], page 52) inside a loop.However, as long as you are not in the END rule, it is quite easy to arrange to immediatelyclose the current input file and then start over with it from the top. For lack of a bettername, we’ll call it rewind:# rewind.awk --- rewind the current file and start overfunction rewind( i){# shift remaining arguments upfor (i = ARGC; i > ARGIND; i--)
Chapter 12: A Library of awk Functions 199ARGV[i] = ARGV[i-1]# make sure gawk knows to keep goingARGC++# make current file next to get doneARGV[ARGIND+1] = FILENAME# do itnextfile}This code relies on the ARGIND variable (see Section 6.5.2 [Built-in Variables That ConveyInformation], page 113), which is specific to gawk. If you are not using gawk, you can useideas presented in the previous section to either update ARGIND on your own or modify thiscode as appropriate.The rewind function also relies on the nextfile keyword (see Section 6.4.9 [Using gawk’snextfile Statement], page 109). See Section 12.2.1 [Implementing nextfile as a Function],page 188, for a function version of nextfile.12.3.3 Checking for Readable Data FilesNormally, if you give awk a data file that isn’t readable, it stops with a fatal error. Thereare times when you might want to just ignore such files and keep going. You can do thisby prepending the following program to your awk program:# readable.awk --- library file to skip over unreadable filesBEGIN {for (i = 1; i < ARGC; i++) {if (ARGV[i] ~ /^[A-Za-z_][A-Za-z0-9_]*=.*/ \|| ARGV[i] == "-")continue # assignment or standard inputelse if ((getline junk < ARGV[i]) < 0) # unreadabledelete ARGV[i]elseclose(ARGV[i])}}This works, because the getline won’t be fatal. Removing the element from ARGV withdelete skips the file (since it’s no longer in the list).12.3.4 Checking For Zero-length FilesAll known awk implementations silently skip over zero-length files. This is a by-productof awk’s implicit read-a-record-and-match-against-the-rules loop: when awk tries to read arecord from an empty file, it immediately receives an end of file indication, closes the file,and proceeds on to the next command-line data file, without executing any user-level awkprogram code.Using gawk’s ARGIND variable (see Section 6.5 [Built-in Variables], page 110), it is possibleto detect when an empty data file has been skipped. Similar to the library file presented
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