GAWK: Effective AWK Programming

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142 GAWK: Effective AWK Programmingexisting practice. The proposed rules have special cases that make it possible to produce a‘\’ preceding the matched text. This is shown in Table 8.3.You type sub sees sub generates\\\\\\& \\\& a literal ‘\&’\\\\& \\& a literal ‘\’, followed by the matched text\\& \& a literal ‘&’\\q \q a literal ‘\q’\\\\ \\ \\Table 8.3: Proposed rules for sub and backslashIn a nutshell, at the runtime level, there are now three special sequences of characters(‘\\\&’, ‘\\&’ and ‘\&’) whereas historically there was only one. However, as in the historicalcase, any ‘\’ that is not part of one of these three sequences is not special and appears inthe output literally.gawk 3.0 and 3.1 follow these proposed POSIX rules for sub and gsub. The POSIXstandard took much longer to be revised than was expected in 1996. The 2001 standarddoes not follow the above rules. Instead, the rules there are somewhat simpler. The resultsare similar except for one case.The 2001 POSIX rules state that ‘\&’ in the replacement string produces a literal ‘&’,‘\\’ produces a literal ‘\’, and ‘\’ followed by anything else is not special; the ‘\’ is placedstraight into the output. These rules are presented in Table 8.4.You type sub sees sub generates\\\\\\& \\\& a literal ‘\&’\\\\& \\& a literal ‘\’, followed by the matched text\\& \& a literal ‘&’\\q \q a literal ‘\q’\\\\ \\ \Table 8.4: POSIX 2001 rules for subThe only case where the difference is noticeable is the last one: ‘\\\\’ is seen as ‘\\’ andproduces ‘\’ instead of ‘\\’.Starting with version 3.1.4, gawk follows the POSIX rules when ‘--posix’ is specified(see Section 11.2 [Command-Line Options], page 177). Otherwise, it continues to followthe 1996 proposed rules, since, as of this writing, that has been its behavior for over sevenyears.NOTE: At the next major release, gawk will switch to using the POSIX 2001rules by default.The rules for gensub are considerably simpler. At the runtime level, whenever gawk seesa ‘\’, if the following character is a digit, then the text that matched the corresponding
Chapter 8: Functions 143parenthesized subexpression is placed in the generated output. Otherwise, no matter whatcharacter follows the ‘\’, it appears in the generated text and the ‘\’ does not, as shown inTable 8.5.You type gensub sees gensub generates& & the matched text\\& \& a literal ‘&’\\\\ \\ a literal ‘\’\\\\& \\& a literal ‘\’, then the matched text\\\\\\& \\\& a literal ‘\&’\\q \q a literal ‘q’Table 8.5: Escape Sequence Processing for gensubBecause of the complexity of the lexical and runtime level processing and the specialcases for sub and gsub, we recommend the use of gawk and gensub when you have to dosubstitutions.Advanced Notes: Matching the Null StringIn awk, the ‘*’ operator can match the null string. This is particularly important for thesub, gsub, and gensub functions. For example:$ echo abc | awk ’{ gsub(/m*/, "X"); print }’⊣ XaXbXcXAlthough this makes a certain amount of sense, it can be surprising.8.1.4 Input/Output FunctionsThe following functions relate to input/output (I/O). Optional parameters are enclosed insquare brackets ([ ]):close(filename [, how])Close the file filename for input or output. Alternatively, the argument may bea shell command that was used for creating a coprocess, or for redirecting toor from a pipe; then the coprocess or pipe is closed. See Section 4.8 [ClosingInput and Output Redirections], page 71, for more information.When closing a coprocess, it is occasionally useful to first close one end of thetwo-way pipe and then to close the other. This is done by providing a secondargument to close. This second argument should be one of the two stringvalues "to" or "from", indicating which end of the pipe to close. Case inthe string does not matter. See Section 10.2 [Two-Way Communications withAnother Process], page 170, which discusses this feature in more detail andgives an example.fflush([filename])Flush any buffered output associated with filename, which is either a file openedfor writing or a shell command for redirecting output to a pipe or coprocess.
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