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Chapter 11. A Flock of awks

In the previous four chapters, we have looked at POSIX awk, with only occasional reference to actual awk implementations that you would run. In this chapter, we focus on the different versions of awk that are available, what features they do or do not have, and how you can get them.

First, we'll look at the original V7 version of awk. The original awk lacks many of the features we've described, so this section mostly describes what's not there. Next, we'll look at the three versions whose source code is freely available. All of them have extensions to the POSIX standard. Those that are common to all three versions are discussed first. Finally, we look at three commercial versions of awk.

11.1. Original awk

In each of the sections that follow, we'll take a brief look at how the original awk differs from POSIX awk. Over the years, UNIX vendors have enhanced their versions of original awk; you may need to write small test programs to see exactly what features your old awk has or doesn't have.

11.1.5. Faking Dynamic Regular Expressions

The original awk made it difficult to use patterns dynamically because they had to be fixed when the script was interpreted. You can get around the problem of not being able to use a variable as a regular expression by importing a shell variable inside an awk program. The value of the shell variable will be interpreted by awk as a constant. Here's an example:

$ cat awkro2
#!/bin/sh
# assign shell's $1 to awk search variable
search=$1
awk '$1 ~ /'"$search"'/' acronyms

The first line of the script makes the variable assignment before awk is invoked. To get the shell to expand the variable inside the awk procedure, we enclose it within single, then double, quotation marks.[72] Thus, awk never sees the shell variable and evaluates it as a constant string.

[72]Actually, this is the concatenation of single-quoted text with double-quoted text with more single-quoted text to produce one large quoted string. This trick was used ealier, in Chapter 6.

Here's another version that makes use of the Bourne shell variable substitution feature. Using this feature gives us an easy way to specify a default value for the variable if, for instance, the user does not supply a command-line argument.

search=$1
awk '$1 ~ /'"${search:-.*}"'/' acronyms

The expression "${search:-.*}" tells the shell to use the value of search if it is defined; if not, use ".*" as the value. Here, ".*" is regular-expression syntax specifying any string of characters; therefore, all entries are printed if no entry is supplied on the command line. Because the whole thing is inside double quotes, the shell does not perform a wildcard expansion on ".*".



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