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gawk: Using Shell Variables

 
 7.2 Using Shell Variables in Programs
 =====================================
 
 'awk' programs are often used as components in larger programs written
 in shell.  For example, it is very common to use a shell variable to
 hold a pattern that the 'awk' program searches for.  There are two ways
 to get the value of the shell variable into the body of the 'awk'
 program.
 
    A common method is to use shell quoting to substitute the variable's
 value into the program inside the script.  For example, consider the
 following program:
 
      printf "Enter search pattern: "
      read pattern
      awk "/$pattern/ "'{ nmatches++ }
           END { print nmatches, "found" }' /path/to/data
 
 The 'awk' program consists of two pieces of quoted text that are
 concatenated together to form the program.  The first part is
 double-quoted, which allows substitution of the 'pattern' shell variable
 inside the quotes.  The second part is single-quoted.
 
    Variable substitution via quoting works, but can potentially be
 messy.  It requires a good understanding of the shell's quoting rules
 (Quoting), and it's often difficult to correctly match up the
 quotes when reading the program.
 
 Assignment Options::) to assign the shell variable's value to an 'awk'
 variable.  Then use dynamic regexps to match the pattern (Computed
 Regexps).  The following shows how to redo the previous example using
 this technique:
 
      printf "Enter search pattern: "
      read pattern
      awk -v pat="$pattern" '$0 ~ pat { nmatches++ }
             END { print nmatches, "found" }' /path/to/data
 
 Now, the 'awk' program is just one single-quoted string.  The assignment
 '-v pat="$pattern"' still requires double quotes, in case there is
 whitespace in the value of '$pattern'.  The 'awk' variable 'pat' could
 be named 'pattern' too, but that would be more confusing.  Using a
 variable also provides more flexibility, as the variable can be used
 anywhere inside the program--for printing, as an array subscript, or for
 any other use--without requiring the quoting tricks at every point in
 the program.
 

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