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Using Shell Variables in Programs

@command{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 @command{awk} program searches for. There are two ways to get the value of the shell variable into the body of the @command{awk} program.

The most common method is to use shell quoting to substitute the variable's value into the program inside the script. For example, in the following program:

echo -n "Enter search pattern: "
read pattern
awk "/$pattern/ "'{ nmatches++ }
     END { print nmatches, "found" }' /path/to/data

the @command{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 variable inside the quotes. The second part is single-quoted.

Variable substitution via quoting works, but can be potentially messy. It requires a good understanding of the shell's quoting rules (see section Shell Quoting Issues), and it's often difficult to correctly match up the quotes when reading the program.

A better method is to use @command{awk}'s variable assignment feature (see section Assigning Variables on the Command Line) to assign the shell variable's value to an @command{awk} variable's value. Then use dynamic regexps to match the pattern (see section Using Dynamic Regexps). The following shows how to redo the previous example using this technique:

echo -n "Enter search pattern: "
read pattern
awk -v pat="$pattern" '$0 ~ pat { nmatches++ }
       END { print nmatches, "found" }' /path/to/data

Now, the @command{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 @command{awk} variable pat could be named pattern too, but that would be more confusing. Using a variable also provides more flexibility, since 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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