gawk: Using Shell Variables
7.2 Using Shell Variables in Programs
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'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.