+fio also supports environment variable expansion in job files. Any
+substring of the form "${VARNAME}" as part of an option value (in other
+words, on the right of the `='), will be expanded to the value of the
+environment variable called VARNAME. If no such environment variable
+is defined, or VARNAME is the empty string, the empty string will be
+substituted.
+
+As an example, let's look at a sample fio invocation and job file:
+
+$ SIZE=64m NUMJOBS=4 fio jobfile.fio
+
+; -- start job file --
+[random-writers]
+rw=randwrite
+size=${SIZE}
+numjobs=${NUMJOBS}
+; -- end job file --
+
+This will expand to the following equivalent job file at runtime:
+
+; -- start job file --
+[random-writers]
+rw=randwrite
+size=64m
+numjobs=4
+; -- end job file --
+