-APUE-:
fig15.11, copy file to pager program using popen.
fig15.12, the popen and pclose functions.
fig15.14, filter to convert uppercase characters to lowercase.
fig15.15, invoke uppercase/lowercase filter to read commands.
-APUE-:
fig15.17, simple filter to add two numbers.
fig15.18, program to drive the add2 filter.
fig15.19, filter to add two numbers, using standard I/O.