Step 6 of 6

Challenge: a real command-line tool

Command-line tools follow conventions that scripts and other programs rely on:

• Normal output goes to stdout; error messages go to stderr (fprintf(stderr, ...)), so they don't pollute output that's piped somewhere else.
• The exit code reports success: 0 means everything worked; nonzero means something failed. Scripts check it (if ./tool; then ...). <stdlib.h> names them EXIT_SUCCESS and EXIT_FAILURE.
• Keep going after one bad input when that's useful, but still report failure at the end.

Your turn: write lines FILE..., a tool that prints COUNT NAME for each file and a final COUNT total line. A file that can't be opened prints lines: cannot open NAME to stderr, is skipped, and makes the program exit with code 1 at the end (after the total). With no arguments at all, print usage: lines FILE... to stderr and exit with code 2.

Previous: Command-line arguments