FogOS

Fall 2025 Edition

📋 Overview

This project extends the FogOS/xv6 cat utility with several Linux-style options to demonstrate user-space programming, system-call–based file I/O, and command-line parsing. Implemented flags include -n (number all lines), -b (number nonempty lines), -E (show $ at end of each line), and -s (squeeze consecutive blank lines). The work preserves original cat behavior while adding composable formatting features.

✨ Implemented Features

• -n — Number all output lines
• -b — Number nonempty output lines only
• -E — Display $ at the end of each line
• -s — Squeeze multiple consecutive blank lines into a single blank line

Options can be combined.
Example:

cat -nEs file.txt

🛠 Build Instructions

From the project root directory, run:

make clean
make qemu

▶️ Run Instructions

Inside the FogOS shell:

cat [options] [file...]

If no file is provided, cat reads from standard input.

Multiple options can be combined and multiple files can be listed.

💡 Usage Examples

$ cat -n test.txt
  1  hello
  2  world

$ cat -bE test.txt
  1  hello$
     $
  2  world$

$ echo -e "foo\nbar" | cat -nEs
  1  foo$
  2  bar$

✅ Testing

Manual Tests

The following commands were verified inside QEMU:

cat -n test.txt      # number all lines
cat -b test.txt      # number nonempty lines only
cat -E test.txt      # show $ at end of each line
cat -s test.txt      # squeeze consecutive blank lines
cat -nEs test.txt    # combine options
echo "abc" | cat -n  # read from stdin
cat -n file1 file2   # multiple files

Edge Cases

Empty files

Files with only blank lines

Consecutive blank lines

Large files to confirm no performance regressions

📂 Source Files

• user/cat.c — Enhanced implementation of the cat command
• Makefile — Updated UPROGS list to include the rebuilt cat

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🔖 Notes

• Code follows xv6/FogOS formatting standards (2-space indentation, minimal library usage).
• Performance is equivalent to the original cat (no regressions).
• This README provides all required documentation for building, running, and testing the software.