the series

Rebuilding Unix Tools from Scratch in C: one tool per episode,
one core concept per tool. No stdio.h. No shortcuts.

// published

cat(1) file I/O · read() · write() · POSIX loop

done 02

head(1) read loops · line counting · -n flag

done 03

tail(1) circular buffer · lseek() · reading from the end

done ★A

lines: arbitrary line access range syntax · sort & merge · early exit

done

// in the pipeline

wc(1) state machines · character I/O · multiple flags

planned

echo(1) argc/argv deep dive · escape sequence parsing

planned

tr(1) lookup tables · character sets · -d -s modes

planned

cut(1) delimiter parsing · string tokenization

planned

★B

peek: first/last N lines with gap lseek() as main event · two-pass read · gap output

planned

sort(1) dynamic arrays · qsort() · malloc / realloc

planned

uniq(1) strcmp() · adjacent comparison · -c mode

planned

grep(1) strstr() → regex.h · regcomp() · line buffering

planned

★C

tally: frequency counter linked list in C · unknown data volume · frequency sort

planned

ls(1) opendir / readdir · stat() · permission bitmasks

planned

cp(1) buffered I/O · recursive dir walk · permissions copy

planned

★ Custom Tools

Episodes marked ★ are original tools that do not exist in Unix. Each one bridges a pedagogical gap between the surrounding episodes: a concept that demands its own dedicated build before the next standard tool makes sense.