Reaching Into Archives · Lesson 4 of the course

The Field Guide

The whole course as one decision flow, one cheat sheet, and two tools that cover everything.

You’ve got the model now: the index makes selective extraction possible, the streaming/seekable split says when it’s cheap and when it isn’t, and code adds the in-memory buffer. This capstone compresses all of it into a field guide you’ll actually keep — and ends with a task to lock it in.

Your mission Fluency is recall under time pressure. The point of a field guide is that you don’t re-derive the answer at 11pm with a 2 GB archive in front of you — you glance, decide, type.

The decision flow

One archive, one question at a time:

Four questions — whole or part, which family, in code, through a pipe — settle every case in the course.

The cheat sheet

The three verbs across every format you’ll meet. Bookmark this one.

Format	List	Stream one → stdout	Bulk extract
`.zip`	`unzip -l a.zip`	`unzip -p a.zip f`	`unzip -d out/ a.zip`
`.rar`	`unrar l a.rar`	`unrar p a.rar f`	`unrar x a.rar out/`
`.tar`	`tar -tf a.tar`	`tar -xO -f a.tar f`	`tar -xf a.tar -C out/`
`.tar.gz`	`tar -tzf a.tgz`	`tar -xzO -f a.tgz f`	`tar -xzf a.tgz -C out/`
`.gz` (one file)	—	`zcat f.gz`	`gunzip f.gz`
`.7z`	`7z l a.7z`	`7z e -so a.7z f`	`7z x a.7z`
any (universal)	`bsdtar -tf a.*`	`bsdtar -xOf a.* f`	`bsdtar -xf a.*`

Two tools that cover (almost) everything

Don’t want to remember per-format syntax? Two readers swallow most formats behind one interface:

7z (p7zip) — one binary for .7z, .zip, .tar, .gz, and more. l lists, x extracts keeping paths, and -so writes a member to stdout.1
bsdtar (libarchive) — “compression and format are always detected automatically, and the same API is used for all formats.”2 It reads tar, zip, 7-zip, cpio, iso and more with the same -tf / -xf flags. The closest thing to one tool for all of them.

The catch worth remembering from Lesson 2: a universal tool still obeys the format’s physics. bsdtar -tf on a .tar.gz is still a sequential walk; on a .zip it’s still a seek. One interface — not one cost.

Capstone — reach into your own archive

This is the part that builds the skill. Do it now on a real archive (one of your BSE zips is perfect). Click each step as you finish it:

List, don’t extract. Run unzip -l yourfile.zip and find one member you want.
Stream one to a pipe. unzip -p yourfile.zip THAT.csv | head — confirm nothing was written to disk.
Process without extracting. unzip -p yourfile.zip THAT.csv | wc -l to count its rows.
Do it in Python. Open it with zipfile, z.read("THAT.csv"), and print the first line — no file on disk.
Feel the streaming cost. Find or make a .tar.gz; time tar -tzf on it vs unzip -l on a zip. Notice the difference.
Test it in the wild. Post your favourite one-liner (or a question) to Unix & Linux SE or r/commandline for critique.

Course win

You started by asking “how did you extract those without unzipping them?” You can now answer it cold — the index, the seek, the streaming split, the in-memory buffer — and reach into any .zip, .rar, .tar(.gz), .7z three ways, choosing the cheap one on sight. That’s the fluency the mission was after.

Recall check — everything, interleaved

This one mixes all four lessons on purpose — interleaving is what proves the knowledge is yours, not just fresh. Retrieve each from memory.

Primary source — keep these two

libarchive / bsdtar — the universal reader; skim the front page on automatic format detection. And the 7-Zip command-line manual for l / x / e -so. Together they’re the two-tool kit behind the cheat sheet above.

I’m your teacher — use me. Did the capstone throw an error, or did the timing surprise you? Paste what happened. And when you’re ready, ask me to design Lesson 5 — encrypted archives, zip64 for >4 GB, or wiring all of this into your backtester’s loader. You pick the direction.

← Lesson 3 · 📖 Glossary 🎉 Course complete — pick Lesson 5 with your teacher

Sources

7-Zip manual — -so (write data to stdout) and the command reference (l list, e/x extract).
libarchive — bsdtar. “On read, compression and format are always detected automatically, and the same API is used for all formats.” Reads tar, zip, 7-zip, cpio, iso, and more. See also the bsdtar(1) man page.