Storage / Stage 9 of 11 · step 6 of 7

Restore an app from a tar archive

~20 min

Not running Starting usually ~20 s

Situation

After a failed update, the application in ~/backup-lab/live/app stopped working. There is a nightly archive, /srv/backup-lab/app.tar, that the team trusts. Before switching to the copy, the team wants to check it next to the working one. Last night a colleague started restoring the copy in ~/backup-lab/restore but did not finish.

Task

Prepare a restored copy of the application in ~/backup-lab/restore/app:

the copy has the same files and directories as the archive, hidden ones included, and nothing extra;
every file's contents match the archive;
every file and directory is owned by student, and the modes are the ones stored in the archive;
the archive and the working copy ~/backup-lab/live are left as they were.

Constraints

Leave the working copy live alone: the switch to the restored copy happens later, after review.
Work as your own user; this task needs no sudo.

Lab goals

0 of 4

restore/app has the same files and directories as the archive, hidden ones included, and nothing extra — not checked yet
File contents match the archive — not checked yet
Owner is student, modes match the ones stored in the archive — not checked yet
The archive and the working copy live are left as they were — not checked yet

Hints

0 of 3 open · they open one at a time

Hint 1

Show the solution after 3 failed checks

Files and directories · picked for this lab

tar — archiving and extracting files

Cheat sheet

$_ — insert into the terminal

tar czf a.tar.gz dir create (gzip)
tar czf a.tar.gz -C /base sub without extra top-level directories
tar xzf a.tar.gz -C /dest extract into a directory
tar xzf a.tar.gz --strip-components=1
tar tvzf a.tar.gz view the contents
c=create x=extract t=list z=gzip J=xz v=verbose f=file -C directory

f requires an argument — the archive name — and by convention goes last in a group of letters (czf, not fcz). Before extracting an unfamiliar archive for the first time, look at its contents with tar tzf: if the listing has no common top-level directory, extraction will scatter files straight into the current directory.

Commands

bash

# create
tar czf backup.tar.gz /etc                 # pack /etc with gzip
tar cJf backup.tar.xz dir                  # with xz (stronger, slower)
tar czf b.tar.gz -C /var/www site          # pack site while inside /var/www (no /var/www/ in paths)
tar czf b.tar.gz --exclude='*.log' --exclude='cache' dir

# extract
tar xzf backup.tar.gz                      # extract here
tar xzf backup.tar.gz -C /restore          # extract into a directory
tar xzf b.tar.gz --strip-components=1       # strip the top directory (common with releases)
tar xzf backup.tar.gz etc/hosts            # extract a single file

# view/verify (without extracting)
tar tzf backup.tar.gz | head               # the listing
tar tvzf backup.tar.gz                     # with permissions/sizes/dates

# over the network (no intermediate file):
tar cz dir | ssh host 'tar xz -C /dest'    # send a tree to a host (see ssh-client)

tar: Removing leading '/'

Normal: tar stores relative paths (so it doesn't overwrite the system on extraction). Extract into the right directory with -C.

Extracted it and scattered files into the current directory (a "tar bomb")

An archive with no top-level directory. First check tar tzf a.tar.gz | head; extract into a separate -C /tmp/x. --strip-components=1 removes an extra top-level directory.

Permission denied / owners were lost

Extract system backups as root and with -p (permissions) / --same-owner (owners, the default for root).

Very slow / a large archive

xz (J) compresses harder but is slower; gzip (z) is a balance; for multi-core, use pigz (tar -c dir | pigz > a.tgz). For incremental backups, rsync is better.

Don't overwrite existing files on extraction

--keep-old-files (error if it exists) or --skip-old-files (skip it).

Extracting an unknown archive without previewing it first (tar tzf) can scatter hundreds of files into the current directory on top of existing ones — overwritten files can only be recovered from a backup. Extract unfamiliar archives into a separate empty directory (-C /tmp/x), not into a place that already holds important files.