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Yesterday I was making some experiments on Slitaz. It uses multiple initrd.img's to store files/changes.

I wanted to extract one of its initrd.gz images (which is a cpio archive) to a folder, edit/remove them, repack again.

I used this code:

cat rootfs.img | cpio -idvm

Then all files are extracted to my root filesystem. My whole OS is corrupted. (What an emberrasing situation...)

What should I do to make such operations safely but in an easy way? Chroot? LXC? (VirtualBox is the last resort)

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up vote 5 down vote accepted

Archived with relative paths

I would advise against running that type of command at your root level, /. That's asking for trouble. I always run cpio -idvm related commands in their own directories, and then use mv or cp to put the files where they need to be manually.

You can also use the method I described in this other U&L Q&A titled: How do I install TazPkg in SliTaz Linux, which also makes use of cpio.

Archived with absolute paths

If the archive was built with absolute paths you can tell cpio with the --no-absolute-filenames switch to block it from extracting into /.

$ mkdir /tmp/cpio-root
$ cd /tmp/cpio-root
$ cat rootfs.img | cpio -idvm --no-absolute-filenames


share|improve this answer
No matter where my working directory is, cpio -idvm is always extracting with absolute filenames (beginning from /). --no-absolute-filenames solved my problem. I think my sendboxing requirement will need LXC or VirtualBox. Thank you. – ceremcem Jun 16 '14 at 14:19

You can use the unp utility for this.

unp is a utility for unpacking multiple formats. One of it's features (the -U argument) is the ability to look into the archive and see if it has multiple root elements. If it does, it extracts them into a directory.

For example:

$ echo $RANDOM > a
$ echo $RANDOM > b
$ tar -cf archive.tar a b
$ rm a b
$ unp -U archive.tar
$ ls -l a b archive
ls: cannot access a: No such file or directory
ls: cannot access b: No such file or directory
total 8
-rw-r--r-- 1 root root 5 Jun 15 03:16 a
-rw-r--r-- 1 root root 6 Jun 15 03:16 b

unp works with many different formats (including cpio). It just calls out to the appropriate utility for handling the archive:

# unp -s
Known archive formats and tools:
7z:           p7zip or p7zip-full
ace:          unace
ar,deb:       binutils
arj:          arj
bz2:          bzip2
cab:          cabextract
chm:          libchm-bin or archmage
cpio,afio:    cpio or afio
dat:          tnef
dms:          xdms
exe:          maybe orange or unzip or unrar or unarj or lha 
gz:           gzip
hqx:          macutils
lha,lzh:      lha
lz:           lzip
lzma:         xz-utils or lzma
lzo:          lzop
lzx:          unlzx
mbox:         formail and mpack
pmd:          ppmd
rar:          rar or unrar or unrar-free
rpm:          rpm2cpio and cpio
sea,sea.bin:  macutils
shar:         sharutils
tar:          tar
tar.bz2,tbz2: tar with bzip2
tar.lzip:     tar with lzip
tar.lzop,tzo: tar with lzop
tar.xz,txz:   tar with xz-utils
tar.z:        tar with compress
tgz,tar.gz:   tar with gzip
uu:           sharutils
xz:           xz-utils
zip,cbz,cbr,jar,war,ear,xpi,adf: unzip
zoo:          zoo

The --help output showing what it can do:

# unp --help

   /usr/bin/unp [ options ] file [ files... ]
   file: compressed file(s) to expand/extract

   Use -- [ ARGUMENTS ] to pass arguments to external programs, eg. some tar options:
   unp fastgl.tgz xmnt.tgz -- -C /tmp

   -f Continue even if program availability checks fail or directory collision occurs
   -u Special helper mode.
      For most archive types:
      - create directory <filename without suffix>/
      - extract contents there
      For Debian/Ubuntu packages:
      - extract data.tar.gz after each operation in local directory
      - extract control.tar.gz into control/<package_version_arch>/
   -U Smart mode, acts like -u (see above) if archive contains multiple
      elements but if there is only one file/directory element then it's stored 
      in the current directory.
   -s Show the list of supported formats
   -v More verbosity
   -h Show this help
share|improve this answer
It's good to know that there is such a program simplifying extraction process (extracting multiple files into a directory is a must-have facility for me). But there is no tazpkg for this program. So it's not an option to use it in SliTaz (with ease, at least). – ceremcem Jun 15 '14 at 11:18

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