Mounting an ISO and extracting files from it. Command line techniques with a Debian ISO remaster example.
- Loopmount an ISO Without Administrative Privileges
- Viewing the Contents of an ISO
- Extraction of Files from an ISO
- Remaster an Installation Image
Loopmount an ISO Without Administrative Privileges
Users seeking to view the contents of an ISO file or copy a file from it are frequently advised to mount the ISO with
mount -o loop example.iso /mnt
There are two aspects of this advice to consider:
- The command requires root privileges, which the user might not have.
- Why invoke a root privilege (if available) when it is unnecessary?
The following programs allow loopmounting without administrative privileges.
apt install udisks2
The package contains the udisksctl program. The command
udisksctl loop-setup -r -f example.iso
attaches the ISO to a loop device in /dev. Logging out and back in again might be necessary to have success with this command as a user.
ls -l /dev/loop*
brw-rw---- 1 root disk 7, 0 Nov 16 15:14 /dev/loop0 brw-rw---- 1 root disk 259, 2 Nov 16 15:14 /dev/loop0p1 brw-rw---- 1 root disk 259, 3 Nov 16 15:14 /dev/loop0p2 brw-rw---- 1 root disk 7, 1 Nov 16 15:19 /dev/loop1 brw-rw---- 1 root disk 7, 2 Nov 16 15:14 /dev/loop2 brw-rw---- 1 root disk 7, 3 Nov 16 15:14 /dev/loop3 brw-rw---- 1 root disk 7, 4 Nov 16 15:14 /dev/loop4 brw-rw---- 1 root disk 7, 5 Nov 16 15:14 /dev/loop5 brw-rw---- 1 root disk 7, 6 Nov 16 15:14 /dev/loop6 brw-rw---- 1 root disk 7, 7 Nov 16 15:14 /dev/loop7 crw-rw---- 1 root disk 10, 237 Nov 16 15:14 /dev/loop-control
The ISO's main partition can be mounted and unmounted with the commands
udisksctl mount -b /dev/loop0p1 udisksctl unmount -b /dev/loop0p1
The mount point is at /media/$USER.
Unmounting still leaves example.iso associated with /dev/loop0. To unattach the device do
udisksctl loop-delete -b /dev/loop0
udisks2 appeared in Debian 8 (jessie).
Intended as a udisks2 replacement but udevil can co-exist with it and will work to mount an ISO with or without its recommended packages.
udevil mount example.iso
udevil unmount example.iso
work out of the box. The mount point is in /media.
udevil appeared in Debian 8 (jessie) but has been backported to Debian 7 (wheezy).
archivemount example.iso isomount/
fusermount -u isomount/
Create the directory isomount in the home directory ($HOME) and mount the iso with
fuseiso9660 example.iso isomount/
fusermount -u isomount/
fuseiso9660 deals only with the basic ISO9660 format (see the manual) so filenames are limited to uppercase and 11 characters in an 8.3 format.
Viewing the Contents of an ISO
Mounting the ISO and following up with
ls -l <mount_point>
is one method of seeing what the structure of the ISO is and what it contains. However, there are a number of programs which will allow examination of the contents of an ISO archive without having to mount it and without administration rights.
A Debian 9 (stretch) netinst ISO will be used as an example image in what follows.
bsdtar is a tar implementation which can view the contents of various archive files, including ISO 9660 images. It can also extract files from the image. For a basic view of the contents of an ISO image do
bsdtar -tf debian-9.3.0-i386-netinst.iso
A detailed view would be obtained with
bsdtar -tvf debian-9.3.0-i386-netinst.iso
To look at a particular directory in the archive there is
bsdtar -tf debian-9.3.0-i386-netinst.iso pool/main/a
xorriso is a versatile program which manipulates ISO 9660 filesystem images. It can also extract files from the image.
Commands which correspond to the ones for bsdtar are
xorriso -indev debian-9.3.0-i386-netinst.iso -ls xorriso -indev debian-9.3.0-i386-netinst.iso -lsl xorriso -indev debian-9.3.0-i386-netinst.iso -ls pool/main/b
For basic listing:
isoinfo -J -l -i debian-9.3.0-i386-netinst.iso
For long listing:
isoinfo -J -f -i debian-9.3.0-i386-netinst.iso
7z l debian-9.3.0-i386-netinst.iso
gives a file listing of the archive.
Extraction of Files from an ISO
If debian-9.3.0-i386-netinst.iso has been mounted and it is desired to have all the deb and udeb files copied to other media it should be sufficient to do
cp -a /media/<mount_point>/pool/main DESTINATION
Without explicit mounting, all the programs mentioned above are capable of extracting the same files. The relevant commands are:
bsdtar -C DESTINATION -xf debian-9.3.0-i386-netinst.iso pool/main xorriso -osirrox on -indev debian-9.3.0-i386-netinst.iso -extract pool/main DESTINATION isoinfo -J -x /pool/main/a/adduser/adduser_3.113+nmu3_all.deb -i debian-9.3.0-i386-netinst.iso > DESTINATION/adduser_3.113+nmu3_all.deb 7z x -oDESTINATION -i\!pool/main debian-9.3.0-i386-netinst.iso
- isoinfo only extracts single files and will display a file on stdout (possibly the screen) if it is not redirected somewhere.
7z doesn't like spaces between an option and its argument. Also, the bash shell sees ! as having a special meaning to it. This meaning can be removed by preceding ! with a backslash.
Remaster an Installation Image
Netinst and DVD-1 images are popular choices when it comes to installing Debian. Much of the time it is sufficient simply to boot the image to have everything installed to a user's satisfaction. However, it is also possible to pass boot parameters to the installer to achieve a particular end, either during the installation or on the installed system. Objectives could be to produce a fully automatic installation or to configure part of the system using preseeding.
Appending parameters to the kernel command line to setup, for example, a network connection, can be done relatively easily, but is a repetitive task when installing frequently. On the other hand, the installation image can be altered to contain the needed parameters before burning to CD or DVD or writing to a USB stick.
Manipulating an ISO image in this way is known as remastering. The basic idea will be illustrated using a Debian 9.3.0 i386 netinst image which is planned to be booted from the non-graphical install option with a preseed configuration file, preseed.cfg.
Extracting All Files from an ISO
First make a place to put the extracted files:
The name of the directory is displayed. Something like this should be seen:
Extract all files from the ISO with (for example)
cat firmware-9.3.0-i386-netinst.iso | bsdtar -C "/tmp/tmp.8t03ifDFAx" -xf -
xorriso -osirrox on -indev firmware-9.3.0-i386-netinst.iso -extract / /tmp/tmp.8t03ifDFAx
and make the files in /tmp/tmp.8t03ifDFAx writable:
chmod -R +w /tmp/tmp.8t03ifDFAx
A preseed configuration file can now be put in /tmp/tmp.8t03ifDFAx:
cp preseed.cfg /tmp/tmp.8t03ifDFAx
Appending Boot Parameters to the ISO
Edit /tmp/tmp.8t03ifDFAx/isolinux/txt.cfg to add the required parameters to the end of the append line. What follows is a personal choice but
priority=high locale=en_GB.UTF-8 keymap=gb file=/cdrom/preseed.cfg
shows the intent. The language and keymap questions will not be asked and the rest of the installation will proceed under the guidance of preseed.cfg. preseed.cfg is found by the installer because the image is mounted on /cdrom.
Putting the ISO Back Together
Install genisoimage and do
genisoimage -r -J -b isolinux/isolinux.bin -c isolinux/boot.cat \ -no-emul-boot -boot-load-size 4 -boot-info-table \ -o preseed-firmware-9.3.0-i386-netinst.iso /tmp/tmp.8t03ifDFAx
(Leave out \ when using the command. It is only here to indicate a line-break ).
The original ISO was capable of being put on a USB stick at /dev/sdX (X depends on what lsblk gives when the stick is plugged in) with
cat firmware-9.3.0-i386-netinst.iso > /dev/sdX
and would have been bootable. The ISO can be restored to a bootable state with
Alternatively, the genisoimage and isohybrid commands can be replaced by the single command:
xorriso -as mkisofs -o preseed-firmware-9.3.0-i386-netinst.iso \ -isohybrid-mbr /usr/lib/ISOLINUX/isohdpfx.bin \ -c isolinux/boot.cat -b isolinux/isolinux.bin -no-emul-boot \ -boot-load-size 4 -boot-info-table /tmp/tmp.8t03ifDFAx/
after installing isolinux.
Finally, remove /tmp/tmp.8t03ifDFAx:
rm -r /tmp/tmp.8t03ifDFAx