Alter command to extract iso content , tar cf - . | (cd /tmp/testiso_temp; tar xfp -)

andrewysk · 04-05-2021, 06:39 PM

I have mounted an bootable iso image onto /mnt/testiso

Code:

$df -H
/dev/loop12      17M   17M     0 100% /mnt/testiso

$pwd
/mnt/testiso

As you can see the content of the iso. 
$ ls
boot  boot.catalog  efi.img  mach_kernel  System


$ tar cf - . | (cd /tmp/testiso_temp; tar xfp -)

this is used to extract the content of the iso to a temporary folder (/tmp/testiso_temp).

Can you please explain to me how this tar with all those options extract the content of the mounted iso to a new file for me to modify ?

I tried to understand the tar options, but just can't get it.

Code:

$ tar cf - /mnt/testiso/ | (cd /tmp/testiso_temp; tar xfp -)

when i tried to change the code as above, the output of it is weird.. instead of it extract the files to /tmp/testiso_temp, it even move the whole /mnt/testiso to /tmp/testiso_temp...
why ?

I just want to alter the code so that i can insert iso path instead of have to be in the directory itself..

Thx
Thx

shruggy · 04-05-2021, 06:51 PM

See B.1 Copying directory hierarchies in the GNU tar manual.

The first command, tar cf - /mnt/testiso/, creates a tar archive from files in /mnt/testiso/ and writes it to the standard output.

Pipe feeds that tar archive to the subshell.

In the subshell, cd changes to /tmp/testiso_temp, and tar unpacks the archive fed from the pipe.

computersavvy · 04-05-2021, 08:06 PM

A much simpler way to copy the files to a different location would be to use either the copy command (cp) or rsync.

Code:

cp -ar /mnt/testiso/* your/destination/directory/

or

rsync -av /mnt/testiso/* your/destination/directory/

instead of having the overhead of creating then extracting the tar file.

jefro · 04-05-2021, 08:32 PM

http://isgsp.net.tripod.com/rick/cmds/tar.html

Somewhat similar.

MadeInGermany · 04-06-2021, 12:15 AM

Code:

tar cf - /mnt/testiso/

stores /mnt/testiso/* in the tar file (that exists as a strean). GNU tar omits the leading / so it becomes mnt/testiso/*

Code:

(cd /tmp/testiso_temp; tar xfp -)

extracts the file names from the tar file (stream), e.g. with a leading mnt/iso, so the resulting file names are /tmp/testiso_temp/mnt/testiso/*

Use another cd and store the short file names!

Code:

(cd /mnt/testiso && tar cf - .) | (cd /tmp/testiso_temp && tar xfp -)

Unlike ; the && only continues with the next command if the previous command had a good exit status.

rnturn · 04-06-2021, 02:35 AM

Quote:

Originally Posted by andrewysk

I have mounted an bootable iso image onto /mnt/testiso

Code:

$ tar cf - . | (cd /tmp/testiso_temp; tar xfp -)

this is used to extract the content of the iso to a temporary folder (/tmp/testiso_temp).

Can you please explain to me how this tar with all those options extract the content of the mounted iso to a new file for me to modify ?

It may be in the tar(1) manpage but it's hard to find (and, Ha!, just trying searching for "-") but the "-" used in your tar commands refers to STDIN or STDOUT depending on whether it's used in a tar create or extract operation. By using the pipe symbol you're connecting STDOUT of the process running on the left side of the pipe to STDIN of the process on the right side of the pipe. The neat trick with the parentheses is that they cause a subprocess to be created that, first, switches to the target directory and, then, invokes tar to take input from the pipe and writes its output into the target directory.

Another way to accomplish the directory tree copy operation would be to use cpio(1). See: "info cpio -> Concept Index -> copying directory structures". For your case:

Code:

# cd /mnt/testiso
# find . -depth -print0 | cpio --null -pVd /tmp/testiso_temp

HTH and have fun...