[SOLVED] sha256sum -c: How to limit to just one ISO file?

andrew.comly · 09-03-2016, 08:57 AM

I have this directory "lubuntu14.04", contents shown below:

Code:

$ ll
total 1406360
-rw-rw-r--. 1 a adm 727711744  5月 18  2015 lubuntu-14.04-desktop-amd64.iso
-rwxrwxrwx. 1 a adm 716177408 12月  1  2014 lubuntu-14.04-desktop-i386.iso*
-rw-rw-r--. 1 a a           0  8月 29 09:27 new.sign
-rw-rw-r--. 1 a a        2722  8月  5 04:40 SHA256SUMS
-rw-rw-r--. 1 a a         933  8月  5 04:40 SHA256SUMS.gpg

I would like to generate a sha256 checksum for the downloaded ISO file "lubuntu-14.04-desktop-i386.iso" and compare it to the official sha256 checksum listed in the downloaded "SHA256SUMS" file.

Attempt 1:

Code:

$ sha256sum -c SHA256SUMS 2>&1 | grep OK
lubuntu-14.04-desktop-amd64.iso: OK
lubuntu-14.04-desktop-i386.iso: OK

Unfortunately the command above checks the "*amd64.iso" file in addition, slowing down the whole process. So I check the sha256sum manual file, and all I can find is:

Quote:

-c, --check
read SHA256 sums from the FILEs and
check them

Actually the manual file for sha256sum is so short, that there probably isn't the functionality I am looking for.

In this next attempt I write the ISO's full name after the "SHA256SUMS" file:

Attempt 2:

Code:

lubuntu14.04$ sha256sum -c SHA256SUMS lubuntu-14.04-desktop-i386.iso 2>&1 | grep OK
lubuntu-14.04-desktop-amd64.iso: OK
lubuntu-14.04-desktop-i386.iso: OK

The command sha256sum still checks the unwanted "*amd64.iso" ISO file.

Is there a way to only check the *.i386.iso file when both ISO files are in the directory "lubuntu14.04"?

rknichols · 09-03-2016, 09:26 AM

Only way I know of is to use a filter to select the one line you want from the sums file:

Code:

fgrep lubuntu-14.04-desktop-i386.iso SHA256SUMS | sha256sum -c -
# or
sha256sum -c <(fgrep lubuntu-14.04-desktop-i386.iso SHA256SUMS)

Habitual · 09-03-2016, 01:06 PM

https://help.ubuntu.com/community/HowToSHA256SUM
https://help.ubuntu.com/community/VerifyIsoHowto

may help.

andrew.comly · 09-04-2016, 05:51 AM

Quote:

Originally Posted by rknichols

Only way I know of is to use a filter to select the one line you want from the sums file:

Code:

fgrep lubuntu-14.04-desktop-i386.iso SHA256SUMS | sha256sum -c -

Works perfectly! Thanks a lot!

Andrew

andrew.comly · 09-04-2016, 06:11 AM

Quote:

Originally Posted by rknichols

sha256sum -c <(fgrep lubuntu-14.04-desktop-i386.iso SHA256SUMS)[/code]

Both solutions work, but if the above command works, and additionally when I execute the fgrep/grep command individually:

Code:

$ fgrep lubuntu-14.04-desktop-i386.iso SHA256SUMS
bba48a1d21720725fcc8f0b8461ba25631c8e28e8ca1aff7c14a1e151290da49 *lubuntu-14.04-desktop-i386.iso

Why then when does the following command produce a different result(fail):

Code:

sha256sum -c bba48a1d21720725fcc8f0b8461ba25631c8e28e8ca1aff7c14a1e151290da49 *lubuntu-14.04-desktop-i386.iso

{fail}

sha__sum and gpg commands are quite peculiar.

rknichols · 09-04-2016, 10:03 AM

There is nothing peculiar about those commands. You are just misunderstanding the shell syntax.

When you write

Code:

sha256sum -c <(fgrep lubuntu-14.04-desktop-i386.iso SHA256SUMS)

the shell runs the fgrep command with its output directed to some high-numbered file descriptor and tells the sha256sum command to read from that pseudo-file:

Code:

sha256sum -c /dev/fd/63

That is not the same as building a command line that includes the output from fgrep. You are confusing the syntax "<(some_command)" with "$(some_command)". They are very different. The first passes the name of a file that can be opened. The second just substitutes text into the command line.