Exercise: Compress and verify

Compress and verify

• Compare compression tools by size and time.
• Create and verify a SHA-256 checksum list.
• Detect a modified file.

Start the exercise

On workstation, start the exercise. It installs bzip2, zip and unzip on servera and removes /root/arch of an earlier run.

[student@workstation ~]$ lab start sa-checksums

Later tasks assume that you are logged in to the server they name: ssh student@servera from workstation, then sudo -i for a root shell.

Make the data

On servera as root (sudo -i), install bzip2 and zip if needed. Make an uncompressed archive etc.tar of /etc in /root/arch and note its size.

[student@workstation ~]$ ssh student@servera
[student@servera ~]$ sudo -i
[root@servera ~]# dnf install -y bzip2 zip unzip > /dev/null
[root@servera ~]# mkdir -p /root/arch && cd /root/arch
[root@servera arch]# tar -cf etc.tar /etc 2> /dev/null
[root@servera arch]# ls -l etc.tar
-rw-r--r--. 1 root root 22538240 Oct  3 17:37 etc.tar

2> /dev/null hides the "Removing leading /" notice and any permission warnings. Your size will differ.

Three compressors

Compress copies of it with gzip, bzip2 and xz, keeping the original, and compare the sizes. Which was slowest? (Use time in front of each command.)

[root@servera arch]# time gzip -k etc.tar
real    0m0.648s
[root@servera arch]# time bzip2 -k etc.tar
real    0m1.529s
[root@servera arch]# time xz -k etc.tar
real    0m8.150s
[root@servera arch]# ls -l etc.tar*
-rw-r--r--. 1 root root 22538240 Oct  3 17:37 etc.tar
-rw-r--r--. 1 root root  4056578 Oct  3 17:43 etc.tar.bz2
-rw-r--r--. 1 root root  4904494 Oct  3 17:43 etc.tar.gz
-rw-r--r--. 1 root root  2996996 Oct  3 17:43 etc.tar.xz

The more effort, the smaller the file: xz is smallest and slowest, gzip is biggest and fastest. Your times and sizes will vary.

Decompress and inspect

Look at the gzip and xz files without decompressing them (gzip -l, xz -l). Then decompress the gzip copy back, keeping both, and prove it equals the original with cmp.

[root@servera arch]# gzip -l etc.tar.gz
         compressed        uncompressed  ratio uncompressed_name
            4904494            22538240  78.2% etc.tar
[root@servera arch]# xz -l etc.tar.xz | tail -2
Strms  Blocks   Compressed Uncompressed  Ratio  Check   Filename
    1       1   2926.8 KiB     21.5 MiB  0.133  CRC64   etc.tar.xz
[root@servera arch]# gunzip -c etc.tar.gz > copy.tar
[root@servera arch]# cmp etc.tar copy.tar && echo identical
identical

gunzip -c writes to standard output and leaves the .gz in place.

Zip a directory

Create proj.zip from a small directory of your choice (for example /etc/ssh), list it, and test it.

[root@servera arch]# zip -rq ssh.zip /etc/ssh
[root@servera arch]# unzip -l ssh.zip | tail -2
---------                     -------
   552973                     15 files
[root@servera arch]# unzip -t ssh.zip | tail -1
No errors detected in compressed data of ssh.zip.

Write checksums and verify

Write SHA-256 checksums of the three compressed archives into SHA256SUMS, and verify the list.

[root@servera arch]# sha256sum etc.tar.gz etc.tar.bz2 etc.tar.xz > SHA256SUMS
[root@servera arch]# cut -c1-20,65- SHA256SUMS
71ee37b3318201c5430d  etc.tar.gz
81dde1770eb2a05ffe79  etc.tar.bz2
022455e1145a49f08e06  etc.tar.xz
[root@servera arch]# sha256sum -c SHA256SUMS
etc.tar.gz: OK
etc.tar.bz2: OK
etc.tar.xz: OK

Catch a modification

Change one byte at the end of etc.tar.gz (append a character) and verify again. Which file fails? What is the exit status?

[root@servera arch]# echo x >> etc.tar.gz
[root@servera arch]# sha256sum -c SHA256SUMS
etc.tar.gz: FAILED
etc.tar.bz2: OK
etc.tar.xz: OK
sha256sum: WARNING: 1 computed checksum did NOT match
[root@servera arch]# echo "status=$?"
status=1

One appended character changed the whole fingerprint.

Grade and finish

Check your work with lab grade sa-checksums, then put the exercise away with lab finish sa-checksums.