C2110 UNIX and programming Lesson 13 / Module 1 -1C2110
UNIX and programming
Petr Kulhanek
kulhanek@chemi.muni.cz
National Center for Biomolecular Research, Faculty of Science
Masaryk University, Kamenice 5, CZ-62500 Brno
PS / 2020 Distance form of teaching: Rev1
Lesson 13 / Module 1
C2110 UNIX and programming Lesson 13 / Module 1 -2-
Compression
➢ Lossless
➢ Lossy
C2110 UNIX and programming Lesson 13 / Module 1 -3-
Compression
Compression is a procedure that reduces the size of data (files). It is achieved by searching
redundant or irrelevant information in the data, which is then stored more efficiently.
According to the type of compression algorithm, data compression can be divided into two
basic categories:
• lossy compression - there is an irreversible loss of some irrelevant information, which is
usually tolerated when compressing video or audio data
• lossless compression - there is no loss of the original information, the compressed data
can be restored to its original state, the compression rate is several times lower than
the lossy compression
Recovering compressed data is called decompression.
A compression ratio indicates the quality of the compression. It is given as the ratio of the
size of the original data (in bytes) to the size of the compressed data.
C2110 UNIX and programming Lesson 13 / Module 1 -4Lossy
compression
Lossy compression and decompression programs:
• mplayer
• mencoder
• convert (Image Magick)
• and more ...
Convert PNG (Portable Network Graphics) image to JPEG (Joint Photographic Experts
Group) format:
$ convert input.png -quality number output.jpeg
uses lossless
compression
uses lossy
compression
Quality image of the resulting
image from 1 (worst quality with
the highest compression) to 100
(best quality with the worst
compression)
C2110 UNIX and programming Lesson 13 / Module 1 -5Exercise
I
1. From the directory /home/kulhanek/Documents/C2110/Lesson13 copy the image
test.png to your home directory.
2. What is the size of the image file in bytes?
3. Do a lossy compression of the image to format jpeg. Use quality for compression 10, 50
and 90. Save the resulting images separately.
4. Compare visual quality compressed images (display command).
5. What is a compression ratio for quality 10 and 90?
C2110 UNIX and programming Lesson 13 / Module 1 -6Lossless
compression
Programs for lossless compression and decompression:
• gzip/gunzip
• bzip2/bunzip2
• zip/unzip
• and more ...
Compression text file:
$ gzip file.txt
$ bzip2 file.txt the resulting file will be named
soubor.txt.bz2
the resulting file will be named
file.txt.gz
Decompression of compressed data:
$ gunzip file.txt.gz
$ bunzip2 file.txt.bz2
Compression or decompression can be performed by sending the result to standard
output (the original file then remains unchanged), e.g.:
$ bunzip2 -stdout soubor.txt.bz2 | wc
C2110 UNIX and programming Lesson 13 / Module 1 -7Exercise
II
1. Copy the text file bu6_f.log to your home directory from
/home/kulhanek/Documents/C2110/Lesson13 directory.
2. What is the size of the file in bytes?
3. Perform lossless file compression using programs gzip and bzip2. Which program
achieves a higher compression ratio?
4. Which program compresses the file faster?
C2110 UNIX and programming Lesson 13 / Module 1 -8-
Archives
➢ Types
➢ Creating and unpacking archives
C2110 UNIX and programming Lesson 13 / Module 1 -9Archives
- tar
tar (abbreviation from tape archiver) is a collective name for a file format used to store
many individual files, as well as for single-purpose programs that work with this format.
The format itself originated in the early days of Unix and was later standardized under the
POSIX standard. Originally it helped in archiving files on tape drives, but later its use
expanded and today it is used simply where it is appropriate to merge multiple files into
one so that information about directory structure, access rights, and other attributes,
which the file system normally contains, is preserved for distribution or archiving
purposes.
www.wikipedia.org
Unpack archive:
$ tar xvf archive.tar
Create archive:
$ tar cvf archive.tar directory/
$ cd directory
$ tar cvf /path/to/archive.tar *
if archive name contains .gz or .bz2
ending, the archive is automatically
decompressed or compressed
C2110 UNIX and programming Lesson 13 / Module 1 -10Exercise
III
1. What is the meaning of options cvf of tar command?
2. What is themeaning of options xvf of tar command?
3. Create an archive from files stored in the directory:
/home/kulhanek/Documents/C2110/Lesson13/Archive
4. What is the size of the file containing the archive?
5. Compress the archive. What is the compression ratio?
6. Unzip the archive to directory /scratch/your_login/archive