$ mkdir ~/linux_essentials-3.1
$ cd ~/linux_essentials-3.1
$ mkdir compression archiving
$ cd compression
$ cat /etc/* > bigfile 2> /dev/null
    

$ cp bigfile bigfile2
$ cp bigfile bigfile3
$ cp bigfile bigfile4
$ ls -lh
total 2.8M
-rw-r--r-- 1 emma emma 712K Jun 23 08:08 bigfile
-rw-r--r-- 1 emma emma 712K Jun 23 08:08 bigfile2
-rw-r--r-- 1 emma emma 712K Jun 23 08:08 bigfile3
-rw-r--r-- 1 emma emma 712K Jun 23 08:08 bigfile4
    

$ bzip2 bigfile2
$ gzip bigfile3
$ xz bigfile4
$ ls -lh
total 1.2M
-rw-r--r-- 1 emma emma 712K Jun 23 08:08 bigfile
-rw-r--r-- 1 emma emma 170K Jun 23 08:08 bigfile2.bz2
-rw-r--r-- 1 emma emma 179K Jun 23 08:08 bigfile3.gz
-rw-r--r-- 1 emma emma 144K Jun 23 08:08 bigfile4.xz
    

$ bunzip2 bigfile2.bz2
$ gunzip bigfile3.gz
$ unxz bigfile4.xz
$ ls -lh
total 2.8M
-rw-r--r-- 1 emma emma 712K Jun 23 08:20 bigfile
-rw-r--r-- 1 emma emma 712K Jun 23 08:20 bigfile2
-rw-r--r-- 1 emma emma 712K Jun 23 08:20 bigfile3
-rw-r--r-- 1 emma emma 712K Jun 23 08:20 bigfile4
    

$ cp bigfile bigfile-gz1
$ cp bigfile bigfile-gz9
$ gzip -1 bigfile-gz1
$ gzip -9 bigfile-gz9
$ cp bigfile bigfile-xz1
$ cp bigfile bigfile-xz9
$ xz -1 bigfile bigfile-xz1
$ xz -9 bigfile bigfile-xz9
$ ls -lh bigfile bigfile-* *
total 3.5M
-rw-r--r-- 1 emma emma 712K Jun 23 08:08 bigfile
-rw-r--r-- 1 emma emma 205K Jun 23 13:14 bigfile-gz1.gz
-rw-r--r-- 1 emma emma 178K Jun 23 13:14 bigfile-gz9.gz
-rw-r--r-- 1 emma emma 156K Jun 23 08:08 bigfile-xz1.xz
-rw-r--r-- 1 emma emma 143K Jun 23 08:08 bigfile-xz9.xz
    

$ cd ~/linux_essentials-3.1
$ tar cf archiving/3.1.tar compression
    

$ tar -tf 3.1.tar
compression/
compression/bigfile-xz1.xz
compression/bigfile-gz9.gz
compression/hosts.gz
compression/bigfile2
compression/bigfile
compression/bigfile-gz1.gz
compression/bigfile-xz9.xz
compression/bigfile3
compression/bigfile4
    

$ cd ~/linux_essentials-3.1/archiving
$ ls
3.1.tar
$ tar xf 3.1.tar
$ ls
3.1.tar compression
    

$ cd ~/linux_essentials-3.1/archiving
$ rm -rf compression
$ ls
3.1.tar
$ tar xvf 3.1.tar compression/hosts.gz
compression/
compression/bigfile-xz1.xz
compression/bigfile-gz9.gz
compression/hosts.gz
compression/bigfile2
compression/bigfile
compression/bigfile-gz1.gz
compression/bigfile-xz9.xz
compression/bigfile3
compression/bigfile4
$ ls
3.1.tar compression
$ ls compression
hosts.gz
    

$ cd ~/linux_essentials-3.1/archiving
$ rm -rf compression
$ cd ../compression
$ tar cf ../tar/3.1-nodir.tar *
$ cd ../archiving
$ mkdir untar
$ cd untar
$ tar -xf ../3.1-nodir.tar
$ ls
bigfile bigfile3 bigfile-gz1.gz bigfile-xz1.xz hosts.gz
bigfile2 bigfile4 bigfile-gz9.gz bigfile-xz9.xz
    

$ cd ~/linux_essentials-3.1/compression
$ ls
bigfile bigfile3 bigfile-gz1.gz bigfile-xz1.xz hosts.gz
bigfile2 bigfile4 bigfile-gz9.gz bigfile-xz9.xz
$ tar -czf gzip.tar.gz bigfile bigfile2 bigfile3
$ tar -cjf bzip2.tar.bz2 bigfile bigfile2 bigfile3
$ tar -cJf xz.tar.xz bigfile bigfile2 bigfile3
$ ls -l | grep tar
-rw-r--r-- 1 emma emma 450202 Jun 27 05:56 bzip2.tar.bz2
-rw-r--r-- 1 emma emma 548656 Jun 27 05:55 gzip.tar.gz
-rw-r--r-- 1 emma emma 147068 Jun 27 05:56 xz.tar.xz
    

$ cd ~/linux_essentials-3.1/compression
$ ls
bigfile bigfile3 bigfile-gz1.gz bigfile-xz1.xz bzip2.tar.bz2 hosts.gz
bigfile2 bigfile4 bigfile-gz9.gz bigfile-xz9.xz gzip.tar.gz xz.tar.xz
$ tar cf plain.tar bigfile bigfile2 bigfile3
$ tar tf plain.tar
bigfile
bigfile2
bigfile3
$ tar uf plain.tar bigfile4
$ tar tf plain.tar
bigfile
bigfile2
bigfile3
bigfile4
$ tar uzf gzip.tar.gz bigfile4
tar: Cannot update compressed archives
Try 'tar --help' or 'tar --usage' for more information.
    

$ cd ~/linux_essentials-3.1
$ mkdir zip
$ cd zip/
$ mkdir dir
$ touch dir/file1 dir/file2
    

$ zip -r zipfile.zip dir
  adding: dir/ (stored 0%)
  adding: dir/file1 (stored 0%)
  adding: dir/file2 (stored 0%)
$ rm -rf dir
    

$ ls
zipfile.zip
$ unzip zipfile.zip
Archive: zipfile.zip
  creating: dir/
 extracting: dir/file1
 extracting: dir/file2
$ find
.
./zipfile.zip
./dir
./dir/file1
./dir/file2
    

שם הקובץ      tar  gzip  bzip2  xz
archive.tar   
archive.tgz   
archive.tar.xz
    

שם הקובץ      ארכיון   דחוס
file.tar      
file.tar.bz2  
file.zip      
file.xz       
    

שם הקובץ       tar  gzip  bzip2  xz
archive.tar    X
archive.tgz    X    X
archive.tar.xz X          X
    

שם הקובץ       ארכיון   דחוס
file.tar       X
file.tar.bz2   X       X
file.zip       X       X
file.xz        X
    

-P, --absolute-names
  אל תסיר את הסלאשים המובילים משמות הקבצים בעת יצירת ארכיונים
    

-#
(-0, -1, -2, -3, -4, -5, -6, -7, -8, -9)
  מווסת את מהירות הדחיסה באמצעות הספרה #,
  כאשר -0 מציין ללא דחיסה, -1 מציין את מהירות הדחיסה המהירה ביותר (פחות דחיסה), ו-9 מציין את מהירות הדחיסה האיטית ביותר (דחיסה אופטימלית).
    

$ tar xf tarfile.tar --wildcards dir/file*
$ tar --wildcards -xf tarfile.tar dir/file*
    

$ gunzip archive.tgz
$ tar xf archive.tar
    

Introduction

Compression is used to reduce the amount of space a specific set of data consumes. Compression is

commonly used for reducing the amount of space that is needed to store a file. Another common use

is to reduce the amount of data sent over a network connection.

Compression works by replacing repetitive patterns in data. Suppose you have a novel. Some words

are extremely common but have multiple characters, such as the word “the”. You could reduce the

size of the novel significantly if you were to replace these common multi character words and

patterns with single character replacements. E.g., replace “the” with a greek letter that is not used

elsewhere in the text. Data compression algorithms are similar to this but more complex.

Compression comes in two varieties, lossless and lossy. Things compressed with a lossless algorithm

can be decompressed back into their original form. Data compressed with a lossy algorithm cannot

be recovered. Lossy algorithms are often used for images, video, and audio where the quality loss is

imperceptible to humans, irrelevant to the context, or the loss is worth the saved space or network

throughput.

Archiving tools are used to bundle up files and directories into a single file. Some common uses are

backups, bundling software source code, and data retention.

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Archive and compression are commonly used together. Some archiving tools even compress their

contents by default. Others can optionally compress their contents. A few archive tools must be used

in conjunction with stand-alone compression tools if you wish to compress the contents.

The most common tool for archiving files on Linux systems is tar. Most Linux distributions ship

with the GNU version of tar, so it is the one that will be covered in this lesson. tar on its own only

manages the archiving of files but does not compress them.

There are lots of compression tools available on Linux. Some common lossless ones are bzip2, gzip,

and xz. You will find all three on most systems. You may encounter an old or very minimal system

where xz or bzip is not installed. If you become a regular Linux user, you will likely encounter files

compressed with all three of these. All three of them use different algorithms, so a file compressed

with one tool can’t be decompressed by another. Compression tools have a trade off. If you want a

high compression ratio, it will take longer to compress and decompress the file. This is because

higher compression requires more work finding more complex patterns. All of these tools compress

data but can not create archives containing multiple files.

Stand-alone compression tools aren’t typically available on Windows systems. Windows archiving

and compression tools are usually bundled together. Keep this in mind if you have Linux and

Windows systems that need to share files.

Linux systems also have tools for handling .zip files commonly used on Windows system. They are

called zip and unzip. These tools are not installed by default on all systems, so if you need to use

them you may have to install them. Fortunately, they are typically found in distributions’ package

repositories.

Compression Tools

How much disk space is saved by compressing files depends on a few factors. The nature of the data

you are compressing, the algorithm used to compress the data, and the compression level. Not all

algorithms support different compression levels.

Let’s start with setting up some test files to compress:

$ mkdir ~/linux_essentials-3.1

$ cd ~/linux_essentials-3.1

$ mkdir compression archiving

$ cd compression

$ cat /etc/* > bigfile 2> /dev/null

Now we create three copies of this file:

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$ cp bigfile bigfile2

$ cp bigfile bigfile3

$ cp bigfile bigfile4

$ ls -lh

total 2.8M

-rw-r–r– 1 emma emma 712K Jun 23 08:08 bigfile

-rw-r–r– 1 emma emma 712K Jun 23 08:08 bigfile2

-rw-r–r– 1 emma emma 712K Jun 23 08:08 bigfile3

-rw-r–r– 1 emma emma 712K Jun 23 08:08 bigfile4

Now we are going to compress the files with each aforementioned compression tool:

$ bzip2 bigfile2

$ gzip bigfile3

$ xz bigfile4

$ ls -lh

total 1.2M

-rw-r–r– 1 emma emma 712K Jun 23 08:08 bigfile

-rw-r–r– 1 emma emma 170K Jun 23 08:08 bigfile2.bz2

-rw-r–r– 1 emma emma 179K Jun 23 08:08 bigfile3.gz

-rw-r–r– 1 emma emma 144K Jun 23 08:08 bigfile4.xz

Compare the sizes of the compressed files to the uncompressed file named bigfile. Also notice how

the compression tools added extensions to the file names and removed the uncompressed files.

Use bunzip2, gunzip, or unxz to decompress the files:

$ bunzip2 bigfile2.bz2

$ gunzip bigfile3.gz

$ unxz bigfile4.xz

$ ls -lh

total 2.8M

-rw-r–r– 1 emma emma 712K Jun 23 08:20 bigfile

-rw-r–r– 1 emma emma 712K Jun 23 08:20 bigfile2

-rw-r–r– 1 emma emma 712K Jun 23 08:20 bigfile3

-rw-r–r– 1 emma emma 712K Jun 23 08:20 bigfile4

Notice again that now the compressed file is deleted once it is decompressed.

Some compression tools support different compression levels. A higher compression level usually

requires more memory and CPU cycles, but results in a smaller compressed file. The opposite is true

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for a lower level. Below is a demonstration with xz and gzip:

$ cp bigfile bigfile-gz1

$ cp bigfile bigfile-gz9

$ gzip -1 bigfile-gz1

$ gzip -9 bigfile-gz9

$ cp bigfile bigfile-xz1

$ cp bigfile bigfile-xz9

$ xz -1 bigfile bigfile-xz1

$ xz -9 bigfile bigfile-xz9

$ ls -lh bigfile bigfile-* *

total 3.5M

-rw-r–r– 1 emma emma 712K Jun 23 08:08 bigfile

-rw-r–r– 1 emma emma 205K Jun 23 13:14 bigfile-gz1.gz

-rw-r–r– 1 emma emma 178K Jun 23 13:14 bigfile-gz9.gz

-rw-r–r– 1 emma emma 156K Jun 23 08:08 bigfile-xz1.xz

-rw-r–r– 1 emma emma 143K Jun 23 08:08 bigfile-xz9.xz

It is not necessary to decompress a file every time you use it. Compression tools typically come with

special versions of common tools used to read text files. For example, gzip has a version of cat, grep,

diff, less, more, and a few others. For gzip, the tools are prefixed with a z, while the prefix bz exists

for bzip2 and xz exists for xz. Below is an example of using zcat to read display a file compressed

with gzip:

$ cp /etc/hosts ./

$ gzip hosts

$ zcat hosts.gz

127.0.0.1 localhost

# The following lines are desirable for IPv6 capable hosts

::1 localhost ip6-localhost ip6-loopback

ff02::1 ip6-allnodes

ff02::2 ip6-allrouters

Archiving Tools

The tar program is probably the most widely used archiving tool on Linux systems. In case you are

wondering why it is named how it is, it as an abbreviation for “tape archive”. Files created with tar

are often called tar balls. It is very common for applications distributed as source code to be in tar

balls.

The GNU version of tar that Linux distributions ship with has a lot of options. This lesson is going
to cover the most commonly used subset.
Let’s start off by creating an archive of the files used for compression:
$ cd ~/linux_essentials-3.1
$ tar cf archiving/3.1.tar compression
The c option instructs tar to create a new archive file and the f option is the name of the file to
create. The argument immediately following the options is always going to be the name of the file to
work on. The rest of the arguments are the paths to any files or directories you wish to add to, list,
or extract from the file. In the example, we are adding the directory compression and all of its
contents to the archive.
To view the contents of a tar ball, use the t option of tar:
$ tar -tf 3.1.tar
compression/
compression/bigfile-xz1.xz
compression/bigfile-gz9.gz
compression/hosts.gz
compression/bigfile2
compression/bigfile
compression/bigfile-gz1.gz
compression/bigfile-xz9.xz
compression/bigfile3
compression/bigfile4
Notice how the options are preceded with -. Unlike most programs, with tar, the – isn’t required
when specifying options, although it doesn’t cause any harm if it is used.
NOTE You can use the -v option to let tar output the names of files it operates on when
creating or extracting an archive.
Now let’s extract the file:
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$ cd ~/linux_essentials-3.1/archiving
$ ls
3.1.tar
$ tar xf 3.1.tar
$ ls
3.1.tar compression
Suppose you only need one file out of the archive. If this is the case, you can specify it after the
archive’s file name. You can specify multiple files if necessary:
$ cd ~/linux_essentials-3.1/archiving
$ rm -rf compression
$ ls
3.1.tar
$ tar xvf 3.1.tar compression/hosts.gz
compression/
compression/bigfile-xz1.xz
compression/bigfile-gz9.gz
compression/hosts.gz
compression/bigfile2
compression/bigfile
compression/bigfile-gz1.gz
compression/bigfile-xz9.xz
compression/bigfile3
compression/bigfile4
$ ls
3.1.tar compression
$ ls compression
hosts.gz
With the exception of absolute paths (paths beginning with /), tar files preserve the entire path to
files when they are created. Since the file 3.1.tar was created with a single directory, that directory
will be created relative to your current working directory when extracted. Another example should
clarify this:
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$ cd ~/linux_essentials-3.1/archiving
$ rm -rf compression
$ cd ../compression
$ tar cf ../tar/3.1-nodir.tar *
$ cd ../archiving
$ mkdir untar
$ cd untar
$ tar -xf ../3.1-nodir.tar
$ ls
bigfile bigfile3 bigfile-gz1.gz bigfile-xz1.xz hosts.gz
bigfile2 bigfile4 bigfile-gz9.gz bigfile-xz9.xz
TIP If you wish to use the absolute path in a tar file, you must use the P option. Be aware
that this may overwrite important files and might cause errors on your system.
The tar program can also manage compression and decompression of archives on the fly. tar does
so by calling one of the compression tools discussed earlier in this section. It is as simple as adding
the option appropriate to the compression algorithm. The most commonly used ones are j, J, and z
for bzip2, xz, and gzip, respectively. Below are examples using the aforementioned algorithms:
$ cd ~/linux_essentials-3.1/compression
$ ls
bigfile bigfile3 bigfile-gz1.gz bigfile-xz1.xz hosts.gz
bigfile2 bigfile4 bigfile-gz9.gz bigfile-xz9.xz
$ tar -czf gzip.tar.gz bigfile bigfile2 bigfile3
$ tar -cjf bzip2.tar.bz2 bigfile bigfile2 bigfile3
$ tar -cJf xz.tar.xz bigfile bigfile2 bigfile3
$ ls -l | grep tar
-rw-r–r– 1 emma emma 450202 Jun 27 05:56 bzip2.tar.bz2
-rw-r–r– 1 emma emma 548656 Jun 27 05:55 gzip.tar.gz
-rw-r–r– 1 emma emma 147068 Jun 27 05:56 xz.tar.xz
Notice how in the example the .tar files have different sizes. This shows that they were successfully
compressed. If you create compressed .tar archives, you should always add a second file extension
denoting the algorithm you used. They are .xz, .bz, and .gz for xz, bzip2, and gzip, respectively.
Sometimes shortened extensions such as .tgz are used.
It is possible to add files to already existing uncompressed tar archives. Use the u option to do this. If
you attempt to add to a compressed archive, you will get an error.
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$ cd ~/linux_essentials-3.1/compression
$ ls
bigfile bigfile3 bigfile-gz1.gz bigfile-xz1.xz bzip2.tar.bz2 hosts.gz
bigfile2 bigfile4 bigfile-gz9.gz bigfile-xz9.xz gzip.tar.gz xz.tar.xz
$ tar cf plain.tar bigfile bigfile2 bigfile3
$ tar tf plain.tar
bigfile
bigfile2
bigfile3
$ tar uf plain.tar bigfile4
$ tar tf plain.tar
bigfile
bigfile2
bigfile3
bigfile4
$ tar uzf gzip.tar.gz bigfile4
tar: Cannot update compressed archives
Try ‘tar –help’ or ‘tar –usage’ for more information.

Managing ZIP files
Windows machines often don’t have applications to handle tar balls or many of the compression
tools commonly found on Linux systems. If you need to interact with Windows systems, you can use
ZIP files. A ZIP file is an archive file similar to a compressed tar file.
The zip and unzip programs can be used to work with ZIP files on Linux systems. The example
below should be all you need to get started using them. First we create a set of files:
$ cd ~/linux_essentials-3.1
$ mkdir zip
$ cd zip/
$ mkdir dir
$ touch dir/file1 dir/file2
Now we use zip to pack these files into a ZIP file:
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$ zip -r zipfile.zip dir
adding: dir/ (stored 0%)
adding: dir/file1 (stored 0%)
adding: dir/file2 (stored 0%)
$ rm -rf dir
Finally, we unpack the ZIP file again:
$ ls
zipfile.zip
$ unzip zipfile.zip
Archive: zipfile.zip
creating: dir/
extracting: dir/file1
extracting: dir/file2
$ find
.
./zipfile.zip
./dir
./dir/file1
./dir/file2
When adding directories to ZIP files, the -r option causes zip to include a directory’s contents.
Without it, you would have an empty directory in the ZIP file.
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Guided Exercises
1. According to the extensions, which of the following tools were used to create these files?
Filename tar gzip bzip2 xz
archive.tar
archive.tgz
archive.tar.xz
2. According to the extensions, which of these files are archives and which are compressed?
Filename Archive Compressed
file.tar
file.tar.bz2
file.zip
file.xz
3. How would you add a file to a gzip compressed tar file?
4. Which tar option instructs tar to include the leading / in absolute paths?
5. Does zip support different compression levels?
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Explorational Exercises
1. When extracting files, does tar support globs in the file list?
2. How can you make sure a decompressed file is identical to the file before it was compressed?
3. What happens if you try to extract a file from a tar archive that already exists on your
filesystem?
4. How would you extract the file archive.tgz without using the tar z option?
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Summary
Linux systems have several compression and archiving tools available. This lesson covered the most
common ones. The most common archiving tool is tar. If interacting with Windows systems is
necessary, zip and unzip can create and extract ZIP files.
The tar command has a few options that are worth memorizing. They are x for extract, c for create,
t for view contents, and u to add or replace files. The v option lists the files which are processed by
tar while creating or extracting an archive.
The typical Linux distribution’s repository has many compression tools. The most common are gzip,
bzip2, and xz. Compression algorithms often support different levels that allow you to optimize for
speed or file size. Files can be decompressed with gunzip, bunzip2, and unxz.
Compression tools commonly have programs that behave like common text file tools, with the
difference being they work on compressed files. A few of them are zcat, bzcat, and xzcat.
Compression tools typically ship with programs with the functionality of grep, more, less, diff, and
cmp.
Commands used in the exercises:
bunzip2
Decompress a bzip2 compressed file.
bzcat
Output the contents of a bzip compressed file.
bzip2
Compress files using the bzip2 algorithm and format.
gunzip
Decompress a gzip compressed file.
gzip
Compress files using the gzip algorithm and format.
tar
Create, update, list and extract tar archives.
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unxz
Decompress a xz compressed file.
unzip
Decompress and extract content from a ZIP file.
xz Compress files using the xz algorithm and format.
zcat
Output the contents of a gzip compressed file.
zip
Create and compress ZIP archives.

Answers to Guided Exercises
1. According to the extensions, which of the following tools were used to create these files?
Filename tar gzip bzip2 xz
archive.tar X
archive.tgz X X
archive.tar.xz X X
2. According to the extensions, which of these files are archives and which are compressed?
Filename Archive Compressed
file.tar X
file.tar.bz2 X X
file.zip X X
file.xz X
3. How would you add a file to a gzip compressed tar file?
You would decompress the file with gunzip, add the file with tar uf, and then compress it with
gzip
4. Which tar option instructs tar to include the leading / in absolute paths?
The -P option. From the man page:
-P, –absolute-names
Don’t strip leading slashes from file names when creating archives
5. Does zip support different compression levels?
Yes. You would use -#, replacing # with a number from 0-9. From the man page:
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-#
(-0, -1, -2, -3, -4, -5, -6, -7, -8, -9)
Regulate the speed of compression using the specified digit #,
where -0 indicates no compression (store all files), -1 indi‐
cates the fastest compression speed (less compression) and -9
indicates the slowest compression speed (optimal compression,
ignores the suffix list). The default compression level is -6.
Though still being worked, the intention is this setting will
control compression speed for all compression methods. Cur‐
rently only deflation is controlled.
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Answers to Explorational Exercises
1. When extracting files, does tar support globs in the file list?
Yes, you would use the –wildcards option. –wildcards must be placed right after the tar file
when using the no dash style of options. For example:
$ tar xf tarfile.tar –wildcards dir/file*
$ tar –wildcards -xf tarfile.tar dir/file*
2. How can you make sure a decompressed file is identical to the file before it was compressed?
You don’t need to do anything with the tools covered in this lesson. All three of them include
checksums in their file format that is verified when they are decompressed.
3. What happens if you try to extract a file from a tar archive that already exists on your
filesystem?
The file on your filesystem is overwritten with the version that is in the tar file.
4. How would you extract the file archive.tgz without using the tar z option?
You would decompress it with gunzip first.
$ gunzip archive.tgz
$ tar xf archive.tar