In this chapter, you will learn how to put a group of files together into a single archive. You will also learn how to compress an archive file using various compression methods.
Creating an archive
Let's create a backup for all the bash scripts in the /home/elliot directory. As the root user, create a directory named backup in /root:
root@ubuntu-linux:~# mkdir /root/backup
To create an archive, we use the tape archive command tar. The general syntax to create an archive is as follows:
tar -cf archive_name files
The -c option is the shorthand notation of --create, which creates the archive. The -f option is the shorthand notation of --file, which specifies the archive name.
Now let's create an archive named scripts.tar in /root/backup for all the bash scripts in /home/elliot. To do that, we first change to the /home/elliot directory:
root@ubuntu-linux:~# cd /home/elliot
root@ubuntu-linux:/home/elliot#
Then we run the command:
root@ubuntu-linux:/home/elliot# tar -cf /root/backup/scripts.tar *.sh
This will create the archive file scripts.tar in /root/backup, and there will be no command output:
root@ubuntu-linux:/home/elliot# ls -l /root/backup/scripts.tar
-rw-r--r-- 1 root root 20480 Nov 1 23:12 /root/backup/scripts.tar
We could have also added the verbose option -v to see the files that are being archived:
root@ubuntu-linux:/home/elliot# tar -cvf /root/backup/scripts.tar *.sh
3x10.sh
detect.sh
empty.sh
filetype.sh
fun1.sh
game.sh
hello20.sh
hello2.sh
hello3.sh
hello.sh
math.sh
mydate.sh
noweb.sh
numbers.sh
rename.sh
size2.sh
size3.sh
size.sh
Viewing archive contents
You may want to see the contents of an archive. To do that, you can use the -t option along with the -f option followed by the archive you wish to view:
tar -tf archive
For example, to view the contents of the archive scripts.tar that we just created, you can run the command:
root@ubuntu-linux:/home/elliot# tar -tf /root/backup/scripts.tar
3x10.sh
detect.sh
empty.sh
filetype.sh
fun1.sh
game.sh
hello20.sh
hello2.sh
hello3.sh
hello.sh
math.sh
mydate.sh
noweb.sh
numbers.sh
rename.sh
size2.sh
size3.sh
size.sh
As you can see, it listed all the files in the scripts.tar archive.
Extracting archive files
You may also want to extract files from an archive. To demonstrate, let's create a directory named myscripts in /root:
root@ubuntu-linux:/# mkdir /root/myscripts
To extract files from an archive, we use the -x option along with the -f option, followed by the archive name. Then, we use the -C option followed by the destination directory as follows:
tar -xf archive -C destination
So to extract all the files in the scripts.tar archive to the /root/myscripts directory, you can run the following command:
root@ubuntu-linux:/# tar -xf /root/backup/scripts.tar -C /root/myscripts
The -x option is the shorthand notation of --extract, which extracts the files from the archive. We also used the -C option, which basically changes to the /root/myscripts directory before carrying out any operation, and thus the files are extracted to /root/myscripts instead of the current directory.
Now let's verify that the files were indeed extracted to the /root/myscripts directory:
root@ubuntu-linux:/# ls /root/myscripts
3x10.sh
empty.sh
fun1.sh
hello20.sh
hello3.sh
math.sh
noweb.sh
rename.sh
size3.sh
detect.sh
filetype.sh
game.sh
hello2.sh
hello.sh
mydate.sh
numbers.sh
size2.sh
size.sh
And sure enough, we see all our bash scripts in the /root/myscripts directory!
Compressing with gzip
Grouping files in an archive doesn't save disk space on its own. We would need to compress an archive to save disk space. Numerous compression methods are available for us to use on Linux. However, we are only going to cover the three most popular compression methods.
The most popular compression method on Linux is arguably gzip, and the upside is that it's really fast. You can compress an archive file with gzip by using the -z option with the tar command as follows:
tar -czf compressed_archive archive_name
So to compress the scripts.tar archive into a gzip-compressed archive named scripts.tar.gz, you first need to change to the /root/backup directory and then run the following command:
root@ubuntu-linux:~/backup# tar -czf scripts.tar.gz scripts.tar
Now if you list the contents of the backup directory, you will see the newly created gzip-compressed archive scripts.tar.gz:
root@ubuntu-linux:~/backup# ls
scripts.tar scripts.tar.gz
The magic happened by using the -z option, which compressed the archive with the gzip compression method. And that's it! Notice how it's very similar to creating an archive: we just added the -z option – that's the only difference.
Now let's run the file command on both archives:
root@ubuntu-linux:~/backup# file scripts.tar
scripts.tar: POSIX tar archive (GNU)
root@ubuntu-linux:~/backup# file scripts.tar.gz
scripts.tar.gz: gzip compressed data, last modified: Sat Nov 2 22:13:44 2019,
from Unix
As you can see, the file command detects the type of both archives. Now let’s compare the size (in bytes) of both archives:
root@ubuntu-linux:~/backup# du -b scripts.tar scripts.tar.gz
20480 scripts.tar
1479 scripts.tar.gz
The compressed archive scripts.tar.gz is way smaller in size as we expected compared to the uncompressed archive scripts.tar. If you want to extract the files in the compressed archive scripts.tar.gz to /root/myscripts, you can run:
root@ubuntu-linux:~/backup# tar -xf scripts.tar.gz -C /root/myscripts
Notice it is exactly the same as the way that you would extract the contents of an uncompressed archive.
Compressing with bzip2
bzip2 is another popular compression method used on Linux. On average, bzip2 is slower than gzip; however, bzip2 does a better job of compressing files to smaller sizes.
You can compress an archive with bzip2 compression by using the -j option with the tar command as follows:
tar -cjf compressed_archive archive_name
Notice the only difference here is that we use the -j option for bzip2 compression instead of -z for gzip compression.
So to compress the scripts.tar archive into a bzip2-compressed archive named scripts.tar.bz2, you first need to change to the /root/backup directory and then run the following command:
root@ubuntu-linux:~/backup# tar -cjf scripts.tar.bz2 scripts.tar
Now if you list the contents of the backup directory, you will see the newly created bzip2-compressed archive scripts.tar.bz2:
root@ubuntu-linux:~/backup# ls
scripts.tar scripts.tar.bz2 scripts.tar.gz
Let's run the file command on the bzip2-compressed archive scripts.tar.bz2:
root@ubuntu-linux:~/backup# file scripts.tar.bz2
scripts.tar.bz2: bzip2 compressed data, block size = 900k
It correctly detects the type of compression method used for the archive scripts.tar.bz2. Awesome – now let's compare the size (in bytes) of the gzip-compressed archive scripts.tar.gz and the bzip2-compressed archive scripts.tar.bz2:
root@ubuntu-linux:~/backup# du -b scripts.tar.bz2 scripts.tar.gz
1369 scripts.tar.bz2
1479 scripts.tar.gz
Notice that the bzip2-compressed archive scripts.tar.bz2 is smaller than the gzip-compressed archive scripts.tar.gz. If you want to extract the files in the compressed archive scripts.tar.bz2 to /root/myscripts, you can run:
root@ubuntu-linux:~/backup# tar -xf scripts.tar.bz2 -C /root/myscripts
Notice it is exactly the same as the way that you would extract the contents of a gzip-compressed archive.
Compressing with xz
The xz compression method is yet another popular compression method used on Linux. On average, xz compression does the best job out of all three compression methods in reducing (compressing) the file sizes.
You can compress an archive with xz compression by using the -J option with the tar command as follows:
tar -cJf compressed_name archive_name
Notice here we use the uppercase letter J with xz compression. So to compress the scripts.tar archive into an xz-compressed archive named scripts.tar.xz, you first need to change to the /root/backup directory and then run the following command:
root@ubuntu-linux:~/backup# tar -cJf scripts.tar.xz scripts.tar
Now if you list the contents of the backup directory, you will see the newly created xz-compressed archive scripts.tar.xz:
root@ubuntu-linux:~/backup# ls
scripts.tar scripts.tar.bz2 scripts.tar.gz scripts.tar.xz
Let's run the file command on the xz-compressed archive scripts.tar.xz:
root@ubuntu-linux:~/backup# file scripts.tar.xz
scripts.tar.xz: XZ compressed data
It correctly detects the type of compression method used for the archive scripts.tar.xz.
Measuring performance
You can use the time command to measure the time it takes a command (or a program) to finish executing. The general syntax for the time command is as follows:
time command_or_program
For example, to measure how long it takes for the date command to finish executing, you can run the following command:
root@ubuntu-linux:~# time date
Sun Nov 3 16:36:33 CST 2019
real 0m0.004s
user 0m0.003s
sys 0m0.000s
It just took four milliseconds to run the date command on my system; this is quite fast!
The gzip compression method is the fastest of all three compression methods; well, let's see if I am lying or telling the truth! Change to the /root/backup directory:
root@ubuntu-linux:~# cd /root/backup
root@ubuntu-linux:~/backup#
Now let's see how long it takes to create a gzip-compressed archive file for all the files in /boot:
root@ubuntu-linux:~/backup# time tar -czf boot.tar.gz /boot
real 0m4.717s
user 0m4.361s
sys 0m0.339s
On my system, it took gzip 4.717 seconds to run! Now let's measure the time it takes to create a bzip2-compressed archive of the same directory /boot:
root@ubuntu-linux:~/backup# time tar -cjf boot.tar.bz2 /boot
real 0m19.306s
user 0m18.809s
sys 0m0.359s
It took bzip2 an enormous 19.306 seconds to run! You can see how gzip compression is much faster than bzip2. Now let's see the time it takes to create an xz-compressed archive of the same directory /boot:
root@ubuntu-linux:~/backup# time tar -cJf boot.tar.xz /boot
real 0m53.745s
user 0m52.679s
sys 0m0.873s
It almost took xz a full minute! We can conclude that gzip is definitely the fastest of all three compression methods we have discussed.
Finally, let's check the size (in bytes) of the three compressed archives:
root@ubuntu-linux:~/backup# du -b boot.*
97934386 boot.tar.bz2
98036178 boot.tar.gz
94452156 boot.tar.xz
As you can see, xz did the best job of compressing the files. bzip2 claimed second place, and gzip came in last.
Knowledge check
For the following exercises, open up your Terminal and try to solve the following tasks:
- Create a gzip archive named var.tar.gz in /root for all the files in /var.
- Create a bzip2 archive named tmp.tar.bz2 in /root for all the files in /tmp.
- Create an xz archive named etc.tar.xz in /root for all the files in /etc.