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Let’s see some archiving and compression tool used in Linux to manage filesystem space. This is free necessaary to recover and free the filesystem where lots of files are generated every day.

TAR: A GNU archiving utility.

Format:

tar [options] archive filesToArchive

Mostly used Options:

-r, --append:                              append files to the end of an archive

-v, --verbose:                            verbosely list files processed

-f, --file=[HOSTNAME:]F            use archive file or device F (default /dev/rmt0)

-t, --list                                      list the contents of an archive

-c, --create                                create a new archive

-x, --extract, --get                            extract files from an archive

-k, --keep-old-files                        keep existing files; don't overwrite them from archive

Example1:

tar -rvf testFile.tar testFile

The above command will create an archive of file testFile into testFile.tar. But, we generally don’t create tar archive of a single file. The tar tool is used to create a single archive of several files or directory.

The main advantage is that one can create an archive of hundreds of files in one go with the same file access and permissions. Later one can send it another machine and un-tar it to have the same file structure.

For example:

Lets’ say we have a file named listOffiles.lst which contains 10 files or directories with their absolute path.

cat listOffiles.lst

/home/user1/test/file1

/home/user1/test/file2

/home/user1/test/testDir

------------------------------

/home/user1/test/file10

Now we will create an archive of all these files, we can do it using a “for loop” which will traverse all the files and create a single archive for it.

for i in $(cat listOffiles.lst );  do

                ##The below command will create an archive of all files listed in listOffiles.lst with name      oneArchive.tar

                tar –rvf oneArchive.tar $i;

done

If you want to see the content of the archive, you can use below command:

tar –tvf oneArchive.tar

Now we have the archive ready. We would like to compress this before transferring it to somewhere else. So now we will see how to compress this archive.

gzip tool: used to compress or expand files

GZIP reduces the size of the named files using Lempel-Ziv coding (LZ77).  Whenever possible, each file is replaced by one with the extension .gz, while keeping the same ownership modes, access and modification times.

Format: gzip [options] fileToCompress

Mostly used options:

 -c --stdout --to-stdout              Writes output on standard output; keep original files unchanged.  If there are several input files, the output consists of a sequence of independently compressed members.

-d --decompress --uncompress              Decompress.

-f --force        :      Forces compression or decompression even if the file has multiple links or the corresponding file already exists, or if the compressed data is read from or written to a terminal.

-l --list              For each compressed file, list the following fields:

                        compressed size: size of the compressed file

                        uncompressed size: size of the uncompressed file

                        ratio: compression ratio (0.0% if unknown)

                        uncompressed_name: name of the uncompressed file

-v --verbose     :         Verbose. Display the name and percentage reduction for each file compressed or decompressed.

Example: To create a zip file i.e. to compress a file, we do following:

gzip fileToCompress

But above command will create a zip file with name fileToCompress.gz and will delete the original file. So here we can use –c options to create zip file and send to console while keeping original file unchanged. Later we can redirect the console output to a file named *.gz.

 gzip –c fileToCompress>zipfile.gz

You can see the difference of size between compressed file and original file.

To see the content of zip file, we can use below command:

tar -xvf zipfile.gz

The output is:

/home/user1/test/file1

/home/user1/test/file2

/home/user1/test/file3

------------------------------

/home/user1/test/file10

Now if you want to de-compress it again, you can do it using below command:

gzip -d zipfile.gz

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