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Extracted from Professor Nikolai Bezroukov unpublished lecture notes.
Copyright 2010-2018, Dr. Nikolai Bezroukov. This is a fragment of the copyrighted unpublished work. All rights reserved.
The most common archive format in Linux is compressed tarball. To create such an archive you need to know ins and outs of the tar command . This command was originally designed to stream files to a tape. By default compression is with gzip. You need to specify bzip2 explicitly. You should be able perform three important tasks:
There are four major types are compressed archives in Linux
Tar is a very old (dates back to Version 6 of AT&T UNIX, circa 1975) zero-compression archiver. But it is not easy to replace it with any archiver that has a zero-compression option (for example zip) as with time it got some unique capabilities and became a standard de-facto for zero compression archiver.
Unlike most archivers, tar can be used as the head or tail of a pipeline. tar is very convenient for moving hierarchies directories hierarchies while preserving all attributes and symbolic and hard links between servers.
Archive created by tar is usually called tarball. A tarball historically was a magnetic tape, but now it's usually a disk file. The default device, /dev/rmt0, is seldom used today, the most common is to archive into file that is often processed additionally by gzip, or, for text files, xz (option -J).
The tar command can specify a list of files or directories and can include name substitution characters. The basic form is
tar keystring options -f tarball filenames... .
The keystring is a string of characters starting with one function letter (c, r, t , u, or x) and zero or more function modifiers (letters or digits), depending on the function letter used. You can also specify all of them as options. tarball is specified after option -f. Omitting -f is the most common mistake for novice users of tar.
tar options files_to_include
there are also three options that specify compression program used:
To create an archive, you use the command.
tar -cvf archivename.tar directory_or_list_of_files_or_regular_expression
Always use option -v to see what is happening. To put files in an archive, you need at least read permissions to the file. For example to compress your home directory and pup the resulting archive in /tmp you can use the command
tar -cvf /tmp/joeuser.tar /home/joeuser
Notice the options that are used; the order in these options is important -- the last one should be option f because it has parameter which specifies where to put the archive we are creating.
Similarly you can create archive of etc
tar -cvf -C /etc /root/etc181006.tar .
cd /etc && tar cvf
Originally, tar did not use the dash (-) in front of its options. Modern tar implementations use that dash, as do all other Linux programs, but they still allow the old usage without a dash for backward compatibility.
While managing archives with tar, it is also possible to add a file to an existing archive, or to update an archive. To add a file to an archive, you use the -r options. For example:
tar -rvf /root/etc181006.tar /root/*.cfg /root/.bash*
To update a currently existing archive file, you can use the -u option:
tar -uvf /root/etc181006.tar /root/*.cfg /root/.bash*
write newer versions of specified files all files in /home to the end of archive and correct the directory of the archive to to new entries. Older files are not deleted, only directory is corrected, so archive increases in size.
Before extracting a file, it is good to check if file exists and what are its attributes. The option -t can be used to find out. Type, for instance,
tar -tvf /root/etc181006.tar
to see the contents of the tar archive.
While extracting a particular file from the tarball, just giving a file name doesn’t work. You need to give the path of the file within the tarball. For example: I have taken a backup of my home directory called “/home/aziz” and it contains test1 and test2 files.
$ tar -xvf /tmp/aziz.tar test1 — This command does not work while extracting a particular called test1
$ tar -xvf /tmp/aziz.tar home/aziz/test1 — This command works while extracting a file called test1
$ tar –extract –file=/tmp/aziz.tar home/aziz/test1
To extract the contents of an archive, use tar -xvf /archivename. This extracts all the content of the archive into the current directory. That means that if you are in /root when typing tar -xvf /root/homes.tar, and the file contains a directory /home, after extracting you’ll have a new directory /root/home that contains the entire contents of the file. This might not be what you wanted to accomplish.
It is also possible to extract one file out of the archive. If the file is in root directory of the archive it is put into current directory. If it in one of the subdirectories subdirectories are created:
tar -xvf /archivename.tar exact_path_in_listing_of_tar_archive_for_that_file
There are two solutions to put the extracted contents right where you want to have them:
If your archive etc181012.tar lists the file hosts as e ./hosts that you want to extract, for instance, use
tar -xvf /root/etc181012.tar ./hosts
This is a very useful capability, if you mangled the file and want to recover the previous version. Of course, you need a fresh archive to that. That's why it makes sense to create etc archive ach morning when you login to the system, but before any changes.
As etc is the nerve center of the Linux OS and contains all system related configuration files in here or in its sub-directories a good practice is to create etc archive in you on your first login each day. You can do do putting appropriate command in your .bash_profile. You also can use GIT to maintain files in etc181012. There is a package called etckeeper that automatically adds changed file to git database, but still you need to have a tarball to recover git database in case of SNAFU.
A "configuration file" is defined as a local file used to control the operation of a program; it must be static and cannot be an executable binary. For this reason, it's a good idea to backup this directory regularly. It will definitely save you a lot of re-configuration later if you re-install or lose your current installation. Normally, no binaries should be or are located here.
For large files compression should be done via parallel version of the compression program and this typically done via pipe. For small archives such as software you develop you can use option in tar.
tar -cv /etc | pbzip2 > /root/etc181006.tar.bz2
on /etc directory bzip2 provides ~12% better compression then gzip, while xz provides ~30% better compression. So you save one archive space each ten days if you do it daily from cron. if files that contitute the archive are very large (say around 500MB and larger) is makes sense to compress them individually first and only then create tar archive without compression.
Usage pigz or bzip2 is a must in this case. Especially the last, as it provides better compression ration then gzip, while having adequate (although slightly slower then pigz) speed.
For small archives compression is specified via one of tar option -z for gzip, -j for bzip and -J for xz. The result is the same. For example
tar -cvzf /root/etc181006.tgz /etc
Please note that compressed tarball is an atomic entity: you can't extract individual files without decompressing it first. That's a big difference with zip archive format, which is more like virtual file system and allow to extract individual files from the archive without decompression the whole archive.
To decompress files that have been compressed with gzip or bzip2, you can use the gunzip and bunzip2 utilities. Parallel version in this case are not much faster: decompression speed is approximately the same. Of course, if archive was compressed better the decompression speed might be higher and in this sense xz might beat alternatives as it provides the highest compression ration.
Here are compressed sizes of default content of /etc directory in RHEL 7 using those three compression programs:
[root@test01 Archive]# ll total 24600 -rw-r--r--. 1 root root 8580949 Oct 7 17:56 etc181006.tbz -rw-r--r--. 1 root root 9795626 Oct 7 17:47 etc181006.tgz -rw-r--r--. 1 root root 6808940 Oct 7 18:04 etc181006.txz
One of the important tips for aspiring Linux system administrators is to create backup of etc directory each morning on the first login. This can be done automatically you work with the tar command to manage archives.
It is also important to backup home directory. In both cases the simplest way to accomplish those tasks is to use tar.
Midnight Commander presents archive as "virtual file system" -- that means that if you click Enter on an archive it will display its content in panel and all standard operations are instantly available.
Moreover you can program additional operation using so called "User menu" (available if you hit F2). By default it contains seven archive related operations (in bold):
Operation 3-6 work the following way. they compress the current directory content and put the resilting tarball into the parent directory. by default mc user menu uses non parallel version of cursors. You can edit it and change that to parallel version.
You can also add you own operations. For example instead the parent directory you can put archive into the directory defined by passive panel (%D)
## Untar files in Current Directory ##
# tar -xvf public_html-14-09-12.tar
## Untar files in specified Directory ##
# tar -xvf public_html-14-09-12.tar -C /home/public_html/videos/
Extract Group of Files using Wildcard
To extract a group of files we use wildcard based extracting. For example, to extract a group of all files whose pattern begins with .php from a tar, tar.gz and tar.bz2 archive file.# tar -xvf Phpfiles-org.tar --wildcards '*.php' # tar -zxvf Phpfiles-org.tar.gz --wildcards '*.php' # tar -jxvf Phpfiles-org.tar.bz2 --wildcards '*.php' /home/php/iframe_ew.php /home/php/videos_all.php /home/php/rss.php /home/php/index.php /home/php/vendor.php /home/php/video_title.php /home/php/report.php /home/php/video.php
Bhasker Reddy, November 4, 2015 at 3:58 am
While extracting a particular file from the tarball, just giving a file name doesn't work. You need to give the path of the file within the tarball.
For example: I have taken a backup of my home directory called "/home/aziz" and it contains test1 and test2 files.
$ tar -xvf /tmp/aziz.tar test1 - This command does not work while extracting a particular called test1
$ tar -xvf /tmp/aziz.tar home/aziz/test1 - This command works while extracting a file called test1
OR$ tar --extract --file=/tmp/aziz.tar home/aziz/test1
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