Backup and Restore

In this chapter you will learn how to back up and restore your data with Linux.

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Objectives : In this chapter, future Linux administrators will learn how to:

use the tar and cpio command to make a backup;
check their backups and restore data;
compress or decompress their backups.

backup, restore, compression

Knowledge:
Complexity:

Reading time: 40 minutes

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Note

Throughout this chapter the command structures use "device" to specify both a target location for backup, and the source location when restoring. The device can be either external media or a local file. You should get a feel for this as the chapter unfolds, but you can always refer back to this note for clarification if you need to.

The backup will answer a need to conserve and restore data in a sure and effective way.

The backup allows you to protect yourself from the following:

• Destruction: voluntary or involuntary. Human or technical. Virus, ...
• Deletion: voluntary or involuntary. Human or technical. Virus, ...
• Integrity : data becomes unusable.

No system is infallible, no human is infallible, so to avoid losing data, it must be backed up to be able to restore after a problem.

The backup media should be kept in another room (or building) than the server so that a disaster does not destroy the server and the backups.

In addition, the administrator must regularly check that the media are still readable.

Generalities

There are two principles, the backup and the archive.

• The archive destroys the information source after the operation.
• The backup preserves the source of information after the operation.

These operations consist of saving information in a file, on a peripheral or a supported media (tapes, disks, ...).

The process

Backups require a lot of discipline and rigor from the system administrator. It is necessary to ask the following questions:

• What is the appropriate medium?
• What should be backed up?
• How many copies?
• How long will the backup take?
• Method?
• How often?
• Automatic or manual?
• Where to store it?
• How long will it be kept?

Backup methods

• Complete: one or more filesystems are backed up (kernel, data, utilities, ...).
• Partial: one or more files are backed up (configurations, directories, ...).
• Differential: only files modified since the last complete backup are backed up.
• Incremental: only files modified since the last backup are backed up.

Periodicity

• Pre-current : at a given time (before a system update, ...).
• Periodic: Daily, weekly, monthly, ...

Tip

Before a system change, it can be useful to make a backup. However, there is no point in backing up data every day that is only changed every month.

Restoration methods

Depending on the utilities available, it will be possible to perform several types of restorations.

• Complete restoration: trees, ...
• Selective restoration: part of tree, files, ...

It is possible to restore a whole backup but it is also possible to restore only a part of it. However, when restoring a directory, the files created after the backup are not deleted.

Tip

To recover a directory as it was at the time of the backup, it is necessary to completely delete its contents before launching the restoration.

The tools

There are many utilities to make backups.

• editor tools ;
• graphical tools;
• command line tools: tar, cpio, pax, dd, dump, ...

The commands we will use here are tar and cpio.

• tar:
• easy to use ;
• allows adding files to an existing backup.
• cpio :
• retains owners;
• retains groups, dates and rights;
• skips damaged files;
• complete file system.

Note

These commands save in a proprietary and standardized format.

Naming convention

The use of a naming convention makes it possible to quickly target the contents of a backup file and thus avoid hazardous restorations.

• name of the directory;
• utility used;
• options used;
• date.

Tip

The name of the backup must be an explicit name.

Note

The notion of extension under Linux does not exist. In other words, our use of extensions here is for the human operator. If the systems administrator sees a .tar.gz or .tgz file extension, for instance, then he knows how to deal with the file.

Contents of a backup

A backup generally contains the following elements:

• the file;
• the name;
• the owner;
• the size;
• the permissions
• access date.

Note

The inode number is missing.

Storage modes

There are two different storage modes:

• file on disk;
• device.

Tape ArchiveR - tar

The tar command allows saving on several successive media (multi-volume options).

It is possible to extract all or part of a backup.

tar implicitly backs up in relative mode even if the path of the information to be backed up is mentioned in absolute mode. However, backups and restores in absolute mode are possible.

Restoration guidelines

The right questions to ask are:

• what: partial or complete;
• where: the place where the data will be restored;
• how: absolute or relative.

Warning

Before a restoration, it is important to take time to think about and determine the most appropriate method to avoid mistakes.

Restorations are usually performed after a problem has occurred that needs to be resolved quickly. A poor restoration can, in some cases, make the situation worse.

Backing up with tar

The default utility for creating backups on UNIX systems is the tar command. These backups can be compressed by bzip2, xz, lzip, lzma, lzop, gzip, compress or zstd.

tar allows you to extract a single file or a directory from a backup, view its contents or validate its integrity.

Estimate the size of a backup

The following command estimates the size in kilobytes of a possible tar file:

$ tar cf - /directory/to/backup/ | wc -c
20480
$ tar czf - /directory/to/backup/ | wc -c
508
$ tar cjf - /directory/to/backup/ | wc -c
428

Warning

Beware, the presence of "-" in the command line disturbs zsh. Switch to bash!

Naming convention for a tar backup

Here is an example of a naming convention for a tar backup, knowing that the date is to be added to the name.

keys	Files	Suffix	Observation
cvf	home	home.tar	/home in relative mode, uncompressed form
cvfP	/etc	etc.A.tar	/etc in absolute mode, no compression
cvfz	usr	usr.tar.gz	/usr in relative mode, gzip compression
cvfj	usr	usr.tar.bz2	/usr in relative mode, bzip2 compression
cvfPz	/home	home.A.tar.gz	home in absolute mode, gzip compression
cvfPj	/home	home.A.tar.bz2	home in absolute mode, bzip2 compression
…

Create a backup

Create a backup in relative mode

Creating a non-compressed backup in relative mode is done with the cvf keys:

tar c[vf] [device] [file(s)]

Example:

[root]# tar cvf /backups/home.133.tar /home/

Key	Description
c	Creates a backup.
v	Displays the name of the processed files.
f	Allows you to specify the name of the backup (medium).

Tip

The hyphen (-) in front of the tar keys is not necessary!

Create a backup in absolute mode

Creating a non-compressed backup explicitly in absolute mode is done with the cvfP keys:

$ tar c[vf]P [device] [file(s)]

Example:

[root]# tar cvfP /backups/home.133.P.tar /home/

Key	Description
P	Create a backup in absolute mode.

Warning

With the P key, the path of the files to be backed up must be entered as absolute. If the two conditions (key P and path absolute) are not indicated, the backup is in relative mode.

Creating a compressed backup with gzip

Creating a compressed backup with gzip is done with the cvfz keys:

$ tar cvzf backup.tar.gz dirname/

Key	Description
z	Compresses the backup in gzip.

Note

The .tgz extension is an equivalent extension to .tar.gz.

Note

Keeping the cvf (tvf or xvf) keys unchanged for all backup operations and simply adding the compression key to the end of the keys makes the command easier to understand (e.g. cvfz or cvfj, etc.).

Creating a compressed backup with bzip

Creating a compressed backup with bzip is done with the keys cvfj:

$ tar cvfj backup.tar.bz2 dirname/

Key	Description
j	Compresses the backup in bzip2.

Note

The .tbz and .tb2 extensions are equivalent to .tar.bz2 extensions.

Compression compress, gzip, bzip2, lzip and xz

Compression, and consequently decompression, will have an impact on resource consumption (time and CPU usage).

Here is a ranking of the compression of a set of text files, from least to most efficient:

• compress (.tar.Z)
• gzip (.tar.gz)
• bzip2 (.tar.bz2)
• lzip (.tar.lz)
• xz (.tar.xz)

Add a file or directory to an existing backup

It is possible to add one or more items to an existing backup.

tar {r|A}[key(s)] [device] [file(s)]

To add /etc/passwd to the backup /backups/home.133.tar:

[root]# tar rvf /backups/home.133.tar /etc/passwd

Adding a directory is similar. Here add dirtoadd to backup_name.tar:

$ tar rvf backup_name.tar dirtoadd

Key	Description
r	Adds one or more files at the end of a direct access media backup (hard disk).
A	Adds one or more files at the end of a backup on sequential access media (tape).

Note

It is not possible to add files or folders to a compressed backup.

$ tar rvfz backup.tgz filetoadd
tar: Cannot update compressed archives
Try `tar --help' or `tar --usage' for more information.

Note

If the backup was performed in relative mode, add files in relative mode. If the backup was done in absolute mode, add files in absolute mode.

Mixing modes can cause problems when restoring.

List the contents of a backup

Viewing the contents of a backup without extracting it is possible.

tar t[key(s)] [device]

Key	Description
t	Displays the content of a backup (compressed or not).

Examples:

$ tar tvf backup.tar
$ tar tvfz backup.tar.gz
$ tar tvfj backup.tar.bz2

When the number of files in a backup becomes large, it is possible to pipe the result of the tar command to a pager (more, less, most, etc.):

$ tar tvf backup.tar | less

Tip

To list or retrieve the contents of a backup, it is not necessary to mention the compression algorithm used when the backup was created. That is, a tar tvf is equivalent to tar tvfj, to read the contents, and a tar xvf is equivalent to tar xvfj, to extract.

Tip

Always check the contents of a backup.

Check the integrity of a backup

The integrity of a backup can be tested with the W key at the time of its creation:

$ tar cvfW file_name.tar dir/

The integrity of a backup can be tested with the key d after its creation:

$ tar vfd file_name.tar dir/

Tip

By adding a second v to the previous key, you will get the list of archived files as well as the differences between the archived files and those present in the file system.

$ tar vvfd  /tmp/quodlibet.tar .quodlibet/
drwxr-x--- rockstar/rockstar     0 2021-05-21 00:11 .quodlibet/
-rw-r--r-- rockstar/rockstar     0 2021-05-19 00:59 .quodlibet/queue
[…]
-rw------- rockstar/rockstar  3323 2021-05-21 00:11 .quodlibet/config
.quodlibet/config: Mod time differs
.quodlibet/config: Size differs
[…]

The W key is also used to compare the content of an archive against the filesystem:

$ tar tvfW file_name.tar
Verify 1/file1
1/file1: Mod time differs
1/file1: Size differs
Verify 1/file2
Verify 1/file3

The verification with the W key cannot be done with a compressed archive. The key d must be used:

$ tar dfz file_name.tgz
$ tar dfj file_name.tar.bz2

Extract (untar) a backup

Extract (untar]) a *.tar backup is done with the xvf keys:

Extract the etc/exports file from the /savings/etc.133.tar backup into the etc directory of the active directory:

$ tar xvf /backups/etc.133.tar etc/exports

Extract all files from the compressed backup /backups/home.133.tar.bz2 into the active directory:

[root]# tar xvfj /backups/home.133.tar.bz2

Extract all files from the backup /backups/etc.133.P.tar to their original directory:

$ tar xvfP /backups/etc.133.P.tar

Warning

Go to the right place.

Check the contents of the backup.

Key	Description
x	Extract files from the backup, compressed or not.

Extracting a tar-gzipped (*.tar.gz) backup is done with the xvfz keys:

$ tar xvfz backup.tar.gz

Extracting a tar-bzipped (*.tar.bz2) backup is done with the xvfj keys:

$ tar xvfj backup.tar.bz2

Tip

To extract or list the contents of a backup, it is not necessary to mention the compression algorithm used to create the backup. That is, a tar xvf is equivalent to tar xvfj, to extract the contents, and a tar tvf is equivalent to tar tvfj, to list.

Warning

To restore the files in their original directory (key P of a tar xvf), you must have generated the backup with the absolute path. That is, with the P key of a tar cvf.

Extract only a file from a tar backup

To extract a specific file from a tar backup, specify the name of that file at the end of the tar xvf command.

$ tar xvf backup.tar /path/to/file

The previous command extracts only the /path/to/file file from the backup.tar backup. This file will be restored to the /path/to/ directory created, or already present, in the active directory.

$ tar xvfz backup.tar.gz /path/to/file
$ tar xvfj backup.tar.bz2 /path/to/file

Extract a folder from a backup tar

To extract only one directory (including its subdirectories and files) from a backup, specify the directory name at the end of the tar xvf command.

$ tar xvf backup.tar /path/to/dir/

To extract multiple directories, specify each of the names one after the other:

$ tar xvf backup.tar /path/to/dir1/ /path/to/dir2/
$ tar xvfz backup.tar.gz /path/to/dir1/ /path/to/dir2/
$ tar xvfj backup.tar.bz2 /path/to/dir1/ /path/to/dir2/

Extract a group of files from a tar backup using regular expressions (regex)

Specify a regex to extract the files matching the specified selection pattern.

For example, to extract all files with the extension .conf :

$ tar xvf backup.tar --wildcards '*.conf'

keys :

• --wildcards *.conf corresponds to files with the extension .conf.

CoPy Input Output - cpio

The cpio command allows saving on several successive media without specifying any options.

It is possible to extract all or part of a backup.

There is no option, unlike the tar command, to backup and compress at the same time. So it is done in two steps: backup and compression.

To perform a backup with cpio, you have to specify a list of files to backup.

This list is provided with the commands find, ls or cat.

• find : browse a tree, recursive or not;
• ls : list a directory, recursive or not;
• cat : reads a file containing the trees or files to be saved.

Note

ls cannot be used with -l (details) or -R (recursive).

It requires a simple list of names.

Create a backup with cpio command

Syntax of the cpio command:

[files command |] cpio {-o| --create} [-options] [<file-list] [>device]

Example:

With a redirection of the output of cpio:

$ find /etc | cpio -ov > /backups/etc.cpio

Using the name of a backup media :

$ find /etc | cpio -ovF /backups/etc.cpio

The result of the find command is sent as input to the cpio command via a pipe (character |, AltGr + 6).

Here, the find /etc command returns a list of files corresponding to the contents of the /etc directory (recursively) to the cpio command, which performs the backup.

Do not forget the > sign when saving or the F save_name_cpio.

| Options | Description | | -o | Creates a backup (output). | | -v | Displays the name of the processed files. | | -F | Designates the backup to be modified (medium). |

Backup to a media :

$ find /etc | cpio -ov > /dev/rmt0

The support can be of several types:

• tape drive: /dev/rmt0 ;
• a partition: /dev/sda5, /dev/hda5, etc.

Type of backup

Backup with relative path

$ cd /
$ find etc | cpio -o > /backups/etc.cpio

Backup with absolute path

$ find /etc | cpio -o > /backups/etc.A.cpio

Warning

If the path specified in the find command is absolute then the backup will be performed in absolute.

If the path indicated in the find command is relative then the backup will be done in relative.

Add to a backup

[files command |] cpio {-o| --create} -A [-options] [<fic-list] {F|>device}

Example:

$ find /etc/shadow | cpio -o -AF SystemFiles.A.cpio

Adding files is only possible on direct access media.

Option	Description
-A	Adds one or more files to a backup on disk.
-F	Designates the backup to be modified.

Compressing a backup

• Save then compress

$ find /etc | cpio  –o > etc.A.cpio
$ gzip /backups/etc.A.cpio
$ ls /backups/etc.A.cpio*
/backups/etc.A.cpio.gz

• Save and compress

$ find /etc | cpio –o | gzip > /backups/etc.A.cpio.gz

There is no option, unlike the tar command, to save and compress at the same time. So it is done in two steps: saving and compressing.

The syntax of the first method is easier to understand and remember, because it is done in two steps.

For the first method, the backup file is automatically renamed by the gzip utility which adds .gz to the end of the file name. Similarly the bzip2 utility automatically adds .bz2.

Read the contents of a backup

Syntax of the cpio command to read the contents of a cpio backup:

cpio -t [-options] [<fic-list]

Example:

$ cpio -tv </backups/etc.152.cpio | less

Options	Description
-t	Reads a backup.
-v	Displays file attributes.

After making a backup, you need to read its contents to be sure that there were no errors.

In the same way, before performing a restore, you must read the contents of the backup that will be used.

Restore a backup

Syntax of the cpio command to restore a backup:

cpio {-i| --extract} [-E file] [-options] [<device]

Example:

$ cpio -iv </backups/etc.152.cpio | less

Options	Description
-i	Restore a complete backup.
-E file	Restores only the files whose name is contained in file.
--make-directories or -d	Rebuilds the missing tree structure.
-u	Replaces all files even if they exist.
--no-absolute-filenames	Allows to restore a backup made in absolute mode in a relative way.

Warning

By default, at the time of restoration, files on the disk whose last modification date is more recent or equal to the date of the backup are not restored (in order to avoid overwriting recent information with older information).

The u option, on the other hand, allows you to restore older versions of the files.

Examples:

• Absolute restoration of an absolute backup

$ cpio –ivF home.A.cpio

• Absolute restoration on an existing tree structure

The u option allows you to overwrite existing files at the location where the restore takes place.

$ cpio –iuvF home.A.cpio

• Restore an absolute backup in relative mode

The long option no-absolute-filenames allows a restoration in relative mode. Indeed the / at the beginning of the path will be removed.

$ cpio --no-absolute-filenames -divuF home.A.cpio

Tip

The creation of directories is perhaps necessary, hence the use of the d option

• Restore a relative backup

$ cpio –iv <etc.cpio

• Absolute restoration of a file or directory

The restoration of a particular file or directory requires the creation of a list file that must then be deleted.

echo "/etc/passwd" > tmp
cpio –iuE tmp -F etc.A.cpio
rm -f tmp

Compression - decompression utilities

Using compression at the time of a backup can have a number of drawbacks:

• Lengthens the backup time as well as the restore time.
• It makes it impossible to add files to the backup.

Note

It is therefore better to make a backup and compress it than to compress it during the backup.

Compressing with gzip

The gzip command compresses data.

Syntax of the gzip command:

gzip [options] [file ...]

Example:

$ gzip usr.tar
$ ls
usr.tar.gz

The file receives the extension .gz.

It keeps the same rights and the same last access and modification dates.

Compressing with bunzip2

The bunzip2 command also compresses data.

Syntax of the bzip2 command:

bzip2 [options] [file ...]

Example:

$ bzip2 usr.cpio
$ ls
usr.cpio.bz2

The file name is given the extension .bz2.

Compression by bzip2 is better than compression by gzip but it takes longer to execute.

Decompressing with gunzip

The gunzip command decompresses compressed data.

Syntax of the gunzip command:

gunzip [options] [file ...]

Example:

$ gunzip usr.tar.gz
$ ls
usr.tar

The file name is truncated by gunzip and the extension .gz is removed.

gunzip also decompresses files with the following extensions:

• .z ;
• -z ;
• _z .

Decompressing with bunzip2

The bunzip2 command decompresses compressed data.

Syntax of the bzip2 command:

bzip2 [options] [file ...]

Example:

$ bunzip2 usr.cpio.bz2
$ ls
usr.cpio

The file name is truncated by bunzip2 and the extension .bz2 is removed.

bunzip2 also decompresses the file with the following extensions:

• -bz ;
• .tbz2 ;
• tbz .