SIMPLE SOLUTIONS

# RDIFF-BACKUP(1) - man page online | user commands

Local/remote mirror and incremental backup.

Chapter
March 2009
RDIFF-BACKUP(1)                            User Manuals                           RDIFF-BACKUP(1)

NAME
rdiff-backup - local/remote mirror and incremental backup

SYNOPSIS
rdiff-backup [options] [[[user@]host1.foo]::source_directory] [[[user@]host2.foo]::desti‐
nation_directory]

rdiff-backup {{ -l | --list-increments } | --remove-older-than time_interval | --list-at-
time time | --list-changed-since time | --list-increment-sizes | --verify | --verify-at-
time time} [[[user@]host2.foo]::destination_directory]

rdiff-backup --calculate-average statfile1 statfile2 ...

rdiff-backup --test-server [user1]@host1.net1::path [[user2]@host2.net2::path] ...

DESCRIPTION
rdiff-backup is a script, written in python(1) that backs up one directory to another.
The target directory ends up a copy (mirror) of the source directory, but extra reverse
diffs are stored in a special subdirectory of that target directory, so you can still
recover files lost some time ago.  The idea is to combine the best features of a mirror
and an incremental backup.  rdiff-backup also preserves symlinks, special files,
hardlinks, permissions, uid/gid ownership, and modification times.

rdiff-backup can also operate in a bandwidth efficient manner over a pipe, like rsync(1).
Thus you can use ssh and rdiff-backup to securely back a hard drive up to a remote loca‐
tion, and only the differences will be transmitted.  Using the default settings, rdiff-
backup requires that the remote system accept ssh connections, and that rdiff-backup is
installed in the user's PATH on the remote system.  For information on other options, see
the section on REMOTE OPERATION.

Note that you should not write to the mirror directory except with rdiff-backup.  Many of
the increments are stored as reverse diffs, so if you delete or modify a file, you may
lose the ability to restore previous versions of that file.

Finally, this man page is intended more as a precise description of the behavior and syn‐
tax of rdiff-backup.  New users may want to check out the examples.html file included in
the rdiff-backup distribution.

OPTIONS
-b, --backup-mode
Force backup mode even if first argument appears to be an increment or mirror file.

--calculate-average
Enter calculate average mode.  The arguments should be a number of statistics
files.  rdiff-backup will print the average of the listed statistics files and
exit.

--carbonfile
Enable backup of MacOS X carbonfile information.

--check-destination-dir
If an rdiff-backup session fails, running rdiff-backup with this option on the des‐
tination dir will undo the failed directory.  This happens automatically if you
attempt to back up to a directory and the last backup failed.

--compare
This is equivalent to '--compare-at-time now'

--compare-at-time time
Compare a directory with the backup set at the given time.  This can be useful to
see how archived data differs from current data, or to check that a backup is cur‐
rent.  This only compares metadata, in the same way rdiff-backup decides whether a
file has changed.

--compare-full
This is equivalent to '--compare-full-at-time now'

--compare-full-at-time time
Compare a directory with the backup set at the given time.  To compare regular
files, the repository data will be copied in its entirety to the source side and
compared byte by byte.  This is the slowest but most complete compare option.

--compare-hash
This is equivalent to '--compare-hash-at-time now'

--compare-hash-at-time time
Compare a directory with the backup set at the given time.  Regular files will be
compared by computing their SHA1 digest on the source side and comparing it to the

--create-full-path
Normally only the final directory of the destination path will be created if it
does not exist. With this option, all missing directories on the destination path
will be created. Use this option with care: if there is a typo in the remote path,
the remote filesystem could fill up very quickly (by creating a duplicate backup
tree). For this reason this option is primarily aimed at scripts which automate
backups.

--current-time seconds
This option is useful mainly for testing.  If set, rdiff-backup will use it for the
current time instead of consulting the clock.  The argument is the number of sec‐
onds since the epoch.

--exclude shell_pattern
Exclude the file or files matched by shell_pattern.  If a directory is matched,
then files under that directory will also be matched.  See the FILE SELECTION sec‐

--exclude-device-files
Exclude all device files.  This can be useful for security/permissions reasons or
if rdiff-backup is not handling device files correctly.

--exclude-fifos
Exclude all fifo files.

--exclude-filelist filename
Excludes the files listed in filename.  If filename is handwritten you probably
want --exclude-globbing-filelist instead.  See the FILE SELECTION section for more
information.

--exclude-filelist-stdin
Like --exclude-filelist, but the list of files will be read from standard input.

--exclude-globbing-filelist filename
Like --exclude-filelist but each line of the filelist will be interpreted according
to the same rules as --include and --exclude.

--exclude-globbing-filelist-stdin
Like --exclude-globbing-filelist, but the list of files will be read from standard
input.

--exclude-other-filesystems
Exclude files on file systems (identified by device number) other than the file
system the root of the source directory is on.

--exclude-regexp regexp
Exclude files matching the given regexp.  Unlike the --exclude option, this option
does not match files in a directory it matches.  See the FILE SELECTION section for

--exclude-special-files
Exclude all device files, fifo files, socket files, and symbolic links.

--exclude-sockets
Exclude all socket files.

Exclude all symbolic links. This option is automatically enabled if the backup
source is running on native Windows to avoid backing-up NTFS reparse points.

--exclude-if-present filename
Exclude directories if filename is present. This option needs to come before any
other include or exclude options.

--force
Authorize a more drastic modification of a directory than usual (for instance, when
overwriting of a destination path, or when removing multiple sessions with
--remove-older-than).  rdiff-backup will generally tell you if it needs this.
WARNING: You can cause data loss if you mis-use this option.  Furthermore, do NOT
use this option when doing a restore, as it will DELETE FILES, unless you abso‐
lutely know what you are doing.

--group-mapping-file filename
Map group names and ids according the the group mapping file filename.  See the

--include shell_pattern
Similar to --exclude but include matched files instead.  Unlike --exclude, this
option will also match parent directories of matched files (although not necessar‐

--include-filelist filename
Like --exclude-filelist, but include the listed files instead.  If filename is
handwritten you probably want --include-globbing-filelist instead.  See the FILE

--include-filelist-stdin
Like --include-filelist, but read the list of included files from standard input.

--include-globbing-filelist filename
Like --include-filelist but each line of the filelist will be interpreted according
to the same rules as --include and --exclude.

--include-globbing-filelist-stdin
Like --include-globbing-filelist, but the list of files will be read from standard
input.

--include-regexp regexp
Include files matching the regular expression regexp.  Only files explicitly
matched by regexp will be included by this option.  See the FILE SELECTION section

--include-special-files
Include all device files, fifo files, socket files, and symbolic links.

--list-at-time time
List the files in the archive that were present at the given time.  If a directory
in the archive is specified, list only the files under that directory.

--list-changed-since time
List the files that have changed in the destination directory since the given time.
See TIME FORMATS for the format of time.  If a directory in the archive is speci‐
fied, list only the files under that directory.  This option does not read the
source directory; it is used to compare the contents of two different rdiff-backup
sessions.

-l, --list-increments
List the number and date of partial incremental backups contained in the specified
destination directory.  No backup or restore will take place if this option is
given.

--list-increment-sizes
List the total size of all the increment and mirror files by time.  This may be
helpful in deciding how many increments to keep, and when to --remove-older-than.
Specifying a subdirectory is allowable; then only the sizes of the mirror and
increments pertaining to that subdirectory will be listed.

--max-file-size size
Exclude files that are larger than the given size in bytes

--min-file-size size
Exclude files that are smaller than the given size in bytes

--never-drop-acls
Exit with error instead of dropping acls or acl entries.  Normally this may happen
(with a warning) because the destination does not support them or because the rele‐
vant user/group names do not exist on the destination side.

--no-acls
No Access Control Lists - disable backup of ACLs

--no-carbonfile
Disable backup of MacOS X carbonfile information

--no-compare-inode
This option prevents rdiff-backup from flagging a hardlinked file as changed when
its device number and/or inode changes.  This option is useful in situations where
the source filesystem lacks persistent device and/or inode numbering.  For example,
network filesystems may have mount-to-mount differences in their device number (but
possibly stable inode numbers); USB/1394 devices may come up at different device
numbers each remount (but would generally have same inode number); and there are
filesystems which don't even have the same inode numbers from use to use.  Without
the option rdiff-backup may generate unnecessary numbers of tiny diff files.

--no-compression
Disable the default gzip compression of most of the .snapshot and .diff increment
files stored in the rdiff-backup-data directory.  A backup volume can contain com‐
pressed and uncompressed increments, so using this option inconsistently is fine.

--no-compression-regexp  regexp
Do not compress increments based on files whose filenames match regexp.  The
default includes many common audiovisual and archive files, and may be found in
Globals.py.

--no-eas
No Extended Attributes support - disable backup of EAs.

--no-file-statistics
This will disable writing to the file_statistics file in the rdiff-backup-data
directory.  rdiff-backup will run slightly quicker and take up a bit less space.

Don't replicate hard links on destination side.  If many hard-linked files are
present, this option can drastically decrease memory usage.  This option is enabled
by default if the backup source or restore destination is running on native Win‐
dows.

--null-separator
Use nulls (\0) instead of newlines (\n) as line separators, which may help when
dealing with filenames containing newlines.  This affects the expected format of
the files specified by the --{include|exclude}-filelist[-stdin] switches as well as
the format of the directory statistics file.

--parsable-output
If set, rdiff-backup's output will be tailored for easy parsing by computers,
instead of convenience for humans.  Currently this only applies when listing incre‐
ments using the -l or --list-increments switches, where the time will be given in
seconds since the epoch.

--override-chars-to-quote
If the filesystem to which we are backing up is not case-sensitive, automatic
'quoting' of characters occurs. For example, a file 'Developer.doc' will be con‐
verted into ';068eveloper.doc'. To override this behavior, you need to specify this
option.

--preserve-numerical-ids
If set, rdiff-backup will preserve uids/gids instead of trying to preserve unames

--print-statistics
If set, summary statistics will be printed after a successful backup.  If not set,
this information will still be available from the session statistics file.  See the

-r, --restore-as-of restore_time
Restore the specified directory as it was as of restore_time.  See the TIME FORMATS
section for more information on the format of restore_time, and see the RESTORING

--remote-cmd cmd

--remote-schema schema
Specify an alternate method of connecting to a remote computer.  This is necessary
to get rdiff-backup not to use ssh for remote backups, or if, for instance, rdiff-
backup is not in the PATH on the remote side.  See the REMOTE OPERATION section for

--remote-tempdir path
Adds the --tempdir option with argument path when invoking remote instances of
rdiff-backup.

--remove-older-than time_spec
Remove the incremental backup information in the destination directory that has
been around longer than the given time.  time_spec can be either an absolute time,
like "2002-01-04", or a time interval.  The time interval is an integer followed by
the character s, m, h, D, W, M, or Y, indicating seconds, minutes, hours, days,
weeks, months, or years respectively, or a number of these concatenated.  For exam‐
ple, 32m means 32 minutes, and 3W2D10h7s means 3 weeks, 2 days, 10 hours, and 7
seconds.  In this context, a month means 30 days, a year is 365 days, and a day is
always 86400 seconds.

rdiff-backup cannot remove-older-than and back up or restore in a single session.
In order to both backup a directory and remove old files in it, you must run rdiff-
backup twice.

By default, rdiff-backup will only delete information from one session at a time.
To remove two or more sessions at the same time, supply the --force option (rdiff-
backup will tell you if --force is required).

Note that snapshots of deleted files are covered by this operation.  Thus if you
deleted a file two weeks ago, backed up immediately afterwards, and then ran rdiff-
backup with --remove-older-than 10D today, no trace of that file would remain.
Finally, file selection options such as --include and --exclude don't affect
--remove-older-than.

--restrict path
Require that all file access be inside the given path.  This switch, and the fol‐
lowing two, are intended to be used with the --server switch to provide a bit more
protection when doing automated remote backups.  They are not intended as your only

Like --restrict, but also reject all write requests.

--restrict-update-only path
Like --restrict, but only allow writes as part of an incremental backup.  Requests
for other types of writes (for instance, deleting path) will be rejected.

--server
Enter server mode (not to be invoked directly, but instead used by another rdiff-
backup process on a remote computer).

--ssh-no-compression
When running ssh, do not use the -C option to enable compression.  --ssh-no-com‐
pression is ignored if you specify a new schema using --remote-schema.

--tempdir path
Sets the directory that rdiff-backup uses for temporary files to the given path.
The environment variables TMPDIR, TEMP, and TMP can also be used to set the tempo‐
rary files directory. See the documentation of the Python tempfile module for more
information.

--terminal-verbosity [0-9]
Select which messages will be displayed to the terminal.  If missing the level
defaults to the verbosity level.

--test-server
Test for the presence of a compatible rdiff-backup server as specified in the fol‐
lowing host::filename argument(s).  The filename section will be ignored.

--user-mapping-file filename
Map user names and ids according to the user mapping file filename.  See the USERS

-v[0-9], --verbosity [0-9]
Specify verbosity level (0 is totally silent, 3 is the default, and 9 is noisiest).
This determines how much is written to the log file.

--verify
This is short for --verify-at-time now

--verify-at-time now
Check all the data in the repository at the given time by computing the SHA1 hash
of all the regular files and comparing them with the hashes stored in the metadata
file.

-V, --version
Print the current version and exit

RESTORING
There are two ways to tell rdiff-backup to restore a file or directory.  Firstly, you can
run rdiff-backup on a mirror file and use the -r or --restore-as-of options.  Secondly,
you can run it on an increment file.

For example, suppose in the past you have run:

rdiff-backup /usr /usr.backup

to back up the /usr directory into the /usr.backup directory, and now want a copy of the
/usr/local directory the way it was 3 days ago placed at /usr/local.old.

One way to do this is to run:

rdiff-backup -r 3D /usr.backup/local /usr/local.old

where above the "3D" means 3 days (for other ways to specify the time, see the TIME FOR‐
MATS section).  The /usr.backup/local directory was selected, because that is the direc‐
tory containing the current version of /usr/local.

Note that the option to --restore-as-of always specifies an exact time.  (So "3D" refers
to the instant 72 hours before the present.)  If there was no backup made at that time,
rdiff-backup restores the state recorded for the previous backup.  For instance, in the
above case, if "3D" is used, and there are only backups from 2 days and 4 days ago,
/usr/local as it was 4 days ago will be restored.

The second way to restore files involves finding the corresponding increment file.  It
would be in the /backup/rdiff-backup-data/increments/usr directory, and its name would be
something like "local.2002-11-09T12:43:53-04:00.dir" where the time indicates it is from 3
days ago.  Note that the increment files all end in ".diff", ".snapshot", ".dir", or
".missing", where ".missing" just means that the file didn't exist at that time (finally,
some of these may be gzip-compressed, and have an extra ".gz" to indicate this).  Then
running:

rdiff-backup /backup/rdiff-backup-data/increments/usr/local.<time>.dir
/usr/local.old

would also restore the file as desired.

If you are not sure exactly which version of a file you need, it is probably easiest to
either restore from the increments files as described immediately above, or to see which
increments are available with -l/--list-increments, and then specify exact times into
-r/--restore-as-of.

TIME FORMATS
rdiff-backup uses time strings in two places.  Firstly, all of the increment files rdiff-
backup creates will have the time in their filenames in the w3 datetime format as
described in a w3 note at http://www.w3.org/TR/NOTE-datetime.  Basically they look like
"2001-07-15T04:09:38-07:00", which means what it looks like.  The "-07:00" section means
the time zone is 7 hours behind UTC.

Secondly, the -r, --restore-as-of, and --remove-older-than options take a time string,
which can be given in any of several formats:

1.     the string "now" (refers to the current time)

2.     a sequences of digits, like "123456890" (indicating the time in seconds after the
epoch)

3.     A string like "2002-01-25T07:00:00+02:00" in datetime format

4.     An interval, which is a number followed by one of the characters s, m, h, D, W, M,
or Y (indicating seconds, minutes, hours, days, weeks, months, or years respec‐
tively), or a series of such pairs.  In this case the string refers to the time
that preceded the current time by the length of the interval.  For instance,
"1h78m" indicates the time that was one hour and 78 minutes ago.  The calendar here
is unsophisticated: a month is always 30 days, a year is always 365 days, and a day
is always 86400 seconds.

5.     A date format of the form YYYY/MM/DD, YYYY-MM-DD, MM/DD/YYYY, or MM-DD-YYYY, which
indicates midnight on the day in question, relative to the current timezone set‐
tings.  For instance, "2002/3/5", "03-05-2002", and "2002-3-05" all mean March 5th,
2002.

6.     A backup session specification which is a non-negative integer followed by 'B'.
For instance, '0B' specifies the time of the current mirror, and '3B' specifies the
time of the 3rd newest increment.

REMOTE OPERATION
In order to access remote files, rdiff-backup opens up a pipe to a copy of rdiff-backup
running on the remote machine.  Thus rdiff-backup must be installed on both ends.  To open
this pipe, rdiff-backup first splits the filename into host_info::pathname.  It then sub‐
stitutes host_info into the remote schema, and runs the resulting command, reading its
input and output.

The default remote schema is 'ssh -C %s rdiff-backup --server' where host_info is substi‐
tuted for '%s'.  So if the host_info is @host.net, then rdiff-backup runs 'ssh
@host.net rdiff-backup --server'.  Using --remote-schema, rdiff-backup can invoke an
arbitrary command in order to open up a remote pipe.  For instance,
rdiff-backup --remote-schema 'cd /usr; %s' foo 'rdiff-backup --server'::bar
is basically equivalent to (but slower than)
rdiff-backup foo /usr/bar

Concerning quoting, if for some reason you need to put two consecutive colons in the
host_info section of a host_info::pathname argument, or in the pathname of a local file,
you can quote one of them by prepending a backslash.  So in 'a\::b::c', host_info is
'a::b' and the pathname is 'c'.  Similarly, if you want to refer to a local file whose
filename contains two consecutive colons, like 'strange::file', you'll have to quote one
of the colons as in 'strange\::file'.  Because the backslash is a quote character in these
circumstances, it too must be quoted to get a literal backslash, so 'foo\::\\bar' evalu‐
ates to 'foo::\bar'.  To make things more complicated, because the backslash is also a
common shell quoting character, you may need to type in '\\\\' at the shell prompt to get
a literal backslash (if it makes you feel better, I had to type in 8 backslashes to get
that in this man page...).  And finally, to include a literal % in the string specified by
--remote-schema, quote it with another %, as in %%.

Although ssh itself may be secure, using rdiff-backup in the default way presents some
security risks.  For instance if the server is run as root, then an attacker who compro‐
mised the client could then use rdiff-backup to overwrite arbitrary server files by "back‐
ing up" over them.  Such a setup can be made more secure by using the sshd configuration
option command="rdiff-backup --server" possibly along with the --restrict* options to
rdiff-backup.  For more information, see the web page, the wiki, and the entries for the
--restrict* options on this man page.

FILE SELECTION
rdiff-backup has a number of file selection options.  When rdiff-backup is run, it
searches through the given source directory and backs up all the files matching the speci‐
fied options.  This selection system may appear complicated, but it is supposed to be
flexible and easy-to-use.  If you just want to learn the basics, first look at the selec‐
tion examples in the examples.html file included in the package, or on the web at
http://rdiff-backup.nongnu.org/examples.html

rdiff-backup's selection system was originally inspired by rsync(1), but there are many
differences.  (For instance, trailing backslashes have no special significance.)

The file selection system comprises a number of file selection conditions, which are set
using one of the following command line options: --exclude, --exclude-filelist, --exclude-
globbing-filelist, --exclude-globbing-filelist-stdin, --exclude-filelist-stdin, --exclude-
regexp, --exclude-special-files, --include, --include-filelist, --include-globbing-
filelist, --include-globbing-filelist-stdin, --include-filelist-stdin, and --include-reg‐
exp.  Each file selection condition either matches or doesn't match a given file.  A given
file is excluded by the file selection system exactly when the first matching file selec‐
tion condition specifies that the file be excluded; otherwise the file is included.  When
backing up, if a file is excluded, rdiff-backup acts as if that file does not exist in the
source directory.  When restoring, an excluded file is considered not to exist in either
the source or target directories.

For instance,

rdiff-backup --include /usr --exclude /usr /usr /backup

is exactly the same as

rdiff-backup /usr /backup

because the include and exclude directives match exactly the same files, and the --include
comes first, giving it precedence.  Similarly,

rdiff-backup --include /usr/local/bin --exclude /usr/local /usr /backup

would backup the /usr/local/bin directory (and its contents), but not /usr/local/doc.

The include, exclude, include-globbing-filelist, and exclude-globbing-filelist options
accept extended shell globbing patterns.  These patterns can contain the special patterns
*, **, ?, and [...].  As in a normal shell, * can be expanded to any string of characters
not containing "/", ?  expands to any character except "/", and [...]  expands to a single
character of those characters specified (ranges are acceptable).  The new special pattern,
**, expands to any string of characters whether or not it contains "/".  Furthermore, if
the pattern starts with "ignorecase:" (case insensitive), then this prefix will be removed
and any character in the string can be replaced with an upper- or lowercase version of
itself.

If you need to match filenames which contain the above globbing characters, they may be
escaped using a backslash "\". The backslash will only escape the character following it
so for ** you will need to use "\*\*" to avoid escaping it to the * globbing character.

Remember that you may need to quote these characters when typing them into a shell, so the
shell does not interpret the globbing patterns before rdiff-backup sees them.

The --exclude pattern option matches a file iff:

1.     pattern can be expanded into the file's filename, or

2.     the file is inside a directory matched by the option.

Conversely, --include pattern matches a file iff:

1.     pattern can be expanded into the file's filename,

2.     the file is inside a directory matched by the option, or

3.     the file is a directory which contains a file matched by the option.

For example,

--exclude /usr/local

matches /usr/local, /usr/local/lib, and /usr/local/lib/netscape.  It is the same as
--exclude /usr/local --exclude '/usr/local/**'.

--include /usr/local

specifies that /usr, /usr/local, /usr/local/lib, and /usr/local/lib/netscape (but not
/usr/doc) all be backed up.  Thus you don't have to worry about including parent directo‐
ries to make sure that included subdirectories have somewhere to go.  Finally,

--include ignorecase:'/usr/[a-z0-9]foo/*/**.py'

would match a file like /usR/5fOO/hello/there/world.py.  If it did match anything, it
would also match /usr.  If there is no existing file that the given pattern can be
expanded into, the option will not match /usr.

The --include-filelist, --exclude-filelist, --include-filelist-stdin, and --exclude-
filelist-stdin options also introduce file selection conditions.  They direct rdiff-backup
to read in a file, each line of which is a file specification, and to include or exclude
the matching files.  Lines are separated by newlines or nulls, depending on whether the
--null-separator switch was given.  Each line in a filelist is interpreted similarly to
the way extended shell patterns are, with a few exceptions:

1.     Globbing patterns like *, **, ?, and [...]  are not expanded.

2.     Include patterns do not match files in a directory that is included.  So /usr/local
in an include file will not match /usr/local/doc.

3.     Lines starting with "+ " are interpreted as include directives, even if found in a
filelist referenced by --exclude-filelist.  Similarly, lines starting with "- "
exclude files even if they are found within an include filelist.

For example, if the file "list.txt" contains the lines:

/usr/local
- /usr/local/doc
/usr/local/bin
+ /var
- /var

then "--include-filelist list.txt" would include /usr, /usr/local, and /usr/local/bin.  It
would exclude /usr/local/doc, /usr/local/doc/python, etc.  It neither excludes nor
includes /usr/local/man, leaving the fate of this directory to the next specification con‐
dition.  Finally, it is undefined what happens with /var.  A single file list should not
contain conflicting file specifications.

The --include-globbing-filelist and --exclude-globbing-filelist options also specify
filelists, but each line in the filelist will be interpreted as a globbing pattern the way
--include and --exclude options are interpreted (although "+ " and "- " prefixing is still
allowed).  For instance, if the file "globbing-list.txt" contains the lines:

dir/foo
+ dir/bar
- **

Then "--include-globbing-filelist globbing-list.txt" would be exactly the same as specify‐
ing "--include dir/foo --include dir/bar --exclude **" on the command line.

Finally, the --include-regexp and --exclude-regexp allow files to be included and excluded
if their filenames match a python regular expression.  Regular expression syntax is too
complicated to explain here, but is covered in Python's library reference.  Unlike the
--include and --exclude options, the regular expression options don't match files contain‐
ing or contained in matched files.  So for instance

--include '[0-9]{7}(?!foo)'

matches any files whose full pathnames contain 7 consecutive digits which aren't followed
by 'foo'.  However, it wouldn't match /home even if /home/ben/1234567 existed.

USERS AND GROUPS
There can be complications preserving ownership across systems.  For instance the username
that owns a file on the source system may not exist on the destination.  Here is how
rdiff-backup maps ownership on the source to the destination (or vice-versa, in the case
of restoring):

1.     If the --preserve-numerical-ids option is given, the remote files will always have
the same uid and gid, both for ownership and ACL entries.  This may cause unames
and gnames to change.

2.     Otherwise, attempt to preserve the user and group names for ownership and in ACLs.
This may result in files having different uids and gids across systems.

3.     If a name cannot be preserved (e.g. because the username does not exist), preserve
the original id, but only in cases of user and group ownership.  For ACLs, omit any
entry that has a bad user or group name.

4.     The --user-mapping-file and --group-mapping-file options override this behavior.
If either of these options is given, the policy described in 2 and 3 above will be
followed, but with the mapped user and group instead of the original.  If you spec‐
ify both --preserve-numerical-ids and one of the mapping options, the behavior is
undefined.

The user and group mapping files both have the same form:

old_name_or_id1:new_name_or_id1
old_name_or_id2:new_name_or_id2
<etc>

Each line should contain a name or id, followed by a colon ":", followed by another name
or id.  If a name or id is not listed, they are treated in the default way described
above.

When restoring, the above behavior is also followed, but note that the original source
user/group information will be the input, not the already mapped user/group information
present in the backup repository.  For instance, suppose you have mapped all the files
owned by alice in the source so that they are owned by ben in the repository, and now you
want to restore, making sure the files owned originally by alice are still owned by alice.
In this case there is no need to use any of the mapping options.  However, if you wanted
to restore the files so that the files originally owned by alice on the source are now
owned by ben, you would have to use the mapping options, even though you just want the
unames of the repository's files preserved in the restored files.

STATISTICS
Every session rdiff-backup saves various statistics into two files, the session statistics
file at rdiff-backup-data/session_statistics.<time>.data and the directory statistics file
at rdiff-backup-data/directory_statistics.<time>.data.  They are both text files and con‐
tain similar information: how many files changed, how many were deleted, the total size of
increment files created, etc.  However, the session statistics file is intended to be very
readable and only describes the session as a whole.  The directory statistics file is more
compact (and slightly less readable) but describes every directory backed up.  It also may
be compressed to save space.

Statistics-related options include --print-statistics and --null-separator.

Also, rdiff-backup will save various messages to the log file, which is rdiff-backup-
data/backup.log for backup sessions and rdiff-backup-data/restore.log for restore ses‐
sions.  Generally what is written to this file will coincide with the messages displayed
to stdout or stderr, although this can be changed with the --terminal-verbosity option.

The log file is not compressed and can become quite large if rdiff-backup is run with high
verbosity.

EXIT STATUS
If rdiff-backup finishes successfully, the exit status will be 0.  If there is an unrecov‐
erable (critical) error, it will be non-zero (usually 1, but don't depend on this specific
value).  When setting up rdiff-backup to run automatically (as from cron(8) or similar) it
is probably a good idea to check the exit code.

BUGS
The gzip library in versions 2.2 and earlier of python (but fixed in 2.3a1) has trouble
producing files over 2GB in length.  This bug will prevent rdiff-backup from producing
large compressed increments (snapshots or diffs).  A workaround is to disable compression
for large uncompressable files.

AUTHOR
Ben Escoto <@emerose.org>

Feel free to ask me questions or send me bug reports, but you may want to see the web
page, mentioned below, first.

Version 1.2.8                               March 2009                            RDIFF-BACKUP(1)