rclone/docs/content/bisync.md

title	description	versionIntroduced
Bisync	Bidirectional cloud sync solution in rclone	v1.58

Getting started

• Install rclone and setup your remotes.
• Bisync will create its working directory
  at ~/.cache/rclone/bisync on Linux
  or C:\Users\MyLogin\AppData\Local\rclone\bisync on Windows.
  Make sure that this location is writable.
• Run bisync with the --resync flag, specifying the paths
  to the local and remote sync directory roots.
• For successive sync runs, leave off the --resync flag. (Important!)
• Consider using a filters file for excluding
  unnecessary files and directories from the sync.
• Consider setting up the --check-access feature
  for safety.
• On Linux, consider setting up a crontab entry. bisync can
  safely run in concurrent cron jobs thanks to lock files it maintains.

Here is a typical run log (with timestamps removed for clarity):

rclone bisync /testdir/path1/ /testdir/path2/ --verbose
INFO  : Synching Path1 "/testdir/path1/" with Path2 "/testdir/path2/"
INFO  : Path1 checking for diffs
INFO  : - Path1    File is new                         - file11.txt
INFO  : - Path1    File is newer                       - file2.txt
INFO  : - Path1    File is newer                       - file5.txt
INFO  : - Path1    File is newer                       - file7.txt
INFO  : - Path1    File was deleted                    - file4.txt
INFO  : - Path1    File was deleted                    - file6.txt
INFO  : - Path1    File was deleted                    - file8.txt
INFO  : Path1:    7 changes:    1 new,    3 newer,    0 older,    3 deleted
INFO  : Path2 checking for diffs
INFO  : - Path2    File is new                         - file10.txt
INFO  : - Path2    File is newer                       - file1.txt
INFO  : - Path2    File is newer                       - file5.txt
INFO  : - Path2    File is newer                       - file6.txt
INFO  : - Path2    File was deleted                    - file3.txt
INFO  : - Path2    File was deleted                    - file7.txt
INFO  : - Path2    File was deleted                    - file8.txt
INFO  : Path2:    7 changes:    1 new,    3 newer,    0 older,    3 deleted
INFO  : Applying changes
INFO  : - Path1    Queue copy to Path2                 - /testdir/path2/file11.txt
INFO  : - Path1    Queue copy to Path2                 - /testdir/path2/file2.txt
INFO  : - Path2    Queue delete                        - /testdir/path2/file4.txt
NOTICE: - WARNING  New or changed in both paths        - file5.txt
NOTICE: - Path1    Renaming Path1 copy                 - /testdir/path1/file5.txt..path1
NOTICE: - Path1    Queue copy to Path2                 - /testdir/path2/file5.txt..path1
NOTICE: - Path2    Renaming Path2 copy                 - /testdir/path2/file5.txt..path2
NOTICE: - Path2    Queue copy to Path1                 - /testdir/path1/file5.txt..path2
INFO  : - Path2    Queue copy to Path1                 - /testdir/path1/file6.txt
INFO  : - Path1    Queue copy to Path2                 - /testdir/path2/file7.txt
INFO  : - Path2    Queue copy to Path1                 - /testdir/path1/file1.txt
INFO  : - Path2    Queue copy to Path1                 - /testdir/path1/file10.txt
INFO  : - Path1    Queue delete                        - /testdir/path1/file3.txt
INFO  : - Path2    Do queued copies to                 - Path1
INFO  : - Path1    Do queued copies to                 - Path2
INFO  : -          Do queued deletes on                - Path1
INFO  : -          Do queued deletes on                - Path2
INFO  : Updating listings
INFO  : Validating listings for Path1 "/testdir/path1/" vs Path2 "/testdir/path2/"
INFO  : Bisync successful

Command line syntax

$ rclone bisync --help
Usage:
  rclone bisync remote1:path1 remote2:path2 [flags]

Positional arguments:
  Path1, Path2  Local path, or remote storage with ':' plus optional path.
                Type 'rclone listremotes' for list of configured remotes.

Optional Flags:
      --check-access            Ensure expected `RCLONE_TEST` files are found on
                                both Path1 and Path2 filesystems, else abort.
      --check-filename FILENAME Filename for `--check-access` (default: `RCLONE_TEST`)
      --check-sync CHOICE       Controls comparison of final listings:
                                `true | false | only` (default: true)
                                If set to `only`, bisync will only compare listings
                                from the last run but skip actual sync.
      --filters-file PATH       Read filtering patterns from a file
      --max-delete PERCENT      Safety check on maximum percentage of deleted files allowed.
                                If exceeded, the bisync run will abort. (default: 50%)
      --force                   Bypass `--max-delete` safety check and run the sync.
                                Consider using with `--verbose`
      --create-empty-src-dirs   Sync creation and deletion of empty directories. 
                                  (Not compatible with --remove-empty-dirs)
      --remove-empty-dirs       Remove empty directories at the final cleanup step.
  -1, --resync                  Performs the resync run.
                                Warning: Path1 files may overwrite Path2 versions.
                                Consider using `--verbose` or `--dry-run` first.
      --ignore-listing-checksum Do not use checksums for listings 
                                  (add --ignore-checksum to additionally skip post-copy checksum checks)
      --resilient               Allow future runs to retry after certain less-serious errors, 
                                  instead of requiring --resync. Use at your own risk!
      --localtime               Use local time in listings (default: UTC)
      --no-cleanup              Retain working files (useful for troubleshooting and testing).
      --workdir PATH            Use custom working directory (useful for testing).
                                (default: `~/.cache/rclone/bisync`)
      --backup-dir1 PATH        --backup-dir for Path1. Must be a non-overlapping path on the same remote.
      --backup-dir2 PATH        --backup-dir for Path2. Must be a non-overlapping path on the same remote.
  -n, --dry-run                 Go through the motions - No files are copied/deleted.
  -v, --verbose                 Increases logging verbosity.
                                May be specified more than once for more details.
  -h, --help                    help for bisync

Arbitrary rclone flags may be specified on the
bisync command line, for example
rclone bisync ./testdir/path1/ gdrive:testdir/path2/ --drive-skip-gdocs -v -v --timeout 10s
Note that interactions of various rclone flags with bisync process flow
has not been fully tested yet.

Paths

Path1 and Path2 arguments may be references to any mix of local directory
paths (absolute or relative), UNC paths (//server/share/path),
Windows drive paths (with a drive letter and :) or configured
remotes with optional subdirectory paths.
Cloud references are distinguished by having a : in the argument
(see Windows support below).

Path1 and Path2 are treated equally, in that neither has priority for
file changes (except during --resync), and access efficiency does not change whether a remote
is on Path1 or Path2.

The listings in bisync working directory (default: ~/.cache/rclone/bisync)
are named based on the Path1 and Path2 arguments so that separate syncs
to individual directories within the tree may be set up, e.g.:
path_to_local_tree..dropbox_subdir.lst.

Any empty directories after the sync on both the Path1 and Path2
filesystems are not deleted by default, unless --create-empty-src-dirs is specified.
If the --remove-empty-dirs flag is specified, then both paths will have ALL empty directories purged
as the last step in the process.

Command-line flags

--resync

This will effectively make both Path1 and Path2 filesystems contain a
matching superset of all files. Path2 files that do not exist in Path1 will
be copied to Path1, and the process will then copy the Path1 tree to Path2.

The --resync sequence is roughly equivalent to:

rclone copy Path2 Path1 --ignore-existing [--create-empty-src-dirs]
rclone copy Path1 Path2 [--create-empty-src-dirs]

The base directories on both Path1 and Path2 filesystems must exist
or bisync will fail. This is required for safety - that bisync can verify
that both paths are valid.

When using --resync, a newer version of a file on the Path2 filesystem
will be overwritten by the Path1 filesystem version.
(Note that this is NOT entirely symmetrical.)
Carefully evaluate deltas using --dry-run.

For a resync run, one of the paths may be empty (no files in the path tree).
The resync run should result in files on both paths, else a normal non-resync
run will fail.

For a non-resync run, either path being empty (no files in the tree) fails with
Empty current PathN listing. Cannot sync to an empty directory: X.pathN.lst
This is a safety check that an unexpected empty path does not result in
deleting everything in the other path.

Note: --resync should only be used under three specific (rare) circumstances:

1. It is your first bisync run (between these two paths)
2. You've just made changes to your bisync settings (such as editing the contents of your --filters-file)
3. There was an error on the prior run, and as a result, bisync now requires --resync to recover

The rest of the time, you should omit --resync. The reason is because --resync will only copy (not sync) each side to the other.
Therefore, if you included --resync for every bisync run, it would never be possible to delete a file --
the deleted file would always keep reappearing at the end of every run (because it's being copied from the other side where it still exists).
Similarly, renaming a file would always result in a duplicate copy (both old and new name) on both sides.

--check-access

Access check files are an additional safety measure against data loss.
bisync will ensure it can find matching RCLONE_TEST files in the same places
in the Path1 and Path2 filesystems.
RCLONE_TEST files are not generated automatically.
For --check-access to succeed, you must first either:
A) Place one or more RCLONE_TEST files in both systems, or
B) Set --check-filename to a filename already in use in various locations
throughout your sync'd fileset. Recommended methods for A) include:

• rclone touch Path1/RCLONE_TEST (create a new file)
• rclone copyto Path1/RCLONE_TEST Path2/RCLONE_TEST (copy an existing file)
• rclone copy Path1/RCLONE_TEST Path2/RCLONE_TEST --include "RCLONE_TEST" (copy multiple files at once, recursively)
• create the files manually (outside of rclone)
• run bisync once without --check-access to set matching files on both filesystems
  will also work, but is not preferred, due to potential for user error
  (you are temporarily disabling the safety feature).

Note that --check-access is still enforced on --resync, so bisync --resync --check-access
will not work as a method of initially setting the files (this is to ensure that bisync can't
inadvertently circumvent its own safety switch.)

Time stamps and file contents for RCLONE_TEST files are not important, just the names and locations.
If you have symbolic links in your sync tree it is recommended to place
RCLONE_TEST files in the linked-to directory tree to protect against
bisync assuming a bunch of deleted files if the linked-to tree should not be
accessible.
See also the --check-filename flag.

--check-filename

Name of the file(s) used in access health validation.
The default --check-filename is RCLONE_TEST.
One or more files having this filename must exist, synchronized between your
source and destination filesets, in order for --check-access to succeed.
See --check-access for additional details.

--max-delete

As a safety check, if greater than the --max-delete percent of files were
deleted on either the Path1 or Path2 filesystem, then bisync will abort with
a warning message, without making any changes.
The default --max-delete is 50%.
One way to trigger this limit is to rename a directory that contains more
than half of your files. This will appear to bisync as a bunch of deleted
files and a bunch of new files.
This safety check is intended to block bisync from deleting all of the
files on both filesystems due to a temporary network access issue, or if
the user had inadvertently deleted the files on one side or the other.
To force the sync, either set a different delete percentage limit,
e.g. --max-delete 75 (allows up to 75% deletion), or use --force
to bypass the check.

Also see the all files changed check.

--filters-file

By using rclone filter features you can exclude file types or directory
sub-trees from the sync.
See the bisync filters section and generic
--filter-from
documentation.
An example filters file contains filters for
non-allowed files for synching with Dropbox.

If you make changes to your filters file then bisync requires a run
with --resync. This is a safety feature, which prevents existing files
on the Path1 and/or Path2 side from seeming to disappear from view
(since they are excluded in the new listings), which would fool bisync
into seeing them as deleted (as compared to the prior run listings),
and then bisync would proceed to delete them for real.

To block this from happening, bisync calculates an MD5 hash of the filters file
and stores the hash in a .md5 file in the same place as your filters file.
On the next run with --filters-file set, bisync re-calculates the MD5 hash
of the current filters file and compares it to the hash stored in the .md5 file.
If they don't match, the run aborts with a critical error and thus forces you
to do a --resync, likely avoiding a disaster.

--check-sync

Enabled by default, the check-sync function checks that all of the same
files exist in both the Path1 and Path2 history listings. This check-sync
integrity check is performed at the end of the sync run by default.
Any untrapped failing copy/deletes between the two paths might result
in differences between the two listings and in the untracked file content
differences between the two paths. A resync run would correct the error.

Note that the default-enabled integrity check locally executes a load of both
the final Path1 and Path2 listings, and thus adds to the run time of a sync.
Using --check-sync=false will disable it and may significantly reduce the
sync run times for very large numbers of files.

The check may be run manually with --check-sync=only. It runs only the
integrity check and terminates without actually synching.

Note that currently, --check-sync only checks filenames and NOT modtime, size, or hash.
For a more robust integrity check of the current state, consider using check
(or cryptcheck, if at least one path is a crypt remote.)
For example, a possible sequence could look like this:

1. Normally scheduled bisync run:

rclone bisync Path1 Path2 -MPc --check-access --max-delete 10 --filters-file /path/to/filters.txt -v --no-cleanup --ignore-listing-checksum --disable ListR --checkers=16 --drive-pacer-min-sleep=10ms --create-empty-src-dirs --resilient

2. Periodic independent integrity check (perhaps scheduled nightly or weekly):

rclone check -MvPc Path1 Path2 --filter-from /path/to/filters.txt

3. If diffs are found, you have some choices to correct them.
   If one side is more up-to-date and you want to make the other side match it, you could run:

rclone sync Path1 Path2 --filter-from /path/to/filters.txt --create-empty-src-dirs -MPc -v

(or switch Path1 and Path2 to make Path2 the source-of-truth)

Or, if neither side is totally up-to-date, you could run a --resync to bring them back into agreement
(but remember that this could cause deleted files to re-appear.)

*Note also that rclone check does not currently include empty directories,
so if you want to know if any empty directories are out of sync,
consider alternatively running the above rclone sync command with --dry-run added.

See also: Concurrent modifications, --resilient

--ignore-listing-checksum

By default, bisync will retrieve (or generate) checksums (for backends that support them)
when creating the listings for both paths, and store the checksums in the listing files.
--ignore-listing-checksum will disable this behavior, which may speed things up considerably,
especially on backends (such as local) where hashes must be computed on the fly instead of retrieved.
Please note the following:

• While checksums are (by default) generated and stored in the listing files,
  they are NOT currently used for determining diffs (deltas).
  It is anticipated that full checksum support will be added in a future version.
• --ignore-listing-checksum is NOT the same as --ignore-checksum,
  and you may wish to use one or the other, or both. In a nutshell:
  --ignore-listing-checksum controls whether checksums are considered when scanning for diffs,
  while --ignore-checksum controls whether checksums are considered during the copy/sync operations that follow,
  if there ARE diffs.
• Unless --ignore-listing-checksum is passed, bisync currently computes hashes for one path
  even when there's no common hash with the other path
  (for example, a crypt remote.)

--resilient

Caution: this is an experimental feature. Use at your own risk!

By default, most errors or interruptions will cause bisync to abort and
require --resync to recover. This is a safety feature,
to prevent bisync from running again until a user checks things out.
However, in some cases, bisync can go too far and enforce a lockout when one isn't actually necessary,
like for certain less-serious errors that might resolve themselves on the next run.
When --resilient is specified, bisync tries its best to recover and self-correct,
and only requires --resync as a last resort when a human's involvement is absolutely necessary.
The intended use case is for running bisync as a background process (such as via scheduled cron).

When using --resilient mode, bisync will still report the error and abort,
however it will not lock out future runs -- allowing the possibility of retrying at the next normally scheduled time,
without requiring a --resync first. Examples of such retryable errors include
access test failures, missing listing files, and filter change detections.
These safety features will still prevent the current run from proceeding --
the difference is that if conditions have improved by the time of the next run,
that next run will be allowed to proceed.
Certain more serious errors will still enforce a --resync lockout, even in --resilient mode, to prevent data loss.

Behavior of --resilient may change in a future version.

--backup-dir1 and --backup-dir2

As of v1.65, --backup-dir is supported in bisync.
Because --backup-dir must be a non-overlapping path on the same remote,
Bisync has introduced new --backup-dir1 and --backup-dir2 flags to support
separate backup-dirs for Path1 and Path2 (bisyncing between different
remotes with --backup-dir would not otherwise be possible.) --backup-dir1
and --backup-dir2 can use different remotes from each other, but
--backup-dir1 must use the same remote as Path1, and --backup-dir2 must
use the same remote as Path2. Each backup directory must not overlap its
respective bisync Path without being excluded by a filter rule.

The standard --backup-dir will also work, if both paths use the same remote
(but note that deleted files from both paths would be mixed together in the
same dir). If either --backup-dir1 and --backup-dir2 are set, they will
override --backup-dir.

Example:

rclone bisync /Users/someuser/some/local/path/Bisync gdrive:Bisync --backup-dir1 /Users/someuser/some/local/path/BackupDir --backup-dir2 gdrive:BackupDir --suffix -2023-08-26 --suffix-keep-extension --check-access --max-delete 10 --filters-file /Users/someuser/some/local/path/bisync_filters.txt --no-cleanup --ignore-listing-checksum --checkers=16 --drive-pacer-min-sleep=10ms --create-empty-src-dirs --resilient -MvP --drive-skip-gdocs --fix-case

In this example, if the user deletes a file in
/Users/someuser/some/local/path/Bisync, bisync will propagate the delete to
the other side by moving the corresponding file from gdrive:Bisync to
gdrive:BackupDir. If the user deletes a file from gdrive:Bisync, bisync
moves it from /Users/someuser/some/local/path/Bisync to
/Users/someuser/some/local/path/BackupDir.

In the event of a ..path1 / ..path2 rename due to a sync conflict, the
rename is not considered a delete, unless a previous conflict with the same
name already exists and would get overwritten.

See also: --suffix,
--suffix-keep-extension

Operation

Runtime flow details

bisync retains the listings of the Path1 and Path2 filesystems
from the prior run.
On each successive run it will:

• list files on path1 and path2, and check for changes on each side.
  Changes include New, Newer, Older, and Deleted files.
• Propagate changes on path1 to path2, and vice-versa.

Safety measures

• Lock file prevents multiple simultaneous runs when taking a while.
  This can be particularly useful if bisync is run by cron scheduler.
• Handle change conflicts non-destructively by creating
  ..path1 and ..path2 file versions.
• File system access health check using RCLONE_TEST files
  (see the --check-access flag).
• Abort on excessive deletes - protects against a failed listing
  being interpreted as all the files were deleted.
  See the --max-delete and --force flags.
• If something evil happens, bisync goes into a safe state to block
  damage by later runs. (See Error Handling)

Normal sync checks

Type	Description	Result	Implementation
Path2 new	File is new on Path2, does not exist on Path1	Path2 version survives	rclone copy Path2 to Path1
Path2 newer	File is newer on Path2, unchanged on Path1	Path2 version survives	rclone copy Path2 to Path1
Path2 deleted	File is deleted on Path2, unchanged on Path1	File is deleted	rclone delete Path1
Path1 new	File is new on Path1, does not exist on Path2	Path1 version survives	rclone copy Path1 to Path2
Path1 newer	File is newer on Path1, unchanged on Path2	Path1 version survives	rclone copy Path1 to Path2
Path1 older	File is older on Path1, unchanged on Path2	Path1 version survives	rclone copy Path1 to Path2
Path2 older	File is older on Path2, unchanged on Path1	Path2 version survives	rclone copy Path2 to Path1
Path1 deleted	File no longer exists on Path1	File is deleted	rclone delete Path2

Unusual sync checks

Type	Description	Result	Implementation
Path1 new/changed AND Path2 new/changed AND Path1 == Path2	File is new/changed on Path1 AND new/changed on Path2 AND Path1 version is currently identical to Path2	No change	None
Path1 new AND Path2 new	File is new on Path1 AND new on Path2 (and Path1 version is NOT identical to Path2)	Files renamed to _Path1 and _Path2	rclone copy _Path2 file to Path1, rclone copy _Path1 file to Path2
Path2 newer AND Path1 changed	File is newer on Path2 AND also changed (newer/older/size) on Path1 (and Path1 version is NOT identical to Path2)	Files renamed to _Path1 and _Path2	rclone copy _Path2 file to Path1, rclone copy _Path1 file to Path2
Path2 newer AND Path1 deleted	File is newer on Path2 AND also deleted on Path1	Path2 version survives	rclone copy Path2 to Path1
Path2 deleted AND Path1 changed	File is deleted on Path2 AND changed (newer/older/size) on Path1	Path1 version survives	rclone copy Path1 to Path2
Path1 deleted AND Path2 changed	File is deleted on Path1 AND changed (newer/older/size) on Path2	Path2 version survives	rclone copy Path2 to Path1

As of rclone v1.64, bisync is now better at detecting false positive sync conflicts,
which would previously have resulted in unnecessary renames and duplicates.
Now, when bisync comes to a file that it wants to rename (because it is new/changed on both sides),
it first checks whether the Path1 and Path2 versions are currently identical
(using the same underlying function as check.)
If bisync concludes that the files are identical, it will skip them and move on.
Otherwise, it will create renamed ..Path1 and ..Path2 duplicates, as before.
This behavior also improves the experience of renaming directories,
as a --resync is no longer required, so long as the same change has been made on both sides.

All files changed check

If all prior existing files on either of the filesystems have changed
(e.g. timestamps have changed due to changing the system's timezone)
then bisync will abort without making any changes.
Any new files are not considered for this check. You could use --force
to force the sync (whichever side has the changed timestamp files wins).
Alternately, a --resync may be used (Path1 versions will be pushed
to Path2). Consider the situation carefully and perhaps use --dry-run
before you commit to the changes.

Modification times

Bisync relies on file timestamps to identify changed files and will
refuse to operate if backend lacks the modification time support.

If you or your application should change the content of a file
without changing the modification time then bisync will not
notice the change, and thus will not copy it to the other side.

Note that on some cloud storage systems it is not possible to have file
timestamps that match precisely between the local and other filesystems.

Bisync's approach to this problem is by tracking the changes on each side
separately over time with a local database of files in that side then
applying the resulting changes on the other side.

Error handling

Certain bisync critical errors, such as file copy/move failing, will result in
a bisync lockout of following runs. The lockout is asserted because the sync
status and history of the Path1 and Path2 filesystems cannot be trusted,
so it is safer to block any further changes until someone checks things out.
The recovery is to do a --resync again.

It is recommended to use --resync --dry-run --verbose initially and
carefully review what changes will be made before running the --resync
without --dry-run.

Most of these events come up due to an error status from an internal call.
On such a critical error the {...}.path1.lst and {...}.path2.lst
listing files are renamed to extension .lst-err, which blocks any future
bisync runs (since the normal .lst files are not found).
Bisync keeps them under bisync subdirectory of the rclone cache directory,
typically at ${HOME}/.cache/rclone/bisync/ on Linux.

Some errors are considered temporary and re-running the bisync is not blocked.
The critical return blocks further bisync runs.

See also: --resilient

Lock file

When bisync is running, a lock file is created in the bisync working directory,
typically at ~/.cache/rclone/bisync/PATH1..PATH2.lck on Linux.
If bisync should crash or hang, the lock file will remain in place and block
any further runs of bisync for the same paths.
Delete the lock file as part of debugging the situation.
The lock file effectively blocks follow-on (e.g., scheduled by cron) runs
when the prior invocation is taking a long time.
The lock file contains PID of the blocking process, which may help in debug.

Note
that while concurrent bisync runs are allowed, be very cautious
that there is no overlap in the trees being synched between concurrent runs,
lest there be replicated files, deleted files and general mayhem.

Return codes

rclone bisync returns the following codes to calling program:

• 0 on a successful run,
• 1 for a non-critical failing run (a rerun may be successful),
• 2 for a critically aborted run (requires a --resync to recover).

Limitations

Supported backends

Bisync is considered BETA and has been tested with the following backends:

• Local filesystem
• Google Drive
• Dropbox
• OneDrive
• S3
• SFTP
• Yandex Disk

It has not been fully tested with other services yet.
If it works, or sorta works, please let us know and we'll update the list.
Run the test suite to check for proper operation as described below.

First release of rclone bisync requires that underlying backend supports
the modification time feature and will refuse to run otherwise.
This limitation will be lifted in a future rclone bisync release.

Concurrent modifications

When using Local, FTP or SFTP remotes with --inplace, rclone does not create temporary
files at the destination when copying, and thus if the connection is lost
the created file may be corrupt, which will likely propagate back to the
original path on the next sync, resulting in data loss.
It is therefore recommended to omit --inplace.

Files that change during a bisync run may result in data loss.
Prior to rclone v1.65, this was commonly seen in highly dynamic environments, where the filesystem
was getting hammered by running processes during the sync.
As of rclone v1.65, bisync was redesigned to use a "snapshot" model,
greatly reducing the risks from changes during a sync.
Changes that are not detected during the current sync will now be detected during the following sync,
and will no longer cause the entire run to throw a critical error.
There is additionally a mechanism to mark files as needing to be internally rechecked next time, for added safety.
It should therefore no longer be necessary to sync only at quiet times --
however, note that an error can still occur if a file happens to change at the exact moment it's
being read/written by bisync (same as would happen in rclone sync.)
(See also: --ignore-checksum,
--local-no-check-updated)

Empty directories

By default, new/deleted empty directories on one path are not propagated to the other side.
This is because bisync (and rclone) natively works on files, not directories.
However, this can be changed with the --create-empty-src-dirs flag, which works in
much the same way as in sync and copy.
When used, empty directories created or deleted on one side will also be created or deleted on the other side.
The following should be noted:

• --create-empty-src-dirs is not compatible with --remove-empty-dirs. Use only one or the other (or neither).
• It is not recommended to switch back and forth between --create-empty-src-dirs
  and the default (no --create-empty-src-dirs) without running --resync.
  This is because it may appear as though all directories (not just the empty ones) were created/deleted,
  when actually you've just toggled between making them visible/invisible to bisync.
  It looks scarier than it is, but it's still probably best to stick to one or the other,
  and use --resync when you need to switch.

Renamed directories

By default, renaming a folder on the Path1 side results in deleting all files on
the Path2 side and then copying all files again from Path1 to Path2.
Bisync sees this as all files in the old directory name as deleted and all
files in the new directory name as new.

A recommended solution is to use --track-renames,
which is now supported in bisync as of rclone v1.65.
Note that --track-renames is not available during --resync,
as --resync does not delete anything (--track-renames only supports sync, not copy.)

Otherwise, the most effective and efficient method of renaming a directory
is to rename it to the same name on both sides. (As of rclone v1.64,
a --resync is no longer required after doing so, as bisync will automatically
detect that Path1 and Path2 are in agreement.)

--fast-list used by default

Unlike most other rclone commands, bisync uses --fast-list by default,
for backends that support it. In many cases this is desirable, however,
there are some scenarios in which bisync could be faster without --fast-list,
and there is also a known issue concerning Google Drive users with many empty directories.
For now, the recommended way to avoid using --fast-list is to add --disable ListR
to all bisync commands. The default behavior may change in a future version.

Overridden Configs

When rclone detects an overridden config, it adds a suffix like {ABCDE} on the fly
to the internal name of the remote. Bisync follows suit by including this suffix in its listing filenames.
However, this suffix does not necessarily persist from run to run, especially if different flags are provided.
So if next time the suffix assigned is {FGHIJ}, bisync will get confused,
because it's looking for a listing file with {FGHIJ}, when the file it wants has {ABCDE}.
As a result, it throws
Bisync critical error: cannot find prior Path1 or Path2 listings, likely due to critical error on prior run
and refuses to run again until the user runs a --resync (unless using --resilient).
The best workaround at the moment is to set any backend-specific flags in the config file
instead of specifying them with command flags. (You can still override them as needed for other rclone commands.)

Case (and unicode) sensitivity

As of v1.65, case and unicode form differences no longer cause critical errors,
and normalization (when comparing between filesystems) is handled according to the same flags and defaults as rclone sync.
See the following options (all of which are supported by bisync) to control this behavior more granularly:

• --fix-case
• --ignore-case-sync
• --no-unicode-normalization
• --local-unicode-normalization and
  --local-case-sensitive (caution: these are normally not what you want.)

Note that in the (probably rare) event that --fix-case is used AND a file is new/changed on both sides
AND the checksums match AND the filename case does not match, the Path1 filename is considered the winner,
for the purposes of --fix-case (Path2 will be renamed to match it).

Windows support

Bisync has been tested on Windows 8.1, Windows 10 Pro 64-bit and on Windows
GitHub runners.

Drive letters are allowed, including drive letters mapped to network drives
(rclone bisync J:\localsync GDrive:).
If a drive letter is omitted, the shell current drive is the default.
Drive letters are a single character follows by :, so cloud names
must be more than one character long.

Absolute paths (with or without a drive letter), and relative paths
(with or without a drive letter) are supported.

Working directory is created at C:\Users\MyLogin\AppData\Local\rclone\bisync.

Note that bisync output may show a mix of forward / and back \ slashes.

Be careful of case independent directory and file naming on Windows
vs. case dependent Linux

Filtering

See filtering documentation
for how filter rules are written and interpreted.

Bisync's --filters-file flag slightly extends the rclone's
--filter-from
filtering mechanism.
For a given bisync run you may provide only one --filters-file.
The --include*, --exclude*, and --filter flags are also supported.

How to filter directories

Filtering portions of the directory tree is a critical feature for synching.

Examples of directory trees (always beneath the Path1/Path2 root level)
you may want to exclude from your sync:

• Directory trees containing only software build intermediate files.
• Directory trees containing application temporary files and data
  such as the Windows C:\Users\MyLogin\AppData\ tree.
• Directory trees containing files that are large, less important,
  or are getting thrashed continuously by ongoing processes.

On the other hand, there may be only select directories that you
actually want to sync, and exclude all others. See the
Example include-style filters for Windows user directories
below.

Filters file writing guidelines

1. Begin with excluding directory trees:
   • e.g. - /AppData/
   • ** on the end is not necessary. Once a given directory level
     is excluded then everything beneath it won't be looked at by rclone.
   • Exclude such directories that are unneeded, are big, dynamically thrashed,
     or where there may be access permission issues.
   • Excluding such dirs first will make rclone operations (much) faster.
   • Specific files may also be excluded, as with the Dropbox exclusions
     example below.
2. Decide if it's easier (or cleaner) to:
   • Include select directories and therefore exclude everything else -- or --
   • Exclude select directories and therefore include everything else
3. Include select directories:
   • Add lines like: + /Documents/PersonalFiles/** to select which
     directories to include in the sync.
   • ** on the end specifies to include the full depth of the specified tree.
   • With Include-style filters, files at the Path1/Path2 root are not included.
     They may be included with + /*.
   • Place RCLONE_TEST files within these included directory trees.
     They will only be looked for in these directory trees.
   • Finish by excluding everything else by adding - ** at the end
     of the filters file.
   • Disregard step 4.
4. Exclude select directories:
   • Add more lines like in step 1.
     For example: -/Desktop/tempfiles/, or - /testdir/.
     Again, a ** on the end is not necessary.
   • Do not add a - ** in the file. Without this line, everything
     will be included that has not been explicitly excluded.
   • Disregard step 3.

A few rules for the syntax of a filter file expanding on
filtering documentation:

• Lines may start with spaces and tabs - rclone strips leading whitespace.
• If the first non-whitespace character is a # then the line is a comment
  and will be ignored.
• Blank lines are ignored.
• The first non-whitespace character on a filter line must be a + or -.
• Exactly 1 space is allowed between the +/- and the path term.
• Only forward slashes (/) are used in path terms, even on Windows.
• The rest of the line is taken as the path term.
  Trailing whitespace is taken literally, and probably is an error.

Example include-style filters for Windows user directories

This Windows include-style example is based on the sync root (Path1)
set to C:\Users\MyLogin. The strategy is to select specific directories
to be synched with a network drive (Path2).

• - /AppData/ excludes an entire tree of Windows stored stuff
  that need not be synched.
  In my case, AppData has >11 GB of stuff I don't care about, and there are
  some subdirectories beneath AppData that are not accessible to my
  user login, resulting in bisync critical aborts.
• Windows creates cache files starting with both upper and
  lowercase NTUSER at C:\Users\MyLogin. These files may be dynamic,
  locked, and are generally don't care.
• There are just a few directories with my data that I do want synched,
  in the form of + /<path>. By selecting only the directory trees I
  want to avoid the dozen plus directories that various apps make
  at C:\Users\MyLogin\Documents.
• Include files in the root of the sync point, C:\Users\MyLogin,
  by adding the + /* line.
• This is an Include-style filters file, therefore it ends with - **
  which excludes everything not explicitly included.

- /AppData/
- NTUSER*
- ntuser*
+ /Documents/Family/**
+ /Documents/Sketchup/**
+ /Documents/Microcapture_Photo/**
+ /Documents/Microcapture_Video/**
+ /Desktop/**
+ /Pictures/**
+ /*
- **

Note also that Windows implements several "library" links such as
C:\Users\MyLogin\My Documents\My Music pointing to C:\Users\MyLogin\Music.
rclone sees these as links, so you must add --links to the
bisync command line if you which to follow these links. I find that I get
permission errors in trying to follow the links, so I don't include the
rclone --links flag, but then you get lots of Can't follow symlink…
noise from rclone about not following the links. This noise can be
quashed by adding --quiet to the bisync command line.

Example exclude-style filters files for use with Dropbox

• Dropbox disallows synching the listed temporary and configuration/data files.
  The - <filename> filters exclude these files where ever they may occur
  in the sync tree. Consider adding similar exclusions for file types
  you don't need to sync, such as core dump and software build files.
• bisync testing creates /testdir/ at the top level of the sync tree,
  and usually deletes the tree after the test. If a normal sync should run
  while the /testdir/ tree exists the --check-access phase may fail
  due to unbalanced RCLONE_TEST files.
  The - /testdir/ filter blocks this tree from being synched.
  You don't need this exclusion if you are not doing bisync development testing.
• Everything else beneath the Path1/Path2 root will be synched.
• RCLONE_TEST files may be placed anywhere within the tree, including the root.

Example filters file for Dropbox

# Filter file for use with bisync
# See https://rclone.org/filtering/ for filtering rules
# NOTICE: If you make changes to this file you MUST do a --resync run.
#         Run with --dry-run to see what changes will be made.

# Dropbox won't sync some files so filter them away here.
# See https://help.dropbox.com/installs-integrations/sync-uploads/files-not-syncing
- .dropbox.attr
- ~*.tmp
- ~$*
- .~*
- desktop.ini
- .dropbox

# Used for bisync testing, so excluded from normal runs
- /testdir/

# Other example filters
#- /TiBU/
#- /Photos/

How --check-access handles filters

At the start of a bisync run, listings are gathered for Path1 and Path2
while using the user's --filters-file. During the check access phase,
bisync scans these listings for RCLONE_TEST files.
Any RCLONE_TEST files hidden by the --filters-file are not in the
listings and thus not checked during the check access phase.

Troubleshooting

Reading bisync logs

Here are two normal runs. The first one has a newer file on the remote.
The second has no deltas between local and remote.

2021/05/16 00:24:38 INFO  : Synching Path1 "/path/to/local/tree/" with Path2 "dropbox:/"
2021/05/16 00:24:38 INFO  : Path1 checking for diffs
2021/05/16 00:24:38 INFO  : - Path1    File is new                         - file.txt
2021/05/16 00:24:38 INFO  : Path1:    1 changes:    1 new,    0 newer,    0 older,    0 deleted
2021/05/16 00:24:38 INFO  : Path2 checking for diffs
2021/05/16 00:24:38 INFO  : Applying changes
2021/05/16 00:24:38 INFO  : - Path1    Queue copy to Path2                 - dropbox:/file.txt
2021/05/16 00:24:38 INFO  : - Path1    Do queued copies to                 - Path2
2021/05/16 00:24:38 INFO  : Updating listings
2021/05/16 00:24:38 INFO  : Validating listings for Path1 "/path/to/local/tree/" vs Path2 "dropbox:/"
2021/05/16 00:24:38 INFO  : Bisync successful

2021/05/16 00:36:52 INFO  : Synching Path1 "/path/to/local/tree/" with Path2 "dropbox:/"
2021/05/16 00:36:52 INFO  : Path1 checking for diffs
2021/05/16 00:36:52 INFO  : Path2 checking for diffs
2021/05/16 00:36:52 INFO  : No changes found
2021/05/16 00:36:52 INFO  : Updating listings
2021/05/16 00:36:52 INFO  : Validating listings for Path1 "/path/to/local/tree/" vs Path2 "dropbox:/"
2021/05/16 00:36:52 INFO  : Bisync successful

Dry run oddity

The --dry-run messages may indicate that it would try to delete some files.
For example, if a file is new on Path2 and does not exist on Path1 then
it would normally be copied to Path1, but with --dry-run enabled those
copies don't happen, which leads to the attempted delete on Path2,
blocked again by --dry-run: ... Not deleting as --dry-run.

This whole confusing situation is an artifact of the --dry-run flag.
Scrutinize the proposed deletes carefully, and if the files would have been
copied to Path1 then the threatened deletes on Path2 may be disregarded.

Retries

Rclone has built-in retries. If you run with --verbose you'll see
error and retry messages such as shown below. This is usually not a bug.
If at the end of the run, you see Bisync successful and not
Bisync critical error or Bisync aborted then the run was successful,
and you can ignore the error messages.

The following run shows an intermittent fail. Lines 5 and _6- are
low-level messages. Line 6 is a bubbled-up warning message, conveying
the error. Rclone normally retries failing commands, so there may be
numerous such messages in the log.

Since there are no final error/warning messages on line 7, rclone has
recovered from failure after a retry, and the overall sync was successful.

1: 2021/05/14 00:44:12 INFO  : Synching Path1 "/path/to/local/tree" with Path2 "dropbox:"
2: 2021/05/14 00:44:12 INFO  : Path1 checking for diffs
3: 2021/05/14 00:44:12 INFO  : Path2 checking for diffs
4: 2021/05/14 00:44:12 INFO  : Path2:  113 changes:   22 new,    0 newer,    0 older,   91 deleted
5: 2021/05/14 00:44:12 ERROR : /path/to/local/tree/objects/af: error listing: unexpected end of JSON input
6: 2021/05/14 00:44:12 NOTICE: WARNING  listing try 1 failed.                 - dropbox:
7: 2021/05/14 00:44:12 INFO  : Bisync successful

This log shows a Critical failure which requires a --resync to recover from.
See the Runtime Error Handling section.

2021/05/12 00:49:40 INFO  : Google drive root '': Waiting for checks to finish
2021/05/12 00:49:40 INFO  : Google drive root '': Waiting for transfers to finish
2021/05/12 00:49:40 INFO  : Google drive root '': not deleting files as there were IO errors
2021/05/12 00:49:40 ERROR : Attempt 3/3 failed with 3 errors and: not deleting files as there were IO errors
2021/05/12 00:49:40 ERROR : Failed to sync: not deleting files as there were IO errors
2021/05/12 00:49:40 NOTICE: WARNING  rclone sync try 3 failed.           - /path/to/local/tree/
2021/05/12 00:49:40 ERROR : Bisync aborted. Must run --resync to recover.

Denied downloads of "infected" or "abusive" files

Google Drive has a filter for certain file types (.exe, .apk, et cetera)
that by default cannot be copied from Google Drive to the local filesystem.
If you are having problems, run with --verbose to see specifically which
files are generating complaints. If the error is
This file has been identified as malware or spam and cannot be downloaded,
consider using the flag
--drive-acknowledge-abuse.

Google Docs (and other files of unknown size)

As of v1.65, Google Docs
(including Google Sheets, Slides, etc.) are now supported in bisync, subject to
the same options, defaults, and limitations as in rclone sync. When bisyncing
drive with non-drive backends, the drive -> non-drive direction is controlled
by --drive-export-formats (default
"docx,xlsx,pptx,svg") and the non-drive -> drive direction is controlled by
--drive-import-formats (default none.)

For example, with the default export/import formats, a Google Sheet on the
drive side will be synced to an .xlsx file on the non-drive side. In the
reverse direction, .xlsx files with filenames that match an existing Google
Sheet will be synced to that Google Sheet, while .xlsx files that do NOT
match an existing Google Sheet will be copied to drive as normal .xlsx files
(without conversion to Sheets, although the Google Drive web browser UI may
still give you the option to open it as one.)

If --drive-import-formats is set (it's not, by default), then all of the
specified formats will be converted to Google Docs, if there is no existing
Google Doc with a matching name. Caution: such conversion can be quite lossy,
and in most cases it's probably not what you want!

To bisync Google Docs as URL shortcut links (in a manner similar to "Drive for
Desktop"), use: --drive-export-formats url (or
alternatives.)

Note that these link files cannot be edited on the non-drive side -- you will
get errors if you try to sync an edited link file back to drive. They CAN be
deleted (it will result in deleting the corresponding Google Doc.) If you
create a .url file on the non-drive side that does not match an existing
Google Doc, bisyncing it will just result in copying the literal .url file
over to drive (no Google Doc will be created.) So, as a general rule of thumb,
think of them as read-only placeholders on the non-drive side, and make all
your changes on the drive side.

Likewise, even with other export-formats, it is best to only move/rename Google
Docs on the drive side. This is because otherwise, bisync will interpret this
as a file deleted and another created, and accordingly, it will delete the
Google Doc and create a new file at the new path. (Whether or not that new file
is a Google Doc depends on --drive-import-formats.)

Lastly, take note that all Google Docs on the drive side have a size of -1
and no checksum. Therefore, they cannot be reliably synced with the
--checksum or --size-only flags. (To be exact: they will still get
created/deleted, and bisync's delta engine will notice changes and queue them
for syncing, but the underlying sync function will consider them identical and
skip them.) To work around this, use the default (modtime and size) instead of
--checksum or --size-only.

To ignore Google Docs entirely, use
--drive-skip-gdocs.

(Note that all flags starting with --drive are backend-specific, and
therefore will cause the behavior explained in Overridden
Configs.)

Usage examples

Cron

Rclone does not yet have a built-in capability to monitor the local file
system for changes and must be blindly run periodically.
On Windows this can be done using a Task Scheduler,
on Linux you can use Cron which is described below.

The 1st example runs a sync every 5 minutes between a local directory
and an OwnCloud server, with output logged to a runlog file:

# Minute (0-59)
#      Hour (0-23)
#           Day of Month (1-31)
#                Month (1-12 or Jan-Dec)
#                     Day of Week (0-6 or Sun-Sat)
#                         Command
  */5  *    *    *    *   /path/to/rclone bisync /local/files MyCloud: --check-access --filters-file /path/to/bysync-filters.txt --log-file /path/to//bisync.log

See crontab syntax
for the details of crontab time interval expressions.

If you run rclone bisync as a cron job, redirect stdout/stderr to a file.
The 2nd example runs a sync to Dropbox every hour and logs all stdout (via the >>)
and stderr (via 2>&1) to a log file.

0 * * * * /path/to/rclone bisync /path/to/local/dropbox Dropbox: --check-access --filters-file /home/user/filters.txt >> /path/to/logs/dropbox-run.log 2>&1

Sharing an encrypted folder tree between hosts

bisync can keep a local folder in sync with a cloud service,
but what if you have some highly sensitive files to be synched?

Usage of a cloud service is for exchanging both routine and sensitive
personal files between one's home network, one's personal notebook when on the
road, and with one's work computer. The routine data is not sensitive.
For the sensitive data, configure an rclone crypt remote to point to
a subdirectory within the local disk tree that is bisync'd to Dropbox,
and then set up an bisync for this local crypt directory to a directory
outside of the main sync tree.

Linux server setup

• /path/to/DBoxroot is the root of my local sync tree.
  There are numerous subdirectories.
• /path/to/DBoxroot/crypt is the root subdirectory for files
  that are encrypted. This local directory target is setup as an
  rclone crypt remote named Dropcrypt:.
  See rclone.conf snippet below.
• /path/to/my/unencrypted/files is the root of my sensitive
  files - not encrypted, not within the tree synched to Dropbox.
• To sync my local unencrypted files with the encrypted Dropbox versions
  I manually run bisync /path/to/my/unencrypted/files DropCrypt:.
  This step could be bundled into a script to run before and after
  the full Dropbox tree sync in the last step,
  thus actively keeping the sensitive files in sync.
• bisync /path/to/DBoxroot Dropbox: runs periodically via cron,
  keeping my full local sync tree in sync with Dropbox.

Windows notebook setup

• The Dropbox client runs keeping the local tree C:\Users\MyLogin\Dropbox
  always in sync with Dropbox. I could have used rclone bisync instead.
• A separate directory tree at C:\Users\MyLogin\Documents\DropLocal
  hosts the tree of unencrypted files/folders.
• To sync my local unencrypted files with the encrypted
  Dropbox versions I manually run the following command:
  rclone bisync C:\Users\MyLogin\Documents\DropLocal Dropcrypt:.
• The Dropbox client then syncs the changes with Dropbox.

rclone.conf snippet

[Dropbox]
type = dropbox
...

[Dropcrypt]
type = crypt
remote = /path/to/DBoxroot/crypt          # on the Linux server
remote = C:\Users\MyLogin\Dropbox\crypt   # on the Windows notebook
filename_encryption = standard
directory_name_encryption = true
password = ...
...

Testing

You should read this section only if you are developing for rclone.
You need to have rclone source code locally to work with bisync tests.

Bisync has a dedicated test framework implemented in the bisync_test.go
file located in the rclone source tree. The test suite is based on the
go test command. Series of tests are stored in subdirectories below the
cmd/bisync/testdata directory. Individual tests can be invoked by their
directory name, e.g.
go test . -case basic -remote local -remote2 gdrive: -v

Tests will make a temporary folder on remote and purge it afterwards.
If during test run there are intermittent errors and rclone retries,
these errors will be captured and flagged as invalid MISCOMPAREs.
Rerunning the test will let it pass. Consider such failures as noise.

Test command syntax

usage: go test ./cmd/bisync [options...]

Options:
  -case NAME        Name(s) of the test case(s) to run. Multiple names should
                    be separated by commas. You can remove the `test_` prefix
                    and replace `_` by `-` in test name for convenience.
                    If not `all`, the name(s) should map to a directory under
                    `./cmd/bisync/testdata`.
                    Use `all` to run all tests (default: all)
  -remote PATH1     `local` or name of cloud service with `:` (default: local)
  -remote2 PATH2    `local` or name of cloud service with `:` (default: local)
  -no-compare       Disable comparing test results with the golden directory
                    (default: compare)
  -no-cleanup       Disable cleanup of Path1 and Path2 testdirs.
                    Useful for troubleshooting. (default: cleanup)
  -golden           Store results in the golden directory (default: false)
                    This flag can be used with multiple tests.
  -debug            Print debug messages
  -stop-at NUM      Stop test after given step number. (default: run to the end)
                    Implies `-no-compare` and `-no-cleanup`, if the test really
                    ends prematurely. Only meaningful for a single test case.
  -refresh-times    Force refreshing the target modtime, useful for Dropbox
                    (default: false)
  -verbose          Run tests verbosely

Note: unlike rclone flags which must be prefixed by double dash (--), the
test command flags can be equally prefixed by a single - or double dash.

Running tests

• go test . -case basic -remote local -remote2 local
  runs the test_basic test case using only the local filesystem,
  synching one local directory with another local directory.
  Test script output is to the console, while commands within scenario.txt
  have their output sent to the .../workdir/test.log file,
  which is finally compared to the golden copy.
• The first argument after go test should be a relative name of the
  directory containing bisync source code. If you run tests right from there,
  the argument will be . (current directory) as in most examples below.
  If you run bisync tests from the rclone source directory, the command
  should be go test ./cmd/bisync ....
• The test engine will mangle rclone output to ensure comparability
  with golden listings and logs.
• Test scenarios are located in ./cmd/bisync/testdata. The test -case
  argument should match the full name of a subdirectory under that
  directory. Every test subdirectory name on disk must start with test_,
  this prefix can be omitted on command line for brevity. Also, underscores
  in the name can be replaced by dashes for convenience.
• go test . -remote local -remote2 local -case all runs all tests.
• Path1 and Path2 may either be the keyword local
  or may be names of configured cloud services.
  go test . -remote gdrive: -remote2 dropbox: -case basic
  will run the test between these two services, without transferring
  any files to the local filesystem.
• Test run stdout and stderr console output may be directed to a file, e.g.
  go test . -remote gdrive: -remote2 local -case all > runlog.txt 2>&1

Test execution flow

1. The base setup in the initial directory of the testcase is applied
   on the Path1 and Path2 filesystems (via rclone copy the initial directory
   to Path1, then rclone sync Path1 to Path2).
2. The commands in the scenario.txt file are applied, with output directed
   to the test.log file in the test working directory.
   Typically, the first actual command in the scenario.txt file is
   to do a --resync, which establishes the baseline
   {...}.path1.lst and {...}.path2.lst files in the test working
   directory (.../workdir/ relative to the temporary test directory).
   Various commands and listing snapshots are done within the test.
3. Finally, the contents of the test working directory are compared
   to the contents of the testcase's golden directory.

Notes about testing

• Test cases are in individual directories beneath ./cmd/bisync/testdata.
  A command line reference to a test is understood to reference a directory
  beneath testdata. For example,
  go test ./cmd/bisync -case dry-run -remote gdrive: -remote2 local
  refers to the test case in ./cmd/bisync/testdata/test_dry_run.
• The test working directory is located at .../workdir relative to a
  temporary test directory, usually under /tmp on Linux.
• The local test sync tree is created at a temporary directory named
  like bisync.XXX under system temporary directory.
• The remote test sync tree is located at a temporary directory
  under <remote:>/bisync.XXX/.
• path1 and/or path2 subdirectories are created in a temporary
  directory under the respective local or cloud test remote.
• By default, the Path1 and Path2 test dirs and workdir will be deleted
  after each test run. The -no-cleanup flag disables purging these
  directories when validating and debugging a given test.
  These directories will be flushed before running another test,
  independent of the -no-cleanup usage.
• You will likely want to add - /testdir/ to your normal
  bisync --filters-file so that normal syncs do not attempt to sync
  the test temporary directories, which may have RCLONE_TEST miscompares
  in some testcases which would otherwise trip the --check-access system.
  The --check-access mechanism is hard-coded to ignore RCLONE_TEST
  files beneath bisync/testdata, so the test cases may reside on the
  synched tree even if there are check file mismatches in the test tree.
• Some Dropbox tests can fail, notably printing the following message:
  src and dst identical but can't set mod time without deleting and re-uploading
  This is expected and happens due to the way Dropbox handles modification times.
  You should use the -refresh-times test flag to make up for this.
• If Dropbox tests hit request limit for you and print error message
  too_many_requests/...: Too many requests or write operations.
  then follow the
  Dropbox App ID instructions.

Updating golden results

Sometimes even a slight change in the bisync source can cause little changes
spread around many log files. Updating them manually would be a nightmare.

The -golden flag will store the test.log and *.lst listings from each
test case into respective golden directories. Golden results will
automatically contain generic strings instead of local or cloud paths which
means that they should match when run with a different cloud service.

Your normal workflow might be as follows:

1. Git-clone the rclone sources locally
2. Modify bisync source and check that it builds
3. Run the whole test suite go test ./cmd/bisync -remote local
4. If some tests show log difference, recheck them individually, e.g.:
   go test ./cmd/bisync -remote local -case basic
5. If you are convinced with the difference, goldenize all tests at once:
   go test ./cmd/bisync -remote local -golden
6. Use word diff: git diff --word-diff ./cmd/bisync/testdata/.
   Please note that normal line-level diff is generally useless here.
7. Check the difference carefully!
8. Commit the change (git commit) only if you are sure.
   If unsure, save your code changes then wipe the log diffs from git:
   git reset [--hard].

Structure of test scenarios

• <testname>/initial/ contains a tree of files that will be set
  as the initial condition on both Path1 and Path2 testdirs.
• <testname>/modfiles/ contains files that will be used to
  modify the Path1 and/or Path2 filesystems.
• <testname>/golden/ contains the expected content of the test
  working directory (workdir) at the completion of the testcase.
• <testname>/scenario.txt contains the body of the test, in the form of
  various commands to modify files, run bisync, and snapshot listings.
  Output from these commands is captured to .../workdir/test.log
  for comparison to the golden files.

Supported test commands

• test <some message>
  Print the line to the console and to the test.log:
  test sync is working correctly with options x, y, z
• copy-listings <prefix>
  Save a copy of all .lst listings in the test working directory
  with the specified prefix:
  save-listings exclude-pass-run
• move-listings <prefix>
  Similar to copy-listings but removes the source
• purge-children <dir>
  This will delete all child files and purge all child subdirs under given
  directory but keep the parent intact. This behavior is important for tests
  with Google Drive because removing and re-creating the parent would change
  its ID.
• delete-file <file>
  Delete a single file.
• delete-glob <dir> <pattern>
  Delete a group of files located one level deep in the given directory
  with names matching a given glob pattern.
• touch-glob YYYY-MM-DD <dir> <pattern>
  Change modification time on a group of files.
• touch-copy YYYY-MM-DD <source-file> <dest-dir>
  Change file modification time then copy it to destination.
• copy-file <source-file> <dest-dir>
  Copy a single file to given directory.
• copy-as <source-file> <dest-file>
  Similar to above but destination must include both directory
  and the new file name at destination.
• copy-dir <src> <dst> and sync-dir <src> <dst>
  Copy/sync a directory. Equivalent of rclone copy and rclone sync.
• list-dirs <dir>
  Equivalent to rclone lsf -R --dirs-only <dir>
• bisync [options]
  Runs bisync against -remote and -remote2.

Supported substitution terms

• {testdir/} - the root dir of the testcase
• {datadir/} - the modfiles dir under the testcase root
• {workdir/} - the temporary test working directory
• {path1/} - the root of the Path1 test directory tree
• {path2/} - the root of the Path2 test directory tree
• {session} - base name of the test listings
• {/} - OS-specific path separator
• {spc}, {tab}, {eol} - whitespace
• {chr:HH} - raw byte with given hexadecimal code

Substitution results of the terms named like {dir/} will end with
/ (or backslash on Windows), so it is not necessary to include
slash in the usage, for example delete-file {path1/}file1.txt.

Benchmarks

This section is work in progress.

Here are a few data points for scale, execution times, and memory usage.

The first set of data was taken between a local disk to Dropbox.
The speedtest.net download speed was ~170 Mbps,
and upload speed was ~10 Mbps. 500 files (~9.5 MB each) had been already
synched. 50 files were added in a new directory, each ~9.5 MB, ~475 MB total.

Change	Operations and times	Overall run time
500 files synched (nothing to move)	1x listings for Path1 & Path2	1.5 sec
500 files synched with --check-access	1x listings for Path1 & Path2	1.5 sec
50 new files on remote	Queued 50 copies down: 27 sec	29 sec
Moved local dir	Queued 50 copies up: 410 sec, 50 deletes up: 9 sec	421 sec
Moved remote dir	Queued 50 copies down: 31 sec, 50 deletes down: <1 sec	33 sec
Delete local dir	Queued 50 deletes up: 9 sec	13 sec

This next data is from a user's application. They had ~400GB of data
over 1.96 million files being sync'ed between a Windows local disk and some
remote cloud. The file full path length was on average 35 characters
(which factors into load time and RAM required).

• Loading the prior listing into memory (1.96 million files, listing file
  size 140 MB) took ~30 sec and occupied about 1 GB of RAM.
• Getting a fresh listing of the local file system (producing the
  140 MB output file) took about XXX sec.
• Getting a fresh listing of the remote file system (producing the 140 MB
  output file) took about XXX sec. The network download speed was measured
  at XXX Mb/s.
• Once the prior and current Path1 and Path2 listings were loaded (a total
  of four to be loaded, two at a time), determining the deltas was pretty
  quick (a few seconds for this test case), and the transfer time for any
  files to be copied was dominated by the network bandwidth.

References

rclone's bisync implementation was derived from
the rclonesync-V2 project,
including documentation and test mechanisms,
with @cjnaz's full support and encouragement.

rclone bisync is similar in nature to a range of other projects:

• unison
• syncthing
• cjnaz/rclonesync
• ConorWilliams/rsinc
• jwink3101/syncrclone
• DavideRossi/upback

Bisync adopts the differential synchronization technique, which is
based on keeping history of changes performed by both synchronizing sides.
See the Dual Shadow Method section in
Neil Fraser's article.

Also note a number of academic publications by
Benjamin Pierce
about Unison and synchronization in general.

Changelog

v1.65

• Copies and deletes are now handled in one operation instead of two
• --track-renames and --backup-dir are now supported
• Partial uploads known issue on local/ftp/sftp has been resolved (unless using --inplace)
• Final listings are now generated from sync results, to avoid needing to re-list
• Bisync is now much more resilient to changes that happen during a bisync run, and far less prone to critical errors / undetected changes
• Bisync is now capable of rolling a file listing back in cases of uncertainty, essentially marking the file as needing to be rechecked next time.
• A few basic terminal colors are now supported, controllable with --color (AUTO|NEVER|ALWAYS)
• Initial listing snapshots of Path1 and Path2 are now generated concurrently, using the same "march" infrastructure as check and sync,
  for performance improvements and less risk of error.
• Fixed handling of unicode normalization and case insensitivity, support for --fix-case, --ignore-case-sync, --no-unicode-normalization
• --resync is now much more efficient (especially for users of --create-empty-src-dirs)
• Google Docs (and other files of unknown size) are now supported (with the same options as in sync)

v1.64

• Fixed an issue
  causing dry runs to inadvertently commit filter changes
• Fixed an issue
  causing --resync to erroneously delete empty folders and duplicate files unique to Path2
• --check-access is now enforced during --resync, preventing data loss in certain user error scenarios
• Fixed an issue
  causing bisync to consider more files than necessary due to overbroad filters during delete operations
• Improved detection of false positive change conflicts
  (identical files are now left alone instead of renamed)
• Added support for --create-empty-src-dirs
• Added experimental --resilient mode to allow recovery from self-correctable errors
• Added new --ignore-listing-checksum flag
  to distinguish from --ignore-checksum
• Performance improvements for large remotes
• Documentation and testing improvements