Documentation< Previous | Contents | Next >
Synchronizing Files And Directories
A common strategy for maintaining a backup copy of a system involves keeping one or more directories synchronized with another directory (or directories) located on either the local system (usually a removable storage device of some kind) or a remote system. We might, for example, have a local copy of a website under development and synchronize it from time to time with the “live” copy on a remote web server.
In the Unix-like world, the preferred tool for this task is rsync. This program can syn- chronize both local and remote directories by using the rsync remote-update protocol, which allows rsync to quickly detect the differences between two directories and per- form the minimum amount of copying required to bring them into sync. This makes rsync very fast and economical to use, compared to other kinds of copy programs.
rsync is invoked like this:
rsync options source destination
where source and destination are one of the following:
● A local file or directory
● A remote file or directory in the form of [user@]host:path
● A remote rsync server specified with a URI of rsync://[user@]host[:port]/path
Note that either the source or the destination must be a local file. Remote-to-remote copy- ing is not supported.
Let’s try rsync out on some local files. First, let’s clean out our foo directory:
[me@linuxbox ~]$ rm -rf foo/*
[me@linuxbox ~]$ rm -rf foo/*
Next, we’ll synchronize the playground directory with a corresponding copy in foo:
[me@linuxbox ~]$ rsync -av playground foo
[me@linuxbox ~]$ rsync -av playground foo
We’ve included both the -a option (for archiving — causes recursion and preservation of file attributes) and the -v option (verbose output) to make a mirror of the playground directory within foo. While the command runs, we will see a list of the files and directo- ries being copied. At the end, we will see a summary message like this:
sent 135759 bytes received 57870 bytes 387258.00 bytes/sec total size is 3230 speedup is 0.02
sent 135759 bytes received 57870 bytes 387258.00 bytes/sec total size is 3230 speedup is 0.02
indicating the amount of copying performed. If we run the command again, we will see a different result:
[me@linuxbox ~]$ rsync -av playgound foo
building file list ... done
sent 22635 bytes received 20 bytes 45310.00 bytes/sec total size is 3230 speedup is 0.14
[me@linuxbox ~]$ rsync -av playgound foo
building file list ... done
sent 22635 bytes received 20 bytes 45310.00 bytes/sec total size is 3230 speedup is 0.14
Notice that there was no listing of files. This is because rsync detected that there were no differences between ~/playground and ~/foo/playground, and therefore it didn’t need to copy anything. If we modify a file in playground and run rsync again:
[me@linuxbox ~]$ touch playground/dir-099/file-Z [me@linuxbox ~]$ rsync -av playground foo building file list ... done
playground/dir-099/file-Z
sent 22685 bytes received 42 bytes 45454.00 bytes/sec total size is 3230 speedup is 0.14
[me@linuxbox ~]$ touch playground/dir-099/file-Z [me@linuxbox ~]$ rsync -av playground foo building file list ... done
playground/dir-099/file-Z
sent 22685 bytes received 42 bytes 45454.00 bytes/sec total size is 3230 speedup is 0.14
we see that rsync detected the change and copied only the updated file.
As a practical example, let’s consider the imaginary external hard drive that we used ear- lier with tar. If we attach the drive to our system and, once again, it is mounted at /me- dia/BigDisk, we can perform a useful system backup by first creating a directory named /backup on the external drive, and then using rsync to copy the most impor- tant stuff from our system to the external drive:
[me@linuxbox ~]$ mkdir /media/BigDisk/backup
[me@linuxbox ~]$ sudo rsync -av --delete /etc /home /usr/local
/media/BigDisk/backup
[me@linuxbox ~]$ mkdir /media/BigDisk/backup
[me@linuxbox ~]$ sudo rsync -av --delete /etc /home /usr/local
/media/BigDisk/backup
In this example, we copied the /etc, /home, and /usr/local directories from our system to our imaginary storage device. We included the --delete option to remove files that may have existed on the backup device that no longer existed on the source de- vice (this is irrelevant the first time we make a backup, but will be useful on subsequent copies). Repeating the procedure of attaching the external drive and running this rsync command would be a useful (though not ideal) way of keeping a small system backed up. Of course, an alias would be helpful here, too. We could create an alias and add it to our
.bashrc file to provide this feature:
alias backup='sudo rsync -av --delete /etc /home /usr/local
/media/BigDisk/backup'
alias backup='sudo rsync -av --delete /etc /home /usr/local
/media/BigDisk/backup'
Now all we have to do is attach our external drive and run the backup command to do the job.