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Shell Scripting: Using FTP to Automatically Create a Mirror of a Specified Directory of Files on a Remote Server

📄 Contents

  1. Working with FTP On the Command Line
  2. Turning It into a Shell Script
  3. Where to Go Next
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If you think that months of study are required to start customizing your Unix or Linux box, or that writing shell scripts is only for people who type kewl and d00d, you'll want to read this article about scripting the Unix FTP command. In simple terms, Unix expert and popular author Dave Taylor shows you how to create a powerful FTP-based file synchronization tool.

One of the greatest challenges of modern computing is keeping files in sync across multiple computers. Whether you have a local copy of your Web site on your PC or (exactly the opposite) have the master copy of your directory on your local Linux/Unix box and want to make sure that a remote directory has the same files available, it's ridiculously difficult to accomplish.

Which is why it's the perfect task for a shell script! And that's what this article will explore: How to write a simple Unix shell script—suitable for Unix, Linux, or even Mac OS X systems—to keep a local and remote directory synchronized automatically.

To start out, a caveat. I will be demonstrating how to accomplish this task using the FTP program, but I nonetheless don't recommend FTP because it's less secure than its SSH-based cousin SFTP. The problem is, most server administrators leave FTP as the de facto method of connectivity. To switch this entire technique to using SFTP is fairly straightforward, however, so the basic solution will nonetheless be helpful and informative.

Working with FTP On the Command Line

The first step is to figure out what commands you'll need to use with the FTP program itself to get to your remote system. First off, let's see the glorious ugliness that is a command line FTP transaction:

$ ftp laptop.intuitive.com
Connected to 192.168.1.106.
220 local FTP server (lukemftpd 1.1) ready.
Name (192.168.1.106:taylor): taylor
331 Password required for taylor.
Password:
230 User taylor logged in.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> dir
229 Entering Extended Passive Mode (|||49288|)
150 Opening ASCII mode data connection for '/bin/ls'.
total 1
drwx------  10 taylor taylor  340 Mar 17 15:00 Desktop
drwx------  40 taylor taylor 1360 Dec 10 10:26 Documents
drwx------  37 taylor taylor 1258 Feb 20 14:51 Library
drwxr-xr-x  3 taylor taylor  102 Jan 26 2003 Movies
drwxr-xr-x  4 taylor taylor  136 Sep 22 15:30 Music
drwx------  73 taylor taylor 2482 Mar 5 11:07 Pictures
drwxr-xr-x  4 taylor taylor  136 Jan 29 2002 Public
drwxr-xr-x  5 taylor taylor  170 Mar 19 17:59 Sites
drwxrwxrwx  9 taylor taylor  306 Jan 11 2002 bin
drwx------  6 taylor taylor  204 Jul 7 2002 mail
drwxr-xr-x 143 taylor taylor 4862 Sep 26 13:43 scripts
226 Transfer complete.
ftp> quit
221 Thank you for using the FTP service
$

Here you can see that I connected to a remote system, laptop.intuitive.com, used taylor as my account name, and entered the necessary password. Once connected, I requested a directory listing with dir and then quit.

The same task can be done without as much interactivity by using a here document, denoted with the << sequence. By using this, I can essentially type in all my FTP commands at once; then let them be fed one-by-one to the program:

$ ftp laptop.intuitive.com << "EOF"
> taylor
> dir
> quit
> EOF
Connected to 192.168.1.106.
220 local FTP server (lukemftpd 1.1) ready.
Name (192.168.1.106:taylor): 331 Password required for taylor.
Password:
230 User taylor logged in.
Remote system type is UNIX.
Using binary mode to transfer files.
229 Entering Extended Passive Mode (|||49367|)
150 Opening ASCII mode data connection for '/bin/ls'.
total 1
drwx------  13 taylor taylor 442 Mar 25 15:52 Desktop
drwx------  40 taylor taylor 1360 Dec 10 10:26 Documents
drwx------  37 taylor taylor 1258 Mar 26 09:37 Library
drwxr-xr-x  3 taylor taylor 102 Jan 26 2003 Movies
drwxr-xr-x  4 taylor taylor 136 Sep 22 2003 Music
drwx------  73 taylor taylor 2482 Mar 5 11:07 Pictures
drwxr-xr-x  4 taylor taylor 136 Jan 29 2002 Public
drwxr-xr-x  5 taylor taylor 170 Mar 19 17:59 Sites
drwxrwxrwx  9 taylor taylor 306 Jan 11 2002 bin
drwx------  6 taylor taylor 204 Jul 7 2002 mail
drwxr-xr-x 143 taylor taylor 4862 Sep 26 13:43 scripts
226 Transfer complete.
221 Thank you for using the FTP service
$ 

Pay close attention to what I typed in and where: Everything but the password itself can easily be dropped into a shell script. Let's have a look.

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