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Chapter 7: Using the command-line connection tool Plink

Plink (PuTTY Link) is a command-line connection tool similar to UNIX ssh. It is mostly used for automated operations, such as making CVS access a repository on a remote server.

Plink is probably not what you want if you want to run an interactive session in a console window.

Section 7.1: Starting Plink

Plink is a command line application. This means that you cannot just double-click on its icon to run it and instead you have to bring up a console window. In Windows 95, 98, and ME, this is called an "MS-DOS Prompt", and in Windows NT and 2000 it is called a "Command Prompt". It should be available from the Programs section of your Start Menu.

In order to use Plink, the file plink.exe will need either to be on your PATH or in your current directory. To add the directory containing Plink to your PATH environment variable, type into the console window:

set PATH=C:\path\to\putty\directory;%PATH%

This will only work for the lifetime of that particular console window. To set your PATH more permanently on Windows NT, use the Environment tab of the System Control Panel. On Windows 95, 98, and ME, you will need to edit your AUTOEXEC.BAT to include a set command like the one above.

Section 7.2: Using Plink

This section describes the basics of how to use Plink for interactive logins and for automated processes.

Once you've got a console window to type into, you can just type plink on its own to bring up a usage message. This tells you the version of Plink you're using, and gives you a brief summary of how to use Plink:

PuTTY Link: command-line connection utility
Release 0.50
Usage: plink [options] [user@]host [command]
  -v        show verbose messages
  -ssh      force use of ssh protocol
  -P port   connect to specified port
  -pw passw login with specified password

Once this works, you are ready to use Plink.

7.2.1 Using Plink for interactive logins

To make a simple interactive connection to a remote server, just type plink and then the host name:


Debian GNU/Linux 2.2
flunky login:

You should then be able to log in as normal and run a session. The output sent by the server will be written straight to your command prompt window, which will most likely not interpret terminal control codes in the way the server expects it to. So if you run any full-screen applications, for example, you can expect to see strange characters appearing in your window. Interactive connections like this are not the main point of Plink.

In order to connect with a different protocol, you can give the command line options -ssh, -telnet, -rlogin or -raw. To make an SSH connection, for example:

Z:\sysosd>plink -ssh
login as:

If you have already set up a PuTTY saved session, then instead of supplying a host name, you can give the saved session name. This allows you to use public-key authentication, specify a user name, and use most of the other features of PuTTY:

Z:\sysosd>plink my-ssh-session
Sent username "fred"
Authenticating with public key "fred@winbox"
Last login: Thu Dec  6 19:25:33 2001 from :0.0

7.2.2 Using Plink for automated connections

More typically Plink is used with the SSH protocol, to enable you to talk directly to a program running on the server. To do this you have to ensure Plink is using the SSH protocol. You can do this in several ways:

Usually Plink is not invoked directly by a user, but run automatically by another process. Therefore you typically do not want Plink to prompt you for a user name or a password.

To avoid being prompted for a user name, you can:

To avoid being prompted for a password, you should almost certainly set up public-key authentication. (See chapter 8 for a general introduction to public-key authentication.) Again, you can do this in two ways:

Once you have done all this, you should be able to run a remote command on the SSH server machine and have it execute automatically with no prompting:

Z:\sysosd>plink -l fred echo hello, world
hello, world


Or, if you have set up a saved session with all the connection details:

Z:\sysosd>plink mysession echo hello, world
hello, world


Then you can set up other programs to run this Plink command and talk to it as if it were a process on the server machine.

You may also find it useful to use the -batch command-line option; see section

7.2.3 Options

This section describes the command line options that Plink accepts. -v show verbose messages

By default, Plink only displays any password prompts and the output of the remote command. The -v option makes it print extra information about the connection being made, for example:

Server version: SSH-1.5-OpenSSH-1.2.3
We claim version: SSH-1.5-PuTTY
Using SSH protocol version 1
Received public keys
Host key fingerprint is:
      1023 e3:65:44:44:bd:b1:04:59:bc:e2:3d:a1:4d:09:ce:99
Encrypted session key
Using 3DES encryption
Trying to enable encryption...
Successfully started encryption
Sent username "fred".
Sent username "fred"'s password:

This information can be useful for diagnosing problems. Protocol selection options

Plink is most useful when using the SSH protocol. However, it allows you to interface to all the protocols supported by PuTTY. You can specify the option -ssh on the command line to select the SSH protocol; you can also specify -telnet, -rlogin or -raw to select other protocols. -P port connect to specified port

If your server machine is running its SSH service on a port other than the standard one, you can specify an alternative port number to connect to using the -P option, like this:

plink -ssh -P 5022 -pw passw login with specified password

A simple way to automate a remote login is to supply your password on the Plink command line. This is not recommended for reasons of security. If you possibly can, we recommend you set up public-key authentication instead. See chapter 8 for details. -l username login with specified username

As described in section 7.2.2, you can specify the user name to log in as on the remote server using the -l option. For example, plink -l fred. -batch avoid interactive prompts

If you use the -batch option, Plink will never give an interactive prompt while establishing the connection. If the server's host key is invalid, for example (see section 2.2), then the connection will simply be abandoned instead of asking you what to do next.

This may help Plink's behaviour when it is used in automated scripts: using -batch, if something goes wrong at connection time, the batch job will fail rather than hang. -m filename read command from a file

If the command you want to run on the remote server is particularly large, you can read it from a file using the -m option, instead of putting it directly on Plink's command line. On most Unix systems, you can even put multiple lines in this file and execute more than one command in sequence, or a whole shell script. -L and -R set up port forwarding

Plink allows you to use port forwarding just as PuTTY does; if you have set up a PuTTY saved session that specifies port forwardings, and you connect to that session using Plink, then the same port forwardings will be set up.

For convenience, Plink also offers the option to set up port forwarding on the command line. The command-line options work just like the ones in Unix ssh programs.

To forward a local port (say 5110) to a remote destination (say port 110), you can write:

plink mysession -L

And to forward a remote port to a local destination, just use the -R option instead of -L:

plink mysession -R

For general information on port forwarding, see section 3.5.

Section 7.3: Using Plink in batch files and scripts

Once you have set up Plink to be able to log in to a remote server without any interactive prompting (see section 7.2.2), you can use it for lots of scripting and batch purposes. For example, to start a backup on a remote machine, you might use a command like:

plink root@myserver /etc/backups/

Or perhaps you want to fetch all system log lines relating to a particular web area:

plink mysession grep /~fjbloggs/ /var/log/httpd/access.log > fredlogs

Any non-interactive command you could usefully run on the server command line, you can run in a batch file using Plink in this way.

Section 7.4: Using Plink with CVS

To use Plink with CVS, you need to set the environment variable CVS_RSH to point to Plink:

set CVS_RSH=\path\to\plink.exe

You also need to arrange to be able to connect to a remote host without any interactive prompts, as described in section 7.2.2.

You should then be able to run CVS as follows:

cvs -d :ext:user@sessionname:/path/to/repository co module

If you specified a username in your saved session, you don't even need to specify the "user" part of this, and you can just say:

cvs -d :ext:sessionname:/path/to/repository co module

Section 7.5: Using Plink with WinCVS

Plink can also be used with WinCVS. Firstly, arrange for Plink to be able to connect to a remote host non-interactively, as described in section 7.2.2.

Then, in WinCVS, bring up the "Preferences" dialogue box from the Admin menu, and switch to the "Ports" tab. Tick the box there labelled "Check for an alternate rsh name" and in the text entry field to the right enter the full path to plink.exe. Select "OK" on the "Preferences" dialogue box.

Next, select "Command Line" from the WinCVS "Admin" menu, and type a CVS command as in section 7.4, for example:

cvs -d :ext:user@hostname:/path/to/repository co module

Select the folder you want to check out to with the "Change Folder" button, and click "OK" to check out your module. Once you've got modules checked out, WinCVS will happily invoke plink from the GUI for CVS operations.

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