So far in this series of posts on
ssh on macOS:
- Quick Introduction to
sshfor Mac Admins
- SSH Keys, Part 1: Host Verification
- SSH Keys, Part 2: Client Verification
- Transferring files with
- SSH Tunnels (upcoming)
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In the previous posts we looked how to connect with
ssh to a remote computer (host) and how to setup the keys necessary for a secure connection.
Despite the name
ssh does not actually provide a shell or command line interface to the host itself. it ‘merely’ provides a secure connection to connect to the default shell on the host itself.
Even this basic use of
ssh is already very useful and powerful. It allows to open one or more full command line sessions to remote computers as if we sat at their keyboard. You can also send individual commands and receive and process the results.
ssh has a few more powerful tools available.
Copy Files Remotely
You can also use the
ssh connection to copy files to and from a remote host.
The command you use for this is
scp (secure copy) and it use the same basic syntax as the
$ scp source destination
scp can copy from the local computer to a remote host or vice versa, you usually add a bit more information:
$ scp [[user@]host1:]source [[user@]host2:]destination
(The examples will use a file name
hello.txt. To create one quickly, simply type
echo "Hello SSH" > hello.txt in Terminal.)
To simplify this, a few examples:
$ scp hello.txt mac.example.com:"~/Documents/" hello.txt 100% 10 10.8KB/s 00:00
This will copy the local file
sample.txt from the current working directory to the remote host’s
scp will show an ascii progress bar for every file copied. (though with these small files, you will not see much of it) You can suppress the progress display with the
-q (quite) option.
For the destination, the colon
: separates the hostname (DNS) from the file path.
This command will prompt for the user’s password on the remote host, unless you have added your public key to the remote host’s
Since no user name is given before the hostname (separated with an
scp uses the username that you are logged in with on the local computer. If the remote user has a different name, use:
$ scp hello.txt firstname.lastname@example.org:"~/Documents/"
We do not want the local shell to evaluate the
~ to the local home directory, but want the remote computer to evaluate
~ to the remote user’s home directory, so we have to quote the remote path.
cp, when the source is a file and the destination is a directory, then the file will be placed into the destination directory.
If the remote path does not exist, then
scp will present an error:
$ scp hello.txt mac.example.com:"~/DoesNotExist/" scp: /Users/armin/DoesNotExist/: No such file or directory
You can also rename the file while copying:
$ scp hello.txt mac.example.com:"~/Documents/ssh_test.txt"
You can copy files from the remote host to your local machine:
$ scp mac.example.com:"~/Documents/ssh_test.txt" .
In this case we passed
. or the current working directory as the destination. You can also pass a local path:
$ scp mac.example.com:"~/Documents/ssh_test.txt" ~/Downloads
-r option to copy all the contents of a directory:
$ scp -r ~/Projects/greatproject/ mac.example.com:"~/Documents/"
You can copy from one remote host to another.
There are two solutions for this. The first will copy the file to the local computer and then back up to the other remote host. You invoke this version of remote-to-remote with the
$ scp -3 primus:"~/Documents/ssh_test.txt" secundus:"~/Documents/"
(I am shortening the full domain names from
secundus for simplicity.)
Under most circumstances copying a file down to your Mac and then back up to the other remote host is less than ideal. Imagine you are working from home with your laptop and want to copy a large file from one server at work to another.
The other option is to tell the source remote host to
scp the to the other remote host. You could achieve this by
sshing to the remote machine and running
scp from there. Thankfully,
scp is smart enough to attempt exactly that when you type
$ scp primus:"~/Documents/ssh_test.txt" secundus:"~/Documents/"
This command will probably fail right now. It requires a few things to be set up to work:
- either: client key authentication to be setup from
- or: client key authentication from your local computer to
secunduswith agent forwarding enabled
The first option is fairly easy to understand.
scp will connect to
primus and authenticate with your local client key. Then it will tell
primus to connect to
secundus, authenticating using
primus’ client key and the copy the file. It basically works as if you
sshed in to
primus and ran
scp without the extra typing.
There are drawbacks to this. If
ssh-agent is not running on
primus and does not have the passphrase stored yet, then
primus cannot unlock its private key and authenticate to
Also you basically need to prepare all remote hosts to have keys exchanged between each other, which can be painful, if not impossible to manage.
The second option, called ‘Agent Forwarding’, circumvents these problems. This will tell the first remote host (
primus) to ask your local
ssh-agent for a key to authenticate to
secundus. The system does not actually transfer the private key, but asks
ssh-agent on your local computer to encode the authentication challenge for
That way you only need to manage the keys for all remote hosts on your local computer.
You can also use agent forwarding with normal
ssh connections. It is not enabled by default, but you can enable it for an
ssh session with the
-o ForwardAgent=yes option:
$ ssh -o ForwardAgent=yes primus
Since this is hard to type, there is a shortcut:
$ ssh -A primus
When you are logged in to the remote host with agent forwarding enabled, you can then ssh from there to another remote host (
secundus) and it will try to use the keys from your local computer’s
ssh-agent to authenticate.
$ ssh -A primus Last login: ... primus$ ssh secundus Last login: ... secundus$
This can be a useful strategy if direct access to the second remote host (
other.mac.com) is blocked with firewalls.
You can also use this for the
scp remote-to-remote copy. Unfortunately,
scp does not have a convenient
-A option, so you have to use the long parameter form:
$ scp -o ForwardAgent=yes mac.example.com:"~/Documents/ssh_test.txt" other.example.com:"~/Documents/"
When you need agent forwarding regularly for a specific host, you can enable it by default for this particular host in your
~/.ssh/config file. Add the following lines:
Host primus.example.com ForwardAgent yes
Note: There are some security concerns with agent forwarding. A user with root access on the intermediate system can gain access to the connection to your local ssh-agent, thus gaining the ability to encrypt with your private keys. Be aware of this in security sensitive environments.
If you have many files in a complex file structure ,
scp can be a little cumbersome. There is a special interactive mode that you can invoke with the
sftp command (secure file transfer program).
$ sftp mac.example.com
sftp(according to its man page) is ‘similar’ to
ftp but not identical. It also should not be confused with
ftps which is
ftp over SSL.
Obviously, if you have key authentication setup,
sftp will use that.
There are many interactive commands to navigate the local and remote file system and upload (
put) and download (
get) files. You can look at the details in the
sftp man page. However, if you need to use
sftp frequently, then you should use a graphical
sftp application. There are a large number of SFTP client for macOS and iOS. Here is a list of some popular clients: (AppStore links are affiliate links.)
- Transmit (Mac Version, also iOS AppStore): Aside from a great macOS application, Transmit also comes as an iOS App.
- Fetch (Mac AppStore): Fetch offers free licenses for educational and charitable organisations
- CyberDuck (Mac AppStore): CyberDuck has an optional command line tool
- MountainDuck (Mac AppStore): Sibling to CyberDuck, but mounts remote servers in the Finder.
- FileZilla: free open source solution
All of these tools connect to many other server protocols other than
sftp. However, the advantage of
sftp is not just the built-in security, but that you don’t need other software than
sshd running on the server.
Previous Post: Client Verification
- you can use
scp [[user@]host:]source [[user@]host:]destinationto copy files from or to a remote host over
- you can use agent forwarding to simplify key management in triangle setups
sftphelp managing/transferring multiple files over
ssh, there are many UI applications
Next Post: SSH Tunnels