How To Use SFTP to Securely Transfer Files with a Remote Server
How To Use SFTP to Securely Transfer Files with a Remote Server

Tutorial

How To Use SFTP to Securely Transfer Files with a Remote Server

Linux Basics

Introduction

FTP, or “File Transfer Protocol” was a popular unencrypted method of transferring files between two remote systems.

SFTP, which stands for SSH File Transfer Protocol, or Secure File Transfer Protocol, is a separate protocol packaged with SSH that works in a similar way but over a secure connection. The advantage is the ability to leverage a secure connection to transfer files and traverse the filesystem on both the local and remote system.

In almost all cases, SFTP is preferable to FTP because of its underlying security features and ability to piggy-back on an SSH connection. FTP is an insecure protocol that should only be used in limited cases or on networks you trust.

Although SFTP is integrated into many graphical tools, this guide will demonstrate how to use it through its interactive command line interface.

How to Connect with SFTP

By default, SFTP uses the SSH protocol to authenticate and establish a secure connection. Because of this, the same authentication methods are available that are present in SSH.

Although passwords are easy to use and set up by default, we recommend you create SSH keys and transfer your public key to any system that you need to access. This is much more secure and can save you time in the long run.

Please see this guide to set up SSH keys in order to access your server if you have not done so already.

If you can connect to the machine using SSH, then you have completed all of the necessary requirements necessary to use SFTP to manage files. Test SSH access with the following command:

  • ssh sammy@your_server_ip_or_remote_hostname

If that works, exit back out by typing:

  • exit

Now we can establish an SFTP session by issuing the following command:

  • sftp sammy@your_server_ip_or_remote_hostname

You will connect the the remote system and your prompt will change to an SFTP prompt.

If you are working on a custom SSH port (not the default port 22), then you can open an SFTP session as follows:

  • sftp -oPort=custom_port sammy@your_server_ip_or_remote_hostname

This will connect you to the remote system by way of your specified port.

Getting Help in SFTP

The most useful command to learn first is the help command. This gives you access to a summary of the SFTP help. You can call it by typing either of these in the prompt:

  • help

or

  • ?

This will display a list of the available commands:

Output
Available commands: bye Quit sftp cd path Change remote directory to 'path' chgrp grp path Change group of file 'path' to 'grp' chmod mode path Change permissions of file 'path' to 'mode' chown own path Change owner of file 'path' to 'own' df [-hi] [path] Display statistics for current directory or filesystem containing 'path' exit Quit sftp get [-Ppr] remote [local] Download file help Display this help text lcd path Change local directory to 'path' . . .

We will explore some of the commands you see in the following sections.

We can navigate through the remote system’s file hierarchy using a number of commands that function similarly to their shell counterparts.

First, let’s orient ourselves by finding out which directory we are in currently on the remote system. Just like in a typical shell session, we can type the following to get the current directory:

  • pwd
Output
Remote working directory: /home/demouser

We can view the contents of the current directory of the remote system with another familiar command:

  • ls
Output
Summary.txt info.html temp.txt testDirectory

Note that the commands within the SFTP interface are not the normal shell commands and are not as feature-rich, but they do implement some of the more important optional flags:

  • ls -la
Output
drwxr-xr-x 5 demouser demouser 4096 Aug 13 15:11 . drwxr-xr-x 3 root root 4096 Aug 13 15:02 .. -rw------- 1 demouser demouser 5 Aug 13 15:04 .bash_history -rw-r--r-- 1 demouser demouser 220 Aug 13 15:02 .bash_logout -rw-r--r-- 1 demouser demouser 3486 Aug 13 15:02 .bashrc drwx------ 2 demouser demouser 4096 Aug 13 15:04 .cache -rw-r--r-- 1 demouser demouser 675 Aug 13 15:02 .profile . . .

To get to another directory, we can issue this command:

  • cd testDirectory

We can now traverse the remote file system, but what if we need to access our local file system? We can direct commands towards the local file system by preceding them with an l for local.

All of the commands discussed so far have local equivalents. We can print the local working directory:

  • lpwd
Output
Local working directory: /Users/demouser

We can list the contents of the current directory on the local machine:

  • lls
Output
Desktop local.txt test.html Documents analysis.rtf zebra.html

We can also change the directory we wish to interact with on the local system:

  • lcd Desktop

Transferring Files with SFTP

Navigating the remote and local filesystems is of limited usefulness without being able to transfer files between the two.

Transferring Remote Files to the Local System

If we would like download files from our remote host, we can do so by issuing the following command:

  • get remoteFile
Output
Fetching /home/demouser/remoteFile to remoteFile /home/demouser/remoteFile 100% 37KB 36.8KB/s 00:01

As you can see, by default, the get command downloads a remote file to a file with the same name on the local file system.

We can copy the remote file to a different name by specifying the name afterwards:

  • get remoteFile localFile

The get command also takes some option flags. For instance, we can copy a directory and all of its contents by specifying the recursive option:

  • get -r someDirectory

We can tell SFTP to maintain the appropriate permissions and access times by using the -P or -p flag:

  • get -Pr someDirectory

Transferring Local Files to the Remote System

Transferring files to the remote system is just as easily accomplished by using the appropriately named “put” command:

  • put localFile
Output
Uploading localFile to /home/demouser/localFile localFile 100% 7607 7.4KB/s 00:00

The same flags that work with get apply to put. So to copy an entire local directory, you can issue:

  • put -r localDirectory

Note: There is currently a bug in the versions of OpenSSH shipped with current Ubuntu releases (at least 14.04 to 15.10) that prevents the above command from operating correctly. Upon issuing the command above to transfer content to a server using the buggy version of OpenSSH, the following error will be given: Couldn't canonicalise: No such file or directory.

To work around this issue, create the destination directory on the remote end first by typing mkdir localDirectory. Afterwards, the above command should complete without error.

One familiar tool that is useful when downloading and uploading files is the df command, which works similar to the command line version. Using this, you can check that you have enough space to complete the transfers you are interested in:

  • df -h
Output
Size Used Avail (root) %Capacity 19.9GB 1016MB 17.9GB 18.9GB 4%

Please note, that there is no local variation of this command, but we can get around that by issuing the ! command.

The ! command drops us into a local shell, where we can run any command available on our local system. We can check disk usage by typing:

  • !

and then

  • df -h
Output
Filesystem Size Used Avail Capacity Mounted on /dev/disk0s2 595Gi 52Gi 544Gi 9% / devfs 181Ki 181Ki 0Bi 100% /dev map -hosts 0Bi 0Bi 0Bi 100% /net map auto_home 0Bi 0Bi 0Bi 100% /home

Any other local command will work as expected. To return to your SFTP session, type:

  • exit

You should now see the SFTP prompt return.

Simple File Manipulations with SFTP

SFTP allows you to perform the type of basic file maintenance that is useful when working with file hierarchies.

For instance, you can change the owner of a file on the remote system with:

  • chown userID file

Notice how, unlike the system chmod command, the SFTP command does not accept usernames, but instead uses UIDs. Unfortunately, there is no easy way to know the appropriate UID from within the SFTP interface.

An involved workaround could be accomplished with:

  • get /etc/passwd
  • !less passwd
Output
root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh bin:x:2:2:bin:/bin:/bin/sh sys:x:3:3:sys:/dev:/bin/sh sync:x:4:65534:sync:/bin:/bin/sync games:x:5:60:games:/usr/games:/bin/sh man:x:6:12:man:/var/cache/man:/bin/sh . . .

Notice how instead of giving the ! command by itself, we’ve used it as a prefix for a local shell command. This works to run any command available on our local machine and could have been used with the local df command earlier.

The UID will be in the third column of the file, as delineated by colon characters.

Similarly, we can change the group owner of a file with:

  • chgrp groupID file

Again, there is no easy way to get a listing of the remote system’s groups. We can work around it with the following command:

  • get /etc/group
  • !less group
Output
root:x:0: daemon:x:1: bin:x:2: sys:x:3: adm:x:4: tty:x:5: disk:x:6: lp:x:7: . . .

The third column holds the ID of the group associated with name in the first column. This is what we are looking for.

Thankfully, the chmod command works as expected on the remote file system:

  • chmod 777 publicFile
Output
Changing mode on /home/demouser/publicFile

There is no command for manipulating local file permissions, but you can set the local umask, so that any files copied to the local system will have the appropriate permissions.

That can be done with the lumask command:

  • lumask 022
Output
Local umask: 022

Now all regular files downloaded (as long as the -p flag is not used) will have 644 permissions.

SFTP allows you to create directories on both local and remote systems with lmkdir and mkdir respectively. These work as expected.

The rest of the file commands target only the remote filesystem:

  • ln
  • rm
  • rmdir

These commands replicate the basic behavior of the shell versions. If you need to perform these actions on the local file system, remember that you can drop into a shell by issuing this command:

  • !

Or execute a single command on the local system by prefacing the command with ! like so:

  • !chmod 644 somefile

When you are finished with your SFTP session, use exit or bye to close the connection.

  • bye

Conclusion

Although SFTP is a simple tool, it is very useful for administrating servers and transferring files between them.

For example, you can use SFTP to enable particular users to transfer files without SSH access. For more information on this process, check out our tutorial on How To Enable SFTP Without Shell Access.

If you are used to using FTP or SCP to accomplish your transfers, SFTP is a good way to leverage the strengths of both. While it is not appropriate for every situation, it is a flexible tool to have in your repertoire.

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