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

Introduction

FTP, the File Transfer Protocol, was a popular, unencrypted method of transferring files between two remote systems. As of 2022, it has been deprecated by most modern software due to a lack of security, and can mostly only be used in legacy applications.

SFTP, which stands for Secure File Transfer Protocol, is a separate protocol packaged built into SSH that can implement FTP commands over a secure connection. Typically, it can act as a drop-in replacement in any contexts where an FTP server is still needed.

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 you can authenticate with passwords 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 other SFTP commands. 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.

Navigating with SFTP

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 available within the SFTP interface are not a 1:1 match for typical shell syntax and are not as feature-rich. However, they do implement some of the more important optional flags, such as adding -la to ls to view more file metadata and permissions:

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 want to interact with on the local system:

lcd Desktop

Transferring Files with SFTP

If we want to download files from our remote host, we can do so using the get 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 accepts 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 works the same way, but with a 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 run put -r:

put -r localDirectory

One familiar tool that is useful when downloading and uploading files is the df command, which works similarly 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 some kinds of filesystem housekeeping. 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 built-in way to know the appropriate UID from within the SFTP interface.

As a workaround, you can read from the /etc/passwd file, which associates usernames with UIDs in most Linux environments:

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 built-in 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.

The chmod SFTP command works as normal on the remote filesystem:

chmod 777 publicFile

Output
Changing mode on /home/demouser/publicFile

There is no equivalent command for manipulating local file permissions, but you can set the local umask, so that any files copied to the local system will have their corresponding 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 also allows you to create directories on both local and remote systems with lmkdir and mkdir respectively.

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

ln
rm
rmdir

These commands replicate the core behavior of their shell equivalents. 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 syntax is much less comprehensive than modern shell tooling, it can be useful for providing compatibility with legacy FTP syntax or for carefully limiting the functionality available to remote users of some environments.

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.