SFTP

SFTP is normally used as an interactive application/protocol, but also can be put into a batch mode with the -b flag. This can be specified with a file or can be directed stdin from an external script.

The password can not be passed by either process.

sftp [-1Cv] [-b batchfile] [-F SSH_config] [-o SSH_option] [-s subsystem | sftp_server] [-S program] host

sftp [[user@]host[:file [file]]]

sftp [[user@]host[:dir[/]]]

But mostly SFTP is used as an interactive file transfer program.

With FTP one might try to copy up all their JPG files to their image directory like this:

ftp> cd /home/user/images/
ftp> bin
ftp> prompt
ftp> mput *.jpg

With SFTP it is much simpler..

sftp> cd /home/user/images/
sftp> put *.jpgtt>

List of SFTP commands (SFTP will abort if any of the following commands fail):

• get [flags] remote-path [local-path] Retrieve the remote-path and store it on the local machine. If the local path name is not specified, it is given the same name it has on the remote machine.
• put [flags] local-path [local-path] Upload local-path and store it on the remote machine. If the remote path name is not specified, it is given the same name it has on the local machine.
• rename oldpath newpath Rename remote file from oldpath to newpath.
• ln oldpath newpath Create a symbolic link from oldpath to newpath.
• rm path Delete remote file specified by path.
• lmkdir path Create local directory specified by path.

List of other commands:

• bye Quit sftp.
• exit Quit sftp.
• quit Quit sftp.
• cd path Change remote directory to path.
• lcd path Change local directory to path.
• ls [path] Display remote directory listing of either path or current directory if path is not specified.
• pwd Display remote working directory.
• rmdir path Remove remote directory specified by path.
• chgrp grp path Change group of file path to grp. grp must be a numeric GID.
• chmod mode path Change permissions of file path to mode.
• chown own path Change owner of file path to own. own must be a numeric UID.
• symlink oldpath newpath Create a symbolic link from oldpath to newpath.
• mkdir path Create remote directory specified by path.
• lls [ls-options [path]] Display local directory listing of either path or current directory if path is not specified.
• lpwd Print local working directory.
• lumask umask Set local umask to umask.
• ! command Execute command in local shell.
• ! Escape to local shell.
• ? Synonym for help.
• help Display help text.

SFTP may also use many features of SSH, such as public key authentication and compression. SFTP connects and logs into the specified host, then enters an interactive command mode. The second usage format will retrieve files automatically if a noninteractive authentication method is used; otherwise it will do so after successful interactive authentication. The last usage format allows the sftp client to start in a remote directory. 

Here are the options used:

• -1 Specify the use of protocol version 1.
• -b batchfile Batch mode reads a series of commands from an input batchfile instead of stdin. Since it lacks user interaction it should be used in conjunction with non-interactive authentication.
• -C Enables compression (via SSH's -C flag).
• -F SSH_config Specifies an alternative per-user configuration file for SSH. This option is directly passed to SSH.
• -o SSH_option Can be used to pass options to SSH in the format used in the SSH configuration file. This is useful for specifying options for which there is no separate sftp command-line flag.
• For example, to specify an alternate port use: sftp -oPort=26 (UHACC used to use this port config).
• -s subsystem | sftp_server Specifies the SSH2 subsystem or the path for an sftp server on the remote host. A path is useful for using sftp over protocol version 1, or when the remote SSHd does not have an sftp subsystem configured.
• -S program Name of the program to use for the encrypted connection. The program must understand SSH options.
• -v Raise logging level. This option is also passed to SSH. Once in interactive mode, sftp understands a set of commands similar to those of ftp. Commands are case insensitive and pathnames may be enclosed in quotes if they contain spaces.
• -P If the flag is specified, then the file's full permission and access time are copied too.

NEWS CONTENTS

• 20170928 : Transferring Files with Secure FTP ( sftp ) ( Transferring Files with Secure FTP (sftp), Sep 28, 2017 )
• 20100316 : Resuming interrupted sftp transfers in Linux " Free Software ( Resuming interrupted sftp transfers in Linux " Free Software, Mar 16, 2010 )

Old News ;-)

[Sep 28, 2017] Transferring Files with Secure FTP (sftp)

Part of the OpenSSH (Open Source Secure Shell) project, Secure FTP (sftp) is another file-transfer program that works in a similar way to FTP but encrypts the transfer so that it cannot be intercepted or read by intermediary computers. The encryption process increases the amount of data and slows the transfer but provides protection for confidential information.

You must specify the computer and user account on the sftp command line. SFTP prompts you for the password.

$ sftp root@our_web_site.com:/etc/httpd/httpd.conf
Connecting to our_web_site.com...
root@our_web_site.coms password:
Fetching /etc/httpd/httpd.conf to httpd.conf

For security purposes, SFTP normally asks the user for the Linux login password. It doesn't request the password from standard input but from the controlling terminal. This means you can't include the password in the batch file. The solution to this problem is to use SSH's public key authentication using the ssh-keygen command. If you have not already done so, generate a new key pair as follows.

$ ssh-keygen -t rsa
				

A pair of authentication keys are stored under .ssh in your home directory. You must copy the public key (a file ending in .pub) to the remote machine and add it to a text file called ~/.sshd/authorized_keys. Each local login accessing the remote login needs a public key in authorized_keys. If a key pair exists, SFTP automatically uses the keys instead of the Linux login password.

Like FTP, SFTP needs a list of commands to carry out. SFTP includes a -b (batch) switch to specify a separate batch file containing the commands to execute. To use a convenient here file in your script, use a batch file called /dev/stdin.

The commands that SFTP understands are similar to FTP. For purposes of shell scripting, the basic transfer commands are the same. Transfers are always "binary." There is a -v (verbose) switch, but it produces a lot of information. When the -b switch is used, SFTP shows the commands that are executed so the -v switch is not necessary for logging what happened during the transfer.

sftp -C -b /dev/stdin root@our_web_site.com <<!
cd /etc/httpd
get httpd.conf
!
STATUS=$?
if [ $STATUS -ne 0 ] ; then
   printf "%s\n" "Error: SFTP transfer failed" >&2
   exit $STATUS
fi

The -C (compress) option attempts to compress the data for faster transfers.

For more information about ssh, sftp, and related programs, visit http://www.openssh.org/.

[Mar 16, 2010] Resuming interrupted sftp transfers in Linux " Free Software

Even though the Linux version of the sftp client doesn't offer a direct way to resume an interrupted transfer, doing so is quite simple by using common shell tools, as long as you are able to login to the remote server through a console. Assuming that you are transferring data.zip from source_server to target_server and the transfer was interrupted, you can do the following:

• Connect to target_server using ssh, since you will be required to perform some operations there. Navigate to the directory containing the partially transferred file (also called data.zip)
• Check the sizes of the original and the partially transferred files. The easiest way to do that is by using the ls -al data.zip command. Let's assume that data.txt is 8231129 bytes long, and only 2811110 bytes were transferred before the interruption
• Subtract the size of the partially transferred file from the original, to get the remaining size in bytes. In this case, it is 5420019 bytes. In case you didn't know, Linux has a practical command-line calculator, bc, which comes very handy for quick calculations
• In source_server, create a new file consisting of the last 5420019 bytes of the original. You can do this with the tail command: tail -c 5420019 data.zip >data.tail
• Transfer the data.tail file to target_server, using sftp as usually.
• Once the transfer is complete, delete data.tail from source_server to avoid any mistake that would corrupt your original file.
• In target_server, use the cat command to append data.tail to the partially transferred file: cat data.tail >>data.zip (Note the double >>)

This works for both text and binary files. Apparently a better way would be integrating this ability into the sftp client, which is the way some clients such as putty and winscp work, but until that happy day you can use the tips above as a workaround.

Comments

mikeX:

lftp is a very nice command line ftp client, which supports tab completion, directory mirroring and of course resuming of interrupted downloads. It can also work as an sftp client, with the right protocol prefix, e.g. lftp sftp://user@host, and can even be used in batch mode. You can find a copy at http://lftp.yar.ru/. Still, nice trick :)

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