RMT(8) GNU TAR Manual RMT(8)

rmt - remote magnetic tape server


Rmt provides remote access to files and devices for tar(1), cpio(1), and similar backup utilities. It is normally called by running rsh(1) or ssh(1) to the remote machine, optionally using a different login name if one is supplied.

The calling program communicates with rmt by sending requests on its standard input and reading replies from the standard output. A request consists of a request letter followed by an argument (if required) and a newline character. Additional data, if any, are sent after the newline. On success, rmt returns


where number is an ASCII representation of a decimal return code. Additional data are returned after this line. On error, the following response is returned:


where errno is one of the system error codes, as described in errno(3), and error-message is a one-line human-readable description of the error, as printed by perror(3).

Available commands and possible responses are discussed in detail in the subsequent section.

Opens the device with given flags. If a device had already been opened, it is closed before opening the new one.


The name of the device to open.
Flags for open(2): a decimal number, or any valid O_* constant from fcntl.h (the initial O_ may be omitted), or a bitwise or (using |) of any number of these, e.g.:
In addition, a combined form is also allowed, i.e. a decimal mode followed by its symbolic representation. In this case the symbolic representation is given preference.


A0\n on success.


BSD version allows only decimal number as flags.
Close the currently open device.

Any arguments are silently ignored.

A0\n on success.
Performs an lseek(2) on the currently open device with the specified parameters.
Where to measure offset from. Valid values are:
0, SET, SEEK_SET	seek from the file beginning
1, CUR, SEEK_CUR	seek from the current location
2, END, SEEK_END	seek from the file end

Aoffset\n on success. The offset is the new offset in file.
BSD version allows only 0,1,2 as whence.

Read count bytes of data from the current device.
number of bytes to read.

On success:


followed by rdcount bytes of data read from the device.

Writes data onto the current device. The command is followed by count bytes of input data.
Number of bytes to write.

On success: Awrcount\n, where wrcount is the number of bytes actually written.
Perform a MTIOCOP ioctl(2) command with the specified paramedters.
MTIOCOP operation code.

On success: A0\n.
Returns the status of the currently open device, as obtained from a MTIOCGET ioctl(2) call.


On success: Acount\n followed by count bytes of data.


Using this utility as a general-purpose remote file access tool is discouraged.

Report bugs to <bug-tar@gnu.org>.

The rmt command appeared in 4.2BSD. The GNU rmt is written from scratch, using the BSD specification.

Copyright © 2013, 2018 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later http://gnu.org/licenses/gpl.html
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.

March 24, 2018 RMT