rmt - remote magtape protocol module
Rmt is a program used by tar, cpio, mt, and the remote dump and restore
programs in manipulating a magnetic tape drive through an interprocess
communication connection. Rmt is normally started up with an rexec(3) or
rcmd(3) call or the rsh(1) command.
The rmt program accepts requests specific to the manipulation of magnetic
tapes, performs the commands, then responds with a status indication.
All responses are in ASCII and in one of two forms. Successful commands
have responses of:
Number is an ASCII representation of a decimal number. Unsuccessful
commands are responded to with:
Error-number is one of the possible error numbers described in intro(2)
and error-message is the corresponding error string as printed from a
call to perror(3). The protocol is comprised of the following commands,
which are sent as indicated - no spaces are supplied between the command
and its arguments, or between its arguments, and ‘\n’ indicates that a
newline should be supplied:
Open the specified deviceusing the indicated mode.Deviceis a full
pathname and modeis an ASCIIrepresentation of a decimal number
suitable for passing to open(2).If a device had already been
opened, it is closed before a new open is performed.
Close the currently open device. The devicespecified is ignored.
Perform an lseek(2) operation using the specified parameters.
The response value is that returned from the lseek call.
Write data onto the open device. Rmt reads count bytes from the
connection, aborting if a premature end-of-file is encountered.
The response value is that returned from the write(2) call.
Read count bytes of data from the open device. If count exceeds
the size of the data buffer (10 kilobytes), it is truncated to
the data buffer size. rmt then performs the requested read(2)
and responds with Acount-read\n if the read was successful;
otherwise an error in the standard format is returned. If the
read was successful, the data read is then sent.
Perform a MTIOCOP ioctl(2) command using the specified
parameters. The parameters are interpreted as the ASCII
representations of the decimal values to place in the mt_op and
mt_count fields of the structure used in the ioctl call. The
return value is the count parameter when the operation is
S Return the status of the open device, as obtained with a MTIOCGET
ioctl call. If the operation was successful, an ‘‘ack’’ is sent
with the size of the status buffer, then the status buffer is
sent (in binary).
Any other command causes rmt to exit.
All responses are of the form described above.
tar(1), cpio(1), mt(1), rsh(1), rcmd(3), rexec(3), mtio(4), rdump(8),
People should be discouraged from using this for a remote file access
The rmt command appeared in 4.2BSD.