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.Dd $Mdocdate: December 12 2020 $
.Dt CU 1
.Os
.Sh NAME
.Nm cu
.Nd serial terminal emulator
.Sh SYNOPSIS
.Nm
.Op Fl dr
.Op Fl E Ar escape_char
.Op Fl l Ar line
.Op Fl s Ar speed | Fl Ar speed
.Nm
.Op Ar host
.Sh DESCRIPTION
.Nm
is used to connect to another system over a serial link.
In the era before modern networks, it was typically used to
connect to a modem in order to dial in to a remote host.
It is now frequently used for tasks such as attaching to the
serial console of another machine for administrative or
debugging purposes.
.Pp
The options are as follows:
.Bl -tag -width 4n
.It Fl d
Specify that the line is directly connected and
.Nm
should not allow the driver to block waiting for a carrier to be detected.
.It Fl E Ar escape_char
Specify an escape character to use instead of the default tilde.
.It Fl l Ar line
Specify the line to use.
Either of the forms like
.Pa cua00
or
.Pa /dev/cua00
are permitted.
The default is
.Pa /dev/cua00 .
See
.Xr cua 4
for information on terminal devices.
Users in group
.Dq dialer
are permitted to use
.Xr cua 4
devices by default.
.It Fl r
Start
.Nm
in restricted mode.
This prevents all local filesystem operations
.Po
.Cm ~R ,
.Cm ~X ,
and
.Cm ~>
.Pc
and command executions
.Po
.Cm ~C
and
.Cm ~$
.Pc .
.It Fl s Ar speed | Fl Ar speed
Set the speed of the connection.
The default is 9600.
.El
.Pp
If
.Ar host
is given,
.Nm
uses the
.Xr remote 5
database to retrieve the
.Sy dc Pq directly connected ,
.Sy dv Pq device
and
.Sy br Pq baud rate
capabilities for that host.
The
.Nm
utility ignores other capabilities found in that database.
.Pp
Typed characters are normally transmitted directly to the remote
machine (which does the echoing as well).
A tilde
.Pq Ql ~
appearing as the first character of a line is an escape signal; the
following are recognized:
.Bl -tag -offset indent -width Fl
.It Ic ~^D No or Ic ~.
Drop the connection and exit.
Only the connection is dropped \(en the login session is not terminated.
.It Ic ~>
Copy file from local to remote.
.Nm
prompts for the name of a local file to transmit.
.It Ic ~$
Pipe the output from a local
.Ux
process to the remote host.
The command string sent to the local
.Ux
system is processed by the shell.
.It Ic ~#
Send a
.Dv BREAK
to the remote system.
.It Ic ~^Z
Stop
.Nm
(only available with job control).
.It Ic ~C
Fork a child process on the local system to perform special protocols
such as XMODEM.
The child program will be run with the following arrangement of
file descriptors:
.Pp
.Bl -item -compact -offset indent
.It
0 \(<> remote tty in
.It
1 \(<> remote tty out
.It
2 \(<> local tty stderr
.El
.It Ic ~D
Deassert the data terminal ready (DTR) line briefly.
.It Ic ~R
Record all output from the remote system to a file.
If the given file already exists, it is appended to.
If no file is specified, any existing recording is stopped.
.It Ic ~S
Change the speed of the connection.
.It Ic ~X
Send a file with the XMODEM protocol.
.It Ic ~?
Get a summary of the tilde escapes.
.El
.Pp
When
.Nm
prompts for an argument, for example during setup of a file transfer,
the line typed may be edited with the standard erase and kill characters.
A null line in response to a prompt, or an interrupt, will abort the
dialogue and return the user to the remote machine.
.Pp
.Nm
guards against multiple users connecting to a remote system by opening
modems and terminal lines with exclusive access.
.Sh ENVIRONMENT
.Bl -tag -width REMOTEXXX
.It Ev HOST
The default value for
.Ar host
if none is specified via the command line.
.It Ev REMOTE
A system description, or an absolute path to a
.Xr remote 5
system description database.
.El
.Sh FILES
.Bl -tag -width /etc/remote
.It Pa /etc/remote
host description file
.El
.Sh EXIT STATUS
.Ex -std cu
.Sh SEE ALSO
.Xr remote 5
.Sh HISTORY
The
.Nm
command appeared in
.At v7 .
This version was written for
.Ox 5.4
by Nicholas Marriott.