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CU(1) General Commands Manual CU(1)


cuserial terminal emulator


cu [-dr] [-l line] [-s speed | -speed]

cu [host]


cu 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.
The options are as follows:
Specify that the line is directly connected and cu should not allow the driver to block waiting for a carrier to be detected.
-l line
Specify the line to use. Either of the forms like cua00 or /dev/cua00 are permitted. The default is /dev/cua00. See cua(4) for information on terminal devices. Users in group “dialer” are permitted to use cua(4) devices by default.
Start cu in restricted mode. This prevents all local filesystem operations and command executions.
-s speed | -speed
Set the speed of the connection. The default is 9600.
If host is given, cu uses the remote(5) database to retrieve the dc (directly connected), dv (device) and br (baud rate) capabilities for that host. The cu utility ignores other capabilities found in that database.
Typed characters are normally transmitted directly to the remote machine (which does the echoing as well). A tilde (‘~’) appearing as the first character of a line is an escape signal; the following are recognized:
~^D or ~.
Drop the connection and exit. Only the connection is dropped – the login session is not terminated.
Copy file from local to remote. cu prompts for the name of a local file to transmit.
Pipe the output from a local UNIX process to the remote host. The command string sent to the local UNIX system is processed by the shell. This command is not allowed in restricted mode.
Send a BREAK to the remote system.
Stop cu (only available with job control).
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:
0 <-> remote tty in 
1 <-> remote tty out 
2 <-> local tty stderr
This command is not allowed in restricted mode.
Deassert the data terminal ready (DTR) line briefly. This command is not allowed in restricted mode.
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. This command is not allowed in restricted mode.
Change the speed of the connection.
Send a file with the XMODEM protocol. This command is not allowed in restricted mode.
Get a summary of the tilde escapes.
When cu 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.
cu guards against multiple users connecting to a remote system by opening modems and terminal lines with exclusive access.


The default value for host if none is specified via the command line.
A system description, or an absolute path to a remote(5) system description database.


host description file


The cu utility exits 0 on success, and >0 if an error occurs.




The cu command appeared in 4.2BSD. This version was written for OpenBSD 5.4 by Nicholas Marriott.
December 10, 2017 OpenBSD-current