---
name: "cu"
section: "1"
platform: "openbsd"
package: "man"
version: "8.0-current-20261101"
release: "OpenBSD 8.0-current (2026-11-01)"
platforms: ["linux", "freebsd", "openbsd", "macos"]
url: "https://new.carta.tech/cu"
---

# cu(1)

serial terminal emulator

## NAME

**cu** – serial terminal emulator

## SYNOPSIS

**cu** \[**-dr**\] \[**-E** *escape\_char*\] \[**-l** *line*\] \[**-s** *speed* \| **-***speed*\]

**cu** \[*host*\]

## DESCRIPTION

**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:

- **-d**

  Specify that the line is directly connected and **cu** should not allow the driver to block waiting for a carrier to be detected.

- **-E** *escape\_char*

  Specify an escape character to use instead of the default tilde.

- **-l** *line*

  Specify the line to use. Any of the forms *cua00*, */dev/cua00*, or *usb0.1.00002.3* are permitted.

  The default is */dev/cua00*. See [cua(4)](/cua/4.md) for information on terminal devices. Users in group “dialer” are permitted to use [cua(4)](/cua/4.md) devices by default.

  See [sysctl(2)](/sysctl/2.md) *hw.ucomnames* for available USB serial lines.

- **-r**

  Start **cu** in restricted mode. This prevents all local filesystem operations( **~R**, **~X**, and **~\>**) and command executions( **~C** and **~$**).

- **-s** *speed* \| **-***speed*

  Set the speed of the connection. The default is 9600.

If *host* is given, **cu** uses the [remote(5)](/remote/5.md) 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.

- **~#**

  Send a BREAK to the remote system.

- **~^Z**

  Stop **cu** (only available with job control).

- **~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:

  -

    0 ↔ remote tty in

  -

    1 ↔ remote tty out

  -

    2 ↔ local tty stderr

- **~D**

  Deassert the data terminal ready (DTR) line briefly.

- **~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.

- **~S**

  Change the speed of the connection.

- **~X**

  Send a file with the XMODEM protocol.

- **~?**

  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.

## ENVIRONMENT

- HOST

  The default value for *host* if none is specified via the command line.

- REMOTE

  A system description, or an absolute path to a [remote(5)](/remote/5.md) system description database.

## FILES

- */etc/remote*

  host description file

## EXIT STATUS

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

## SEE ALSO

[sysctl(2)](/sysctl/2.md), [cua(4)](/cua/4.md), [remote(5)](/remote/5.md)

## HISTORY

The **cu** (“Call Unix”) command first appeared outside of Bell Labs in PWB/UNIX 1.0. It was reimplemented as part of the **tip** command in 4.1cBSD. The current version was written for OpenBSD 5.4.

## AUTHORS

Nicholas Marriott ⟨*nicm@openbsd.org*⟩