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Manual Pages  — CU


cu – call UNIX



cu [-ehot] [-a acu] [-l line] [-s speed | speed] [phone-number]


The cu utility establishes a full-duplex connection to another machine, giving the appearance of being logged in directly on the remote CPU. It goes without saying that you must have a login on the machine (or equivalent) to which you wish to connect.

The options are as follows:
-a acu
  Set the acu.
  Use even parity. If both -e and -o are given, then no parity is used (the default).
  Echo characters locally (half-duplex mode).
-l line
  Specify the line to use. Either of the forms like cuau0 or /dev/cuau0 are permitted.
  Use odd parity. If both -e and -o are given, then no parity is used (the default).
-s speed | speed
  Set the speed of the connection. The default is 9600.
  Connect via a hard-wired connection to a host on a dial-up line.

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.
~c [name]
  Change directory to name (no argument implies change to home directory).
~! Escape to a shell (exiting the shell will return to cu).
~> Copy file from local to remote. The cu utility prompts for the name of a local file to transmit.
~< Copy file from remote to local. The cu utility prompts first for the name of the file to be sent, then for a command to be executed on the remote machine.
~p from [to]
  Send a file to a remote Unix host. This command causes the remote Unix system to run the following command string, sending it the from file:

    stty -echo; cat > 'to'; stty echo

If the to file is not specified, the from file name is used. This command is actually a Unix specific version of the ~> command.

~t from [to]
  Take a file from a remote Unix host. As in the ~p command, the to file defaults to the from file name if it is not specified. The remote host executes the following command string to send the file to cu:

    cat 'from'; echo '' | tr '\012' '\01'

~| Pipe the output from a remote command to a local Unix process. The command string sent to the local Unix system is processed by the shell.
~$ 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.
~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
~# Send a BREAK to the remote system. For systems which do not support the necessary ioctl() call, the break is simulated by a sequence of line speed changes and DEL characters.
~s Set a variable (see the discussion below).
~v List all variables and their values (if set).
~^Z Stop cu (only available with job control).
~^Y Stop only the "local side" of cu (only available with job control); the "remote side" of cu, the side that displays output from the remote host, is left running.
~? 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.

The cu utility guards against multiple users connecting to a remote system by opening modems and terminal lines with exclusive access, and by honoring the locking protocol used by uucico(8) ( ports/net/freebsd-uucp).

During file transfers cu provides a running count of the number of lines transferred. When using the ~> and ~< commands, the eofread and eofwrite variables are used to recognize end-of-file when reading, and specify end-of-file when writing (see below). File transfers normally depend on hardwareflow or tandem mode for flow control. If the remote system does not support hardwareflow or tandem mode, echocheck may be set to indicate that cu should synchronize with the remote system on the echo of each transmitted character.

When cu must dial a phone number to connect to a system, it will print various messages indicating its actions. The cu utility supports a variety of auto-call units and modems with the at capability in system descriptions.

Support for Ventel 212+ (ventel), Hayes AT-style (hayes), USRobotics Courier (courier), Telebit T3000 (t3000) and Racal-Vadic 831 (vadic) units is enabled by default.

Support for Bizcomp 1031[fw] (biz31[fw]), Bizcomp 1022[fw] (biz22[fw]), DEC DF0[23]-AC (df0[23]), DEC DN-11 (dn11) and Racal-Vadic 3451 (v3451) units can be added by recompiling cu with the appropriate defines.

Note that if support for both the Racal-Vadic 831 and 3451 is enabled, they are referred to as the v831 and v3451, respectively. If only one of the two is supported, it is referred to as vadic.


The cu utility maintains a set of variables which control its operation. Some of these variables are read-only to normal users (root is allowed to change anything of interest). Variables may be displayed and set through the ~s escape. The syntax for variables is patterned after vi(1) and Mail(1). Supplying "all" as an argument to the set command displays all variables readable by the user. Alternatively, the user may request display of a particular variable by attaching a ‘amp;?’ to the end. For example, "escape?" displays the current escape character.

Variables are numeric, string, character, or boolean values. Boolean variables are set merely by specifying their name; they may be reset by prepending a ‘amp;!’ to the name. Other variable types are set by concatenating an ‘=’ and the value. The entire assignment must not have any blanks in it. A single set command may be used to interrogate as well as set a number of variables. Certain common variables have abbreviations. The following is a list of common variables, their abbreviations, and their default values:
  (num) The baud rate at which the connection was established; abbreviated ba.
  (bool) Discard unprintable characters when a session is being scripted; abbreviated be.
  (num) When dialing a phone number, the time (in seconds) to wait for a connection to be established; abbreviated dial.
  (bool) Synchronize with the remote host during file transfer by waiting for the echo of the last character transmitted; default is off.
  (str) The set of characters which signify an end-of-transmission during a ~< file transfer command; abbreviated eofr.
  (str) The string sent to indicate end-of-transmission during a ~> file transfer command; abbreviated eofw.
  (str) The set of characters which indicate an end-of-line. The cu utility will recognize escape characters only after an end-of-line.
  (char) The command prefix (escape) character; abbreviated es; default value is ‘~’.
  (str) The set of characters which should not be discarded due to the beautification switch; abbreviated ex; default value is "\t\n\f\b".
  (char) The character used to force literal data transmission; abbreviated fo; default value is ‘^P’.
  (num) The amount of data (in bytes) to buffer between file system writes when receiving files; abbreviated fr.
  (bool) Whether hardware flow control (CRTSCTS) is enabled for the connection; abbreviated hf; default value is off.
  (str) The name of the host to which you are connected; abbreviated ho.
  (num) The line discipline to use; abbreviated ld.
  (char) The character which indicates an end-of-line on the remote host; abbreviated pr; default value is ‘\n’. This value is used to synchronize during data transfers. The count of lines transferred during a file transfer command is based on receipt of this character.
  (bool) Upper case mapping mode; abbreviated ra; default value is off. When this mode is enabled, all lowercase letters will be mapped to uppercase by cu for transmission to the remote machine.
  (char) The input character used to toggle uppercase mapping mode; abbreviated rc; not set by default.
  (str) The name of the file in which a session script is recorded; abbreviated rec.
  (bool) Session scripting mode; abbreviated sc; default is off. When script is true, cu will record everything transmitted by the remote machine in the script record file specified in record. If the beautify switch is on, only printable ASCII characters will be included in the script file (those characters between 040 and 0177). The variable exceptions is used to indicate characters which are an exception to the normal beautification rules.
  (bool) Expand tabs to spaces during file transfers; abbreviated tab; default value is false. Each tab is expanded to 8 spaces.
  (bool) Use XON/XOFF flow control to throttle data from the remote host; abbreviated ta. The default value is true.
  (bool) Verbose mode; abbreviated verb; default is true. When verbose mode is enabled, cu prints messages while dialing, shows the current number of lines transferred during a file transfer operations, and more.


HOME The home directory to use for the ~c command.
SHELL The name of the shell to use for the ~! command; default value is "/bin/sh".


/var/log/aculog line access log
  lock file to avoid conflicts with uucp(1) ( ports/net/freebsd-uucp)


Connect to the first USB serial port at the speed of 115200 baud:
cu -s 115200 -l /dev/cuaU0




The cu command appeared in BSD 4.2 .


The full set of variables is undocumented and should, probably, be pared down.

CU (1) April 22, 2017

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