CU(1C) Communication Commands CU(1C)


cu - call another UNIX system


cu [-c device | -l line] [-s speed] [-b bits] [-h] [-n]
[-t] [-d] [-o | -e] [-L] [-C] [-H] telno | systemname


The command cu calls up another UNIX system, a terminal, or possibly a
non-UNIX system. It manages an interactive conversation with possible
transfers of files. It is convenient to think of cu as operating in two
phases. The first phase is the connection phase in which the connection
is established. cu then enters the conversation phase. The -d option is
the only one that applies to both phases.


cu accepts many options. The -c, -l, and -s options play a part in
selecting the medium. The remaining options are used in configuring the

-b bits
Forces bits to be the number of bits processed on the line.
bits is either 7 or 8. This allows connection between
systems with different character sizes. By default, the
character size of the line is set to the same value as the
current local terminal, but the character size setting is
affected by LC_CTYPE also.

-c device
Forces cu to use only entries in the "Type" field (the first
field in the /etc/uucp/Devices file) that match the user
specified device, usually the name of a local area network.

Runs the local-cmd specified at the end of the command line
instead of entering interactive mode. The stdin and stdout
of the command that is run refer to the remote connection.

Prints diagnostic traces.

Sets an EVEN data parity. This option designates that EVEN
parity is to be generated for data sent to the remote

Sets communication mode to half-duplex. This option
emulates local echo in order to support calls to other
computer systems that expect terminals to be set to half-
duplex mode.

Ignores one hangup. This allows the user to remain in cu
while the remote machine disconnects and places a call back
to the local machine. This option should be used when
connecting to systems with callback or dialback modems.
Once the callback occurs subsequent hangups will cause cu to
terminate. This option can be specified more than once. For
more information about dialback configuration, see remote(5)
and System Administration Guide: IP Services

-l line
Specifies a device name to use as the communication line.
This can be used to override the search that would otherwise
take place for the first available line having the right
speed. When the -l option is used without the -s option, the
speed of a line is taken from the /etc/uucp/Devices file
record in which line matches the second field (the Line
field). When the -l and -s options are both used together,
cu will search the /etc/uucp/Devices file to check if the
requested speed for the requested line is available. If so,
the connection will be made at the requested speed,
otherwise, an error message will be printed and the call
will not be made. In the general case where a specified
device is a directly connected asynchronous line (for
instance, /dev/term/a), a telephone number (telno) is not
required. The specified device need not be in the /dev
directory. If the specified device is associated with an
auto dialer, a telephone number must be provided.

Goes through the login chat sequence specified in the
/etc/uucp/Systems file. For more information about the chat
sequence, see System Administration Guide: IP Services

Requests user prompt for telephone number. For added
security, this option will prompt the user to provide the
telephone number to be dialed, rather than taking it from
the command line.

Sets an ODD data parity. This option designates that ODD
parity is to be generated for data sent to the remote

-s speed
Specifies the transmission speed (300, 1200, 2400, 4800,
9600, 19200, 38400). The default value is "Any" speed which
will depend on the order of the lines in the
/etc/uucp/Devices file.

Dials a terminal which has been set to auto answer.
Appropriate mapping of carriage-return to carriage-return-
line-feed pairs is set.


The following operands are supported:

When using an automatic dialler, specifies the telephone
number with equal signs for secondary dial tone or minus
signs placed appropriately for delays of 4 seconds.

Specifies a uucp system name, which can be used rather than
a telephone number; in this case, cu will obtain an
appropriate direct line or telephone number from a system


Connection Phase

cu uses the same mechanism that uucp(1C) does to establish a connection.
This means that it will use the uucp control files /etc/uucp/Devices and
/etc/uucp/Systems. This gives cu the ability to choose from several
different media to establish the connection. The possible media include
telephone lines, direct connections, and local area networks (LAN). The
/etc/uucp/Devices file contains a list of media that are available on
your system. The /etc/uucp/Systems file contains information for
connecting to remote systems, but it is not generally readable.

Note: cu determines which /etc/uucp/Systems and /etc/uucp/Devices files
to use based upon the name used to invoke cu. In the simple case, this
name will be "cu", but you could also have created a link to cu with
another name, such as "pppcu", in which case cu would then look for a
"service=pppcu" entry in the /etc/uucp/Sysfiles file to determine which
/etc/uucp/Systems file to use.

The telno or systemname parameter from the command line is used to tell
cu what system you wish to connect to. This parameter can be blank, a
telephone number, a system name, or a LAN specific address.

telephone number
A telephone number is a string consisting of the tone
dial characters (the digits 0 through 9, *, and #)
plus the special characters = and -. The equal sign
designates a secondary dial tone and the minus sign
creates a 4 second delay.

system name
A system name is the name of any computer that uucp
can call; the uuname(1C) command prints a list of
these names.

LAN address
The documentation for your LAN will show the form of
the LAN specific address.

If cu's default behavior is invoked (not using the -c or -l options), cu
will use the telno or systemname parameter to determine which medium to
use. If a telephone number is specified, cu will assume that you wish to
use a telephone line and it will select an automatic call unit (ACU).
Otherwise, cu will assume that it is a system name. cu will follow the
uucp calling mechanism and use the /etc/uucp/Systems and
/etc/uucp/Devices files to obtain the best available connection. Since cu
will choose a speed that is appropriate for the medium that it selects,
you may not use the -s option when this parameter is a system name.

The -c and -l options modify this default behavior. -c is most often used
to select a LAN by specifying a Type field from the /etc/uucp/Devices
file. You must include either a telno or systemname value when using the
-c option. If the connection to systemname fails, a connection will be
attempted using systemname as a LAN specific address. The -l option is
used to specify a device associated with a direct connection. If the
connection is truly a direct connection to the remote machine, then there
is no need to specify a systemname. This is the only case where a telno
or systemname parameter is unnecessary. On the other hand, there may be
cases in which the specified device connects to a dialer, so it is valid
to specify a telephone number. The -c and -l options should not be
specified on the same command line.

Conversation Phase

After making the connection, cu runs as two processes. The transmit
process reads data from the standard input and, except for lines
beginning with ~, passes it to the remote system. The receive process
accepts data from the remote system and, except for lines beginning with
~, passes it to the standard output. Normally, an automatic DC3/DC1
protocol is used to control input from the remote so the buffer is not
overrun. Lines beginning with ~ have special meanings.


The transmit process interprets the following user initiated commands:

Terminates the conversation.

Escapes to an interactive shell on the local

Runs cmd on the local system (via sh -c).

Runs cmd locally and send its output to the
remote system.

Changes the directory on the local system.
Note: ~!cd will cause the command to be run by
a sub-shell, probably not what was intended.

~%take from [to]
Copies file from (on the remote system) to
file to on the local system. If to is omitted,
the from argument is used in both places.

~%put from [to]
Copies file from (on local system) to file to
on remote system. If to is omitted, the from
argument is used in both places.

Sends the line ~ line to the remote system.

Transmits a BREAK to the remote system (which
can also be specified as ~%b).

Toggles the -d debugging option on or off
(which can also be specified as ~%d).

Prints the values of the termio structure
variables for the user's terminal (useful for

Prints the values of the termio structure
variables for the remote communication line
(useful for debugging).

Toggles between DC3/DC1 input control protocol
and no input control. This is useful when the
remote system does not respond properly to the
DC3 and DC1 characters (can also be specified
as ~%nostop).

Toggles the output flow control setting. When
enabled, outgoing data may be flow controlled
by the remote host (can also be specified as

Allows/disallows unsolicited diversions. That
is, diversions not specified by ~%take.

Allows/disallows old style syntax for received

Same as ~%ifc.

The receive process normally copies data from the remote system to the
standard output of the local system. It may also direct the output to
local files.

The use of ~%put requires stty(1) and cat(1) on the remote side. It also
requires that the current erase and kill characters on the remote system
be identical to these current control characters on the local system.
Backslashes are inserted at appropriate places.

The use of ~%take requires the existence of echo(1) and cat(1) on the
remote system, and that the remote system must be using the Bourne shell,
sh. Also, tabs mode (see stty(1)) should be set on the remote system if
tabs are to be copied without expansion to spaces.

When cu is used on system X to connect to system Y and subsequently used
on system Y to connect to system Z, commands on system Y can be executed
by using ~~. Executing a tilde command reminds the user of the local
system uname. For example, uname can be executed on Z, X, and Y as


In general, ~ causes the command to be executed on the original machine.
~~ causes the command to be executed on the next machine in the chain.


Example 1: Dialling a system

To dial a system whose telephone number is 9 1 201 555 1234 using 1200
baud (where dialtone is expected after the 9):

example% cu -s 1200 9=12015551234

If the speed is not specified, "Any" is the default value.

Example 2: Logging in to a system on a direct line

To login to a system connected by a direct line:

example% cu -l /dev/term/b


example% cu -l term/b

Example 3: Dialling a system with specific line and speed

To dial a system with a specific line and speed:

example% cu -s 1200 -l term/b

Example 4: Using a system name

To use a system name:

example% cu systemname


See environ(7) for descriptions of the following environment variables
that affect the execution of cu: LC_CTYPE, LC_MESSAGES, and NLSPATH.


The following exit values are returned:

Successful completion.

An error occurred.


device file

system file

system file

lock file


cat(1), echo(1), stty(1), tip(1), uname(1), ct(1C), uucp(1C), uuname(1C),
remote(5), attributes(7), environ(7)

System Administration Guide: IP Services


The cu utility takes the default action upon receipt of signals, with the
exception of:

Close the connection and terminate.

Forward to the remote system.

Forward to the remote system.

Terminate the cu process without the normal connection closing

The cu command does not do any integrity checking on data it transfers.
Data fields with special cu characters may not be transmitted properly.
Depending on the interconnection hardware, it may be necessary to use a
~. to terminate the conversion, even if stty 0 has been used. Non-
printing characters are not dependably transmitted using either the ~%put
or ~%take commands. ~%put and ~%take cannot be used over multiple links.
Files must be moved one link at a time.

There is an artificial slowing of transmission by cu during the ~%put
operation so that loss of data is unlikely. Files transferred using
~%take or ~%put must contain a trailing newline, otherwise, the operation
will hang. Entering a Control-D command usually clears the hang

May 11, 2001 CU(1C)