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

NAME

cu - call another UNIX system

SYNOPSIS

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

DESCRIPTION

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.

OPTIONS

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 line.

-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.


-C
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.


-d
Prints diagnostic traces.


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


-h
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.


-H
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.


-L
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


-n
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.


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


-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.


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

OPERANDS

The following operands are supported:

telno
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.


systemname
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 file.

USAGE

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.

Commands

The transmit process interprets the following user initiated
commands:

~.
Terminates the conversation.


~!
Escapes to an interactive shell on the
local system.


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


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


~%cd
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.


~~line
Sends the line ~ line to the remote
system.


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


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


~t
Prints the values of the termio structure
variables for the user's terminal (useful
for debugging).


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


~%ifc
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).


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


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


~%old
Allows/disallows old style syntax for
received diversions.


~%nostop
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 follows:

uname
Z
~[X]!uname
X
~~[Y]!uname
Y


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.

EXAMPLES

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


or


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

ENVIRONMENT VARIABLES

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

EXIT STATUS

The following exit values are returned:

0
Successful completion.


>0
An error occurred.

FILES

/etc/uucp/Devices
device file


/etc/uucp/Sysfiles
system file


/etc/uucp/Systems
system file


/var/spool/locks/*
lock file

SEE ALSO

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

NOTES

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

SIGHUP
Close the connection and terminate.


SIGINT
Forward to the remote system.


SIGQUIT
Forward to the remote system.


SIGUSR1
Terminate the cu process without the normal connection
closing sequence.


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 condition.

May 11, 2001 CU(1C)