cu(1bnu)

cu -- call another UNIX system

Synopsis

cu [options] [destination]

Description

The cu command calls up another UNIX system, a terminal, or possibly a non-UNIX system. It manages an interactive conversation with possible transfers of files.

Options

cu takes the following options. The -c, -l, and -s options play a part in selecting the medium; the remaining options are used in configuring the line.

-b n

Force the number of bits processed on the line to be n, where n can be 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 value specified by the CHARSIZE parameter in the Config file (see Config(4bnu)). If no CHARSIZE value is specified in the Config file, the character size of the line is set to the same as the current local terminal.

For example, if no CHARSIZE value is specified, and you are using an 8-bit system to call a 7-bit system, cu assumes the system you want to call is another 8-bit system; unless otherwise specified with the -b 7 on the cu command line.

If CHARSIZE is set to 7 in the Config file, and you want cu to call an 8-bit system, you would specify -b 8 on the cu command line to override both the CHARSIZE value and the local terminal setting.

-c type

Force cu to only use entries in the Devices(4bnu) file having a value in the ``Type'' field that match type. Typically, type specifies the name of a local area network.

-d

Enable debugging. This option logs diagnostic information for both the conversation phase and the connection phase. Output is sent to the terminal screen.

-e

Set an even data parity. This option forces cu to generate an even parity during data transmission. By default, cu uses the value specified by the PARITY parameter in the Config file to establish the parity for data sent to the remote system (see Config(4bnu)). By default, the PARITY parameter is set to the local terminal setting.

-h

Set communication mode to half-duplex. This option echoes characters locally to support calls to other computer systems that expect terminals to be set to half-duplex mode.

-l line

Specify a device name to use as the communication line. This option 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 the line is taken from the Devices file entry having a value in the ``Line'' field that matches the device name line (see Devices(4bnu)). When the -l and -s options are used together, cu searches the Devices file to check if the requested speed for the requested line is available. If available, the connection is made at the requested speed, otherwise, an error message is printed and the call is not made. In the general case where a specified device is a directly connected asynchronous line (for example, /dev/term/ab), a telephone number is not required. The specified device need not be in the /dev directory. If the specified device is associated with an auto dialer, the destination argument must specify a telephone number.

-n

Request 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

Set an odd data parity. This option forces cu to generate an odd parity during data transmission. By default, cu uses the value specified by the PARITY parameter in the Config file to establish the parity for data sent to the remote system (see Config(4bnu)). By default, the PARITY parameter is set to the local terminal setting.

-p

Set data parity to none. This option forces cu to generate a none parity during data transmission. By default, cu uses the value specified by the PARITY parameter in the Config file to establish the parity for data sent to the remote system (see Config(4bnu)). By default, the PARITY parameter is set to the local terminal setting.

-s speed

Specify the transmission speed (300, 1200, 2400, 4800 or 9600). The default value is Any, which will depend on the order of the lines in the Devices(4bnu) file. Most modems are either 2400bps or 9600bps. Directly connected lines may be set to speeds higher than 2400bps.

-t

Set an appropriate mapping of carriage-return to carriage-return-line-feed pairs. This option is used to dial a terminal which has been set to auto answer.

-x

Use chat scripts. By default, cu does not use chat scripts.

Connection phase

cu uses the same mechanism as uucp to establish a connection. This means that it uses the control files /etc/uucp/Devices (see Devices(4bnu)) and /etc/uucp/Systems (see Systems(4bnu)). 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 (LANs). The Devices file contains a list of media that are available on your system. The Systems file contains information for connecting to remote systems, but is not generally readable.

The destination argument lets cu know what system you wish to connect to. destination can be blank, a telephone number, a system name, or a LAN-specific address. 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. A system name is the name of any computer that uucp can call; the uuname command (see uucp(1bnu)) prints a list of these names. The documentation for your LAN will show the form of the LAN-specific address.

If cu's default behavior is invoked (that is, if you do not use the -c or -l options), cu uses destination to determine which medium to use. If destination is a telephone number, then cu assumes that you want to use a telephone line and it selects an automatic call unit (ACU). If destination is not a telephone number, then cu assumes that it is a system name. In this case, cu follows the uucp calling mechanism and uses the Systems and Devices files to obtain the best available connection. Since cu chooses a speed that is appropriate for the medium that it selects, you may not use the -s option when destination 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 Devices file. Here, destination is assumed to be a system name. If the connection attempt to this system name fails, a connection will be attempted using destination 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 destination. This is the only case when a blank destination is allowed. 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 as destination. The -c and -l options should not be specified together on the 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 a tilde (~), passes them to the remote system; the receive process accepts data from the remote system and, except for lines beginning with a tilde, passes them to the standard output. Normally, an automatic DC3/DC1 protocol is used to control input from the remote so that the buffer is not overrun. Lines beginning with a tilde have special meanings.

The transmit process interprets the following user-initiated commands:

~.: terminate the conversation. Note that, until the connection is established, attempting to use ~. to terminate the conversation will have no effect.
~!: escape to an interactive shell on the local system.
~!cmd...: run cmd on the local system (via sh -c).
~$cmd...: run cmd locally and send its output to the remote system.
~+cmd...: run cmd locally and connect its input and output to the remote system.
~%cd: change the directory on the local system. Note that ~!cd will cause the command to be run by a sub-shell.
~%take from [to]: copy 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]: copy file from (on local system) to file to on the remote system. If to is omitted, the from argument is used in both places.
~~line: send the line ~line to the remote system.
~%break: transmit a BREAK to the remote system (which can also be specified as ~%b).
~%debug: toggle the -d debugging option on or off (which can also be specified as ~%d).
~t: print the values of the termio structure variables for the user's terminal (useful for debugging).
~l: print the values of the termio structure variables for the remote communication line (useful for debugging).
~%ifc: toggle 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. (this can also be specified as ~%nostop).
~%ofc: toggle the output flow control setting. When enabled, outgoing data flow may be controlled by the remote host (this can also be specified as ~%noostop).
~%divert: allow/disallow unsolicited diversions. That is, diversions not specified by ~%take.
~%old: allow/disallow old style syntax for received diversions.

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 control characters on the remote system be identical to the current control characters on the local system. Backslashes are inserted at appropriate places for these control characters.

The use of ~%take requires the existence of echo(1) and cat(1) on the remote system. 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 sysX to connect to system sysY and subsequently used on system sysY to connect to system sysZ, 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 sysZ, sysX, and sysY as follows:

uname
sysZ
~[sysX]!uname
sysX
~~[sysY]!uname
sysY

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.

Files

/etc/uucp/Config
/etc/uucp/Devices
/etc/uucp/Sysfiles
/etc/uucp/Systems
/var/spool/locks/*

Usage

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 cu command sets the input and output conversion mode to on or off, as appropriate, to avoid a character conversion on the local system when accessing the remote system.

On the remote system, the input and output conversion should be set manually, as cu cannot know whether input conversion is required or not. In most cases, remote systems can be used with input conversion switched on; however, when transferring files, this should be switched off before invoking the file transfer command in order to avoid unexpected conversion of the file contents.

Diagnostics

Exit code is zero for normal exit, otherwise, one.

Warnings

You cannot use cu to log in as root if root's login shell on the remote system is set to sh(1).

The cu command does not perform 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. cu will not return a login prompt immediately upon connection, when used between an IMBR1 and a PENRIL modem. A carriage return will return the prompt.

~%put and ~%take cannot be used over multiple links. Files must be moved one link at a time.

If cu is invoked without the -b option from the command line in an Xterm window, then problems may occur and ~%take may fail.

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 <Ctrl>d usually clears the hang condition.

Examples

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

cu -s 1200 9=12015551234

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

To log onto a system that is on a Datakit VCS local area network, but which has not been defined in the Systems file:

cu -c dk address

where dk is the name of the Datakit local area network, and address is the Datakit address which is of the form:

/area/exchange/machine

To log onto a system connected by a direct line:

cu -l /dev/term/XX

or

cu -l term/XX

To dial a system with a specific line and speed:

cu -s 1200 -l term/XX

To dial a system using a specific line associated with an auto dialer:

cu -l culXX 9=12015551234

To use a system name:

cu systemname

References

cat(1), Config(4bnu), cs(1Mbnu), ct(1bnu), Devices(4bnu), echo(1), Permissions(4bnu), stty(1), Systems(4bnu), Sysfiles(4bnu), uname(1), uucp(1bnu)