12.6. Communications Commands

Information and Statistics


Searches for information about an Internet host by name or IP address, using DNS.


Verify an Internet e-mail address.


Do an Internet "name server lookup" on a host by IP address. This may be run either interactively or noninteractively, i.e., from within a script.


Similar to nslookup, do an Internet "name server lookup" on a host. May be run either interactively or noninteractively, i.e., from within a script.


Trace the route taken by packets sent to a remote host. This command works within a LAN, WAN, or over the Internet. The remote host may be specified by an IP address. The output of this command may be filtered by grep or sed in a pipe.


Broadcast an "ICMP ECHO_REQUEST" packet to other machines, either on a local or remote network. This is a diagnostic tool for testing network connections, and it should be used with caution.

A successful ping returns an exit status of 0. This can be tested for in a script.

bash$ ping localhost
PING localhost.localdomain ( from : 56(84) bytes of data.
 Warning: time of day goes back, taking countermeasures.
 64 bytes from localhost.localdomain ( icmp_seq=0 ttl=255 time=709 usec
 64 bytes from localhost.localdomain ( icmp_seq=1 ttl=255 time=286 usec

 --- localhost.localdomain ping statistics ---
 2 packets transmitted, 2 packets received, 0% packet loss
 round-trip min/avg/max/mdev = 0.286/0.497/0.709/0.212 ms


Perform a DNS (Domain Name System) lookup. The -h option permits specifying which whois server to query. See Example 5-6.


Retrieve information about a particular user on a network. Optionally, this command can display the user's ~/.plan, ~/.project, and ~/.forward files, if present.

bash$ finger bozo
Login: bozo                             Name: Bozo Bozeman
 Directory: /home/bozo                   Shell: /bin/bash
 On since Fri Aug 31 20:13 (MST) on tty1    1 hour 38 minutes idle
 On since Fri Aug 31 20:13 (MST) on pts/0   12 seconds idle
 On since Fri Aug 31 20:13 (MST) on pts/1
 On since Fri Aug 31 20:31 (MST) on pts/2   1 hour 16 minutes idle
 No mail.
 No Plan.

Out of security considerations, many networks disable finger and its associated daemon. [1]

Remote Host Access

sx, rx

The sx and rx command set serves to transfer files to and from a remote host using the xmodem protocol. These are generally part of a communications package, such as minicom.

sz, rz

The sz and rz command set serves to transfer files to and from a remote host using the zmodem protocol. Zmodem has certain advantages over xmodem, such as greater transmission rate and resumption of interrupted file transfers. Like sx and rx, these are generally part of a communications package.


Utility and protocol for uploading / downloading files to / from a remote host. An ftp session can be automated in a script (see Example 17-7, Example A-4, and Example A-8).


Call Up a remote system and connect as a simple terminal. This is a sort of dumbed-down version of telnet.


UNIX to UNIX copy. This is a communications package for transferring files between UNIX servers. A shell script is an effective way to handle a uucp command sequence.

Since the advent of the Internet and e-mail, uucp seems to have faded into obscurity, but it still exists and remains perfectly workable in situations where an Internet connection is not available or appropriate.


Utility and protocol for connecting to a remote host.


The telnet protocol contains security holes and should therefore probably be avoided.


Remote login, initates a session on a remote host. This command has security issues, so use ssh instead.


Remote shell, executes command(s) on a remote host. This has security issues, so use ssh instead.


Remote copy, copies files between two different networked machines. Using rcp and similar utilities with security implications in a shell script may not be advisable. Consider, instead, using ssh or an expect script.


Secure shell, logs onto a remote host and executes commands there. This secure replacement for telnet, rlogin, rcp, and rsh uses identity authentication and encryption. See its manpage for details.

Local Network


This is a utility for terminal-to-terminal communication. It allows sending lines from your terminal (console or xterm) to that of another user. The mesg command may, of course, be used to disable write access to a terminal

Since write is interactive, it would not normally find use in a script.



This utility automatically replies to e-mails that the intended recipient is on vacation and temporarily unavailable. This runs on a network, in conjunction with sendmail, and is not applicable to a dial-up POPmail account.



A daemon is a background process not attached to a terminal session. Daemons perform designated services either at specified times or explicitly triggered by certain events.

The word "daemon" means ghost in Greek, and there is certainly something mysterious, almost supernatural, about the way UNIX daemons silently wander about behind the scenes, carrying out their appointed tasks.