12.6. Communications Commands

Information and Statistics

host

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

vrfy

Verify an Internet e-mail address.

nslookup

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.

dig

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.

traceroute

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.

ping

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 (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data. Warning: time of day goes back, taking countermeasures. 64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255 time=709 usec 64 bytes from localhost.localdomain (127.0.0.1): 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

whois

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

finger

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.

ftp

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-11).

cu

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

uucp

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.

telnet

Utility and protocol for connecting to a remote host.

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

rlogin

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

rsh

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

rcp

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.

ssh

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

write

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.

Mail

mail

Send an e-mail message to a user.

This stripped-down command-line mail client works fine as a command embedded in a script.

Example 12-29. A script that mails itself

#!/bin/sh # self-mailer.sh: Self-mailing script ARGCOUNT=1 # Need name of addressee. E_WRONGARGS=65 if [ $# -ne "$ARGCOUNT" ] then echo "Usage: `basename $0` addressee" exit $E_WRONGARGS fi # ======================================================================== cat $0 | mail -s "Script \"`basename $0`\" has mailed itself to you." "$1" # ======================================================================== # -------------------------------------------- # Greetings from the self-mailing script. # A mischievous person has run this script, #+ which has caused it to mail itself to you. # Apparently, some people have nothing better #+ to do with their time. # -------------------------------------------- exit 0

vacation

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.