One way of using eval, is to use an external command to set variables that you do not know the name of beforehand. Or a GROUP of variables. A common use of this, is to set terminal-size variables on login:
Technically, the second one is the POSIX-preferred one.echo This is the uptime: `uptime` echo This is the uptime: $(uptime)
In addition to creating dynamic output, this is also very useful for setting variables:
datestring=`date` echo "The current date is: $datestring"
Some people may be familiar with the "curses" library. It is a way to manipulate and move around text on a screen, reguardless of what kind of "terminal" the user is using.
As mentioned, this is a potentially huge topic. So, I'm just going to give you a trivial example, and say "Go read the man-page on tput". Well, okay, actually, you have to read the "tput" manpage, AND either the "terminfo" or "termcap" manpage to figure out what magical 3-5 letter name to use. For example, it should tell you that "cup" is short for the "cursor_address" command. But you must use "cup", NOT "cursor_address", with tput.
The above example clear the screen, prints the given line at a SPECIFIC place on the screen, then puts the cursor back down near the bottom of the screen for you.tput init tput clear tput cup 3 2 print -n "Here is a clean screen, with these words near the top" endline=`tput cols` tput cup $(($endline - 2)) print "and now, back to you" sleep 2
PS: If you've been doing a lot of funky things with the screen, you might want to do a
as the last thing before your shellscript exits.tput reset
This will print out a mini-menu like the following:select word in one two three exit; do echo word is $word echo reply is $REPLY if [[ "$word" = "exit" ]] ; then break; fi done
1) one 2) two 3) three 4) exit #?
Note that this will loop between "do ... done" until you trigger a break somehow! (or until the user control-c's or whatever). So dont forget an exit condition!
Here is a trivial example that just opens up a connection to an SMTP server. Note that the connection is half-duplex: You do NOT see data that you send to the other side.
#!/bin/ksh -p MAILHOST=127.0.0.1 exec 3<>/dev/tcp/$MAILHOST/25 || exit 1 read -r BANNER <&3 echo BANNER is $BANNER print -u3 HELO myhost.com read -r REPLY <&3 echo REPLY is $REPLY
The output will look something like the following:
BANNER is 220 yourhost.domain.com ESMTP Sendmail 8.11.6+Sun/8.11.6; Tue, 3 Dec 2002 17:30:01 -0800 (PST) REPLY is 250 yourhost.domain.com Hello localhost [127.0.0.1], pleased to meet you
Note that we use the "-r" flag to read. In this particular example, it is not neccessary. But in the general case, it will give you the data "raw". Be warned that if the shell cannot open the port, it will kill your entire script, with status 1, automatically
You can also dump the rest of the data waiting on the socket, to whereever you like, by doing
cat <&3 >somefile