If you need to fool a machine into believing that a host:port pair is local, you can use ipchains to redirect traffic. For example, the desired destination is www.example.com:80 and you want it to go to localhost:8080:

# echo '1' > /proc/sys/net/ipv4/ip_forward
# ipchains -A input -j REDIRECT 8080 -p tcp -s 0.0.0.0/0 -d 0.0.0.0/0 80

Note: No one really uses ipchains anymore, but it can be found on older systems.

Sometimes you need to iterate over variable names and access their values. In shell script, you would do something like this to get the values of FOO and BAR:

$ FOO=apple
$ BAR=orange
$ VARS="FOO BAR"
$ for v in $VARS ; do echo ${!v} ; done

The above works in bash. Use this in Zsh:

$ for v in $VARS ; do echo ${(P)v} ; done

(The 'P' flag on ${v} causes a further variable lookup before ${v} is evaluated.)

Thanks to Cliff for the tip!

Let's say you want to check the uptime on a list of servers. We're assuming that you've got a key on each machine otherwise you'll be entering your password often:

$ xargs -i ssh {} uptime < ./server.list
  5:46am  up 155 days, 17:49,  2 users,  load average: 0.12, 0.03, 0.01
  5:46am  up 147 days, 17:14,  2 users,  load average: 0.02, 0.05, 0.01
  5:46am  up 209 days, 17:26,  0 users,  load average: 0.00, 0.00, 0.00
  5:46am  up 89 days,  6:30,  0 users,  load average: 0.00, 0.00, 0.00
  5:46am  up 82 days,  6:40,  0 users,  load average: 0.07, 0.06, 0.01
  5:46am  up 104 days,  9:51,  0 users,  load average: 0.03, 0.03, 0.00
  5:50am  up 68 days,  9:17,  0 users,  load average: 0.00, 0.00, 0.00
  5:48am  up 68 days,  9:15,  0 users,  load average: 0.00, 0.00, 0.00