Issue with grep

Shell Programming and Scripting
1

Hi there,

I need to grep out 1 line of a changing file. Any help would be much appreciated.

code:

xterm -hold -e tail -f /var/lib/dhcp3/dhcpd.leases | grep client-hostname &>/dev/null &

The trouble is it shows the contents of the entire lease file.

I just want to show the line starting with client-hostname or even better just the host name so in the case below it just shows computer01

lease 10.0.0.11 {
  starts 4 2012/04/12 16:25:30;
  ends 4 2012/04/12 16:35:30;
  cltt 4 2012/04/12 16:25:30;
  binding state active;
  next binding state free;
  hardware ethernet XX:XX:XX:XX:XX:XX;
  uid "\001\000\034\263|>\330";
  client-hostname "Computer01";
}

Thank you in advance

DV

2

The | isn't run by xterm, it happens in whatever shell you're running xterm in. So it does nothing to the contents that get printed to the xterm.

You can try this:

xterm -hold -e sh -c 'tail -f /var/lib/dhcp3/dhcpd.leases | grep client-hostname' &>/dev/null &

The sh -c tells it to run the contents of the string after it inside a shell, which you need to understand the |.

And since the | is in a string, it happens inside xterm like you want.

3

Is there anyway i could get grep to just show the connected device and not the whole line? So it would just show Computer01 Thanks Dan

4

grep isn't a programming language, its options for what parts of the match get printed are limited.

How about awk? It is a programming language and understands tokens. Here the program is just 'for all lines that match the regex /client-hostname/, print column two'.

xterm -hold -e sh -c "tail -f /var/lib/dhcp3/dhcpd.leases | awk '/client-hostname/ { print $2 }'" &>/dev/null &
5

It still displays: client-hostname "Computer01"; and not just: Computer01 Thanks for all your help.

6 
xterm -hold -e sh -c "tail -f /var/lib/dhcp3/dhcpd.leases | awk '/client-hostname/ { gsub(/\"/, \"\"); print $2 }'" &>/dev/null &