Using redirection to write a command using a file

Question

I´m starting to use Ubuntu as my secondary OS and I´m creating a script which writes the current IP configuration in a file using redirection (with the command $sudo ifconfig -a > /etc/scripts/current-ipaddress) and the next thing I want to do is using one of the values written in that external file for the next command, if possible.

For example, lets say I run $sudo ifconfig -a > /etc/scripts/current-ipaddress and the IP address for eth0 is 192.168.1.203 and the next thing I want to do is making this IP the static one using the command $sudo ifconfig eth0 x.x.x.x. The idea is substituting this x.x.x.x with the real IP (which will be extracted from the file it was written using redirections previously). How can I make this work?

Answer 1

Try this:

theip=$(cat /etc/scripts/current-ipaddress | awk '/^eth0:/{flag = 1; next} flag == 1 && / inet / {print $2} /^[^ ]/ {flag=0}')

Explanations:

• cat filename: print the file content
• awk: work on text input flow from pipe (the file content)
  • /^eth0:/{flag = 1; next}: search for line who start with eth0: token and put value 1 in variable flag then treat the next line.
  • flag == 1 && / inet / {print $2}: if variable flag equal to 1 and line contains inet token, then print the second field (by default the field separators and spaces)
  • /^[^ ]/ {flag=0}: if line do not starts by a space, put 0 value in the flag variable
• theip=$(...command...): put the command standard output in the variable theip