Not specific to CentOS but I know you guys would be really helpful anyhow. Basically, I have a file which has been editted in the past very similarly to the hosts file only now I want to use it as a hosts file and need to run some fancy sed to massage the data into shape. Currently, the data in the file is in the form of <ip address> <tab> <short hostname> <space> <short hostname alias>. In some cases there may not be any aliases so the end of line would be right after the short hostname (no space at the end either). In other cases there could be many space separated short hostname aliases. What I have been trying to do without success is add our domain name to the first string after the ip address and tab character. As an example,