Diff Tool – How to Print Full File Diff When File is Missing

$ alias diff='diff -W $(( $(tput cols) - 2 ))'

ought to do it. You'll want to add it to ~/.bashrc as well.

The - 2 is mainly paranoia, in case something (embedded double-width Unicode?) expands enough to make the line wrap; if you want, you can just use

$ alias diff='diff -W $(tput cols)'

As Gilles says, you'll need to use a tool that knows about the format of your config files. For your particular example, you can use a short Python script that uses the built-in ConfigParser module.

• First, let's say your original configuration file on the server is original.cfg:

  [config]
  ip=127.0.0.1
  port=22
  

• Now put your changes in a new file called update.cfg:

  [config]
  user=root
  

  This file should have new or changed entries listed under the section heading where you would like them to go.

• Then run a script like this one:

  #!/usr/bin/env python
  
  import ConfigParser
  
  config = ConfigParser.ConfigParser()
  
  # Read the original config file
  with open('original.cfg', 'r') as f:
      config.readfp(f)
  
  # Also read in all the changes we'd like to make
  with open('update.cfg', 'r') as f:
      config.readfp(f)
  
  # Write the full new config file out
  with open('output.cfg', 'w') as f:
      config.write(f)
  

There are, of course, plenty of variations on how to do this. For example, the config changes could be coded directly into the Python script rather than read from a separate update.cfg file. This should give you a good base to get started, though.