Working with config files or with lots of output lines could be quite dificult sometimes, specially if you need to filter out lines based on regular expressions.

How to filter lines that start with empty(blank) space

Let's say we have a file containing IP addresses and some lines start with the IP address and other lines start with "    IP" like the one below:

How to filter (grep out) lines that start directly with 77 or 193 or 64 is easy.  But let's say want to grep out the IP addresses that start with 193 and have an empty space before them:  "   193.24.215.135" (that starts with empty space):

What I did above was to grep out the lines that start with at least one empty space, followed by possibly more space and by 193.

Let's say we need to grep out all the IPs that start with 193 (with or without empty space at the beginning of the line):

Above I used grep to filter the lines that possibly start with empty space followed by 193.

How to output only lines that start with empty space  

Of course, you need to play a little more with grep and regular expressions (a very interesting field, of course) in order to customize the output, depending on your situation.  Man grep and google:regular expressions or yahoo: regular expressions will be very usefull in this case.

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