Useful regular expressions

Below you have some usefull regular expressions that will help you in configuring and troubleshooting a Unix/Linux enviroment:

Use following sed magic to remove both comments and empty lines at the same expense:

#grep -v "^#" file.conf | sed -e '/^$/d'

Or:

# cat file.conf | sed '/ *#/d; /^ *$/d'

Replace upper with lower case:

$ echo "CAPS OFF" | sed -e 's/[A-Z]/\l&/g'
caps off
$

Replace only the first letter:

$ echo "CAPS OFF" | sed -e 's/\<[[:upper:]]/\l&/g'
cAPS oFF
$


To remove the ^M characters at the end of all lines in vi, use:

:%s/^V^M//g

The ^v is a CONTROL-V character and ^m is a CONTROL-M. When you type this, it will look like this:

:%s/^M//g

In UNIX, you can escape a control character by preceeding it with a CONTROL-V. The :%s is a basic search and replace command in vi. It tells vi to replace the regular expression between the first and second slashes (^M) with the text between the second and third slashes (nothing in this case). The g at the end directs vi to search and replace globally (all occurrences).

To be countinued.