Add "A" to the end of the line, where lines contain "$ext" :
sed "/$ext/ s/.*/& A/"
Remove space in the start and teh end of each lines :
sed 's/^[ \t]*//;s/[ \t]*$//'
Remove lines with the word "awk" :
sed '/awk/d' file.txt
Remove blank lines :
sed '/^$/d' file.txt
Remove blank lines and commented lines :
sed -i '/^$/d;/^#/d' file.txt
Comment line containing "deux" :
sed '/deux/ s/^/#/'
Get the IP address of a line :
sed "s/..* \([0-9]\{1,\}\.[0-9]\{1,\}\.[0-9]\{1,\}\.[0-9]\{1,\}\)..*/\1/"
Convert from unix to dos :
sed 's/$/\r/g' file.txt
Grep Email from a file :
grep -oe '\w*.\w*@\w*.\w*.\w\+' file.txt | sort -u
Get SSH attacks :
grep -i 'authentication failure' /var/log/messages | awk '{ print $13 }' | cut -b7- | sort | uniq -c
Get the second ocurence of lines containing "open" :
awk '/open/{print $2}'