Sed

Apply edit command to file and sends to output:

sed -e "<edit command>" <file> > <output>

Apply all edit commands in sedfile:

sed -f <sedfile> <file> > <output>

Some basic edit commands:

s/a/b/       substitute b for a
{range}d     delete lines in range

To append .img to the end of each line:

sed -e "s/$/.img/" <file>

or (if there is already a tab at the end of each line)

sed -e "s/\t$/.img/" <file>

To modify a file in-place, use the -i option:

sed -e 's/\sgutsy/ hardy/g' -i /etc/apt/sources.list