sed

The Stream EDitor
sed  file.txt
Same as  cat  file.txt
sed  -e  '10q'  file.txt
Output first 10 lines
sed  -n  -e  '10,20p'  file.txt
Output lines 10 to 20
sed  -e  's/foo/bar/g'  file.txt
Replace all 'foo' with 'bar'
More info:   man  sed