cat student.txt

3 tom     19  255
1 jack  18  240
2 alice 20  248
4 helen 19  223

sort student.txt

1 jack  18  240
2 alice 20  248
3 tom     19  255
4 helen 19  223

sort -k 3 student.txt

1 jack  18  240
4 helen 19  223
3 tom     19  255
2 alice 20  248

sort -n -t$'\t' -k 4 student.txt

4 helen 19  223
1 jack  18  240
2 alice 20  248
3 tom     19  255

cat fruit.txt

apple:10:2.5
orange:20:3.4
banana:30:5.5
pear:90:2.3

sort -n -t ':' -k 3 fruit.txt

pear:90:2.3
apple:10:2.5
orange:20:3.4
banana:30:5.5