if ... ;then
fi

if ...;then
else
...
fi


if ...; then
elif ...; then
else
...
fi
case ... in
pat1)
   ...
   ;;
pat2)
   ...
   ;;
*)
  ...
  ;;
esac
~]# for i in `ls /etc` ;do echo $i; done
~]# sum=0; for i in {1..100};do let sum=$sum+$i; done;echo $sum

for i in {1..9};do
    for j in `seq $i`;do
        echo -e "${j}x${i}=$[j*i]\t\c"
    done
    echo
done

for i in {1..9};do
    for j in `seq $i`;do
    printf "%sx%s=%s\t" ${j} ${i} $[j*i]
    done
    echo
done


for ((sum=0,i=1;i<=100;i++));do
    let sum+=$i
done
echo sum=$sum
~]# while [ $n -le 10 ];do echo $n; let n++; done
~]# n=0;while (($n<5));do echo $n; let n++; done
~]# df | sed -rn '/^\/dev\/sd/s@^([^ ]+) .* ([0-9]+)% .*@\1 \2@p'|while read DEV PER;do if [ $PER -ge 20 ];then mail -s "warning" root; fi; done
for ((i=0;i<=10;i++));do
    for ((j=1;j<=10;j++));do
        [ $j -eq 5 ] && continue
        echo $j
    done
    echo "---------"
done



for ((i=0;i<=10;i++));do
    for ((j=1;j<=10;j++));do
        [ $j -eq 5 ] && continue 2     #continue 2的意思是退出当次两层循环
        echo $j
    done
    echo "---------"
done