在POSIX shell脚本中执行…while或do…until

有一个众所周知的while condition; do ...; done循环，但是有一个do... while样式循环可以确保至少执行一次该块吗？

一个非常通用的版本do ... while具有以下结构：

while 
      Commands ...
do :; done

一个例子是：

#i=16
while
      echo "this command is executed at least once $i"
      : ${start=$i}              # capture the starting value of i
      # some other commands      # needed for the loop
      (( ++i < 20 ))             # Place the loop ending test here.
do :; done
echo "Final value of $i///$start"
echo "The loop was executed $(( i - start )) times "

照原样（未设置的值i），循环执行20次。
取消注释设置i为16 的行，i=16循环将执行4次。
对于i=16，i=17，i=18和i=19。

如果在同一点（开始）将i设置为（假设为26），则命令仍将在第一次执行（直到测试loop break命令）。

一段时间的测试应该是真实的（退出状态为0）。
测试应反向进行直至循环，即：虚假（退出状态不为0）。

POSIX版本需要更改几个元素才能起作用：

i=16
while
       echo "this command is executed at least once $i"
       : ${start=$i}              # capture the starting value of i
       # some other commands      # needed for the loop
       i="$((i+1))"               # increment the variable of the loop.
       [ "$i" -lt 20 ]            # test the limit of the loop.
do :;  done
echo "Final value of $i///$start"
echo "The loop was executed $(( i - start )) times "

./script.sh
this command is executed at least once 16
this command is executed at least once 17
this command is executed at least once 18
this command is executed at least once 19
Final value of 20///16
The loop was executed 4 times 

没有do ... while或do ... until循环，但是可以像这样完成同一件事：

while true; do
  ...
  condition || break
done

直到：

until false; do
  ...
  condition && break
done