bash while until 循環用法

首先, bash中命令執行成功返回狀態碼 0, 失敗返回非0狀態碼（如下$是終端提示符）
$ true; echo $?
0
$ false; echo $?
1


1. while

$ while true; do echo good; break; done
good
$ while [ 1 -eq 1 ]; do echo good; break; done
good
$ while ps -ef | grep grep; do echo good; break; done
good


2. util

$ until false; do echo good; break; done
good
$ until [ 1 -ne 1 ]; do echo good; break; done
good
$ until ps -ef | grep grep | grep -v grep; do echo good; break; done
good


因而可知，while表示若是條件爲真則進入循環，until表示條件爲真則不進入循環。