#!/bin/bash   # 算术测试.   # (( ... ))结构可以用来计算并测试算术表达式的结果.   # 退出状态将会与[ ... ]结构完全相反!   (( 0 ))   echo "Exit status of \"(( 0 ))\" is $?."         # 1   (( 1 ))   echo "Exit status of \"(( 1 ))\" is $?."         # 0   (( 5 > 4 ))                                      # 真   echo "Exit status of \"(( 5 > 4 ))\" is $?."     # 0   (( 5 > 9 ))                                      # 假   echo "Exit status of \"(( 5 > 9 ))\" is $?."     # 1   (( 5 - 5 ))                                      # 0   echo "Exit status of \"(( 5 - 5 ))\" is $?."     # 1   (( 5 / 4 ))                                      # 除法也可以.   echo "Exit status of \"(( 5 / 4 ))\" is $?."     # 0   (( 1 / 2 ))                                      # 除法的计算结果 < 1.   echo "Exit status of \"(( 1 / 2 ))\" is $?."     # 截取之后的结果为 0.                                                    # 1   (( 1 / 0 )) 2>/dev/null                          # 除数为0, 非法计算.  #           ^^^^^^^^^^^   echo "Exit status of \"(( 1 / 0 ))\" is $?."     # 1   # "2>/dev/null"起了什么作用?   # 如果这句被删除会怎样?   # 尝试删除这句, 然后在运行这个脚本.  exit 0 ====================== root@client.example.com ~/yang # ./calucate.sh Exit status of "(( 0 ))" is 1. Exit status of "(( 1 ))" is 0. Exit status of "(( 5 > 4 ))" is 0. Exit status of "(( 5 > 9 ))" is 1. Exit status of "(( 5 - 5 ))" is 1. Exit status of "(( 5 / 4 ))" is 0. Exit status of "(( 1 / 2 ))" is 1. Exit status of "(( 1 / 0 ))" is 1. root@client.example.com ~/yang #