Bash变量默认值(缺省值)随记

Form 1

${a-defaultvalue}

a如果没有定义，则表达式返回默认值，否则返回a的值；

demo

sanxian@sanxian-pc $ unset test
sanxian@sanxian-pc $ echo ${test-"strstr"}
strstr
sanxian@sanxian-pc $ test=""
sanxian@sanxian-pc $ echo ${test-"strstr"}

sanxian@sanxian-pc $ 

Form 2

${a:-defaultvalue}

a没有定义或者为空字符串，则表达式返回默认值，否则返回a的值；

demo

sanxian@sanxian-pc $ test=1123
sanxian@sanxian-pc $ echo ${test:-"strstr"}
1123
sanxian@sanxian-pc $ test=""
sanxian@sanxian-pc $ echo ${test:-"strstr"}
strstr
sanxian@sanxian-pc $ unset test
sanxian@sanxian-pc $ echo ${test:-"strstr"}
strstr