Bash变量默认值(缺省值)随记

来自WILO & 三线的随记
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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