1. bash手册
搜索: Bash Manual bash手册之变量扩展
2. 变量替换
| Variable | Description |
|---|---|
${parameter:-defaultValue} |
Get default shell variables value |
${parameter:=defaultValue} |
Set default shell variables value |
${parameter:?"Error Message"} |
Display an error message if parameter is not set |
${#var} |
Find the length of the string |
${var%pattern} |
Remove from shortest rear (end) pattern |
${var%%pattern} |
Remove from longest rear (end) pattern |
${var:num1:num2} |
Substring |
${var#pattern} |
Remove from shortest front pattern |
${var##pattern} |
Remove from longest front pattern |
${var/pattern/string} |
Find and replace (only replace first occurrence) |
${var//pattern/string} |
Find and replace all occurrences |
${!prefix*} |
Expands to the names of variables whose names begin with prefix. |
${var,} ${var,pattern} |
Convert first character to lowercase. |
${var,,} ${var,,pattern} |
Convert all characters to lowercase. |
${var^} ${var^pattern} |
Convert first character to uppercase. |
${var^^} ${var^^pattern} |
Convert all character to uppercase. |
2.1. 有没有冒号的区别
${parameter:-defaultValue}
有冒号, 表示如果parameter不存在, 或者为空, 则返回defaultValue${parameter-defaultValue}
没有冒号:, 表示如果parameter不存在, 则返回defaultValue
When not performing substring expansion, using the form described below (e.g., ‘:-’), Bash tests for a parameter that is unset or null. Omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both parameter’s existence and that its value is not null; if the colon is omitted, the operator tests only for existence.
3. 用冒号初始化变量
比如:
: "${LANG_CXX:=true}"
: "${LANG_D:=true}"
: "${LANG_OBJC:=true}"
: "${LANG_GO:=true}"
: "${LANG_FORTRAN:=true}"
: "${LANG_ADA:=true}"
: "${LANG_JIT:=true}"
- 冒号是个空命令, 什么也不干, 永远返回0
- 但冒号命令会展开参数, 最后的效果是给变量赋默认值
- 加冒号的目的是避免shell把默认值当作命令执行
4. export有什么用? 子进程不是继承父进程的环境变量吗?
回答:
- export用于把当前的shell变量export给子进程.
- shell变量并不一定是env变量. 只有export的变量才是环境变量, 没有export的变量压根就不是环境变量, 当然不能被子shell继承.