shell脚本中％和%%有什么区别？(What is the difference between % and %% in shell scripting?)

shell脚本中％和%%有什么区别？(What is the difference between % and %% in shell scripting?)

在bash脚本中，当t="hello.txt"

${t%%.txt}和${t%.txt}返回"hello"

同样适用于${t##*.}和${t#*.}返回"txt" 。

它们之间有区别吗？ 他们是如何工作的？

In bash scripting, when t="hello.txt" both

${t%%.txt} and ${t%.txt} returns "hello"

the same goes for ${t##*.} and ${t#*.} returns "txt".

Is there a difference between them? How do they work?

最满意答案

简而言之， %%尽可能少地删除% 。

# t="hello.world.txt" # echo ${t%.*} hello.world # echo ${t%%.*} hello

从bash手册：

'${PARAMETER%WORD}'
'${PARAMETER%%WORD}'
     The WORD is expanded to produce a pattern just as in filename
     expansion.  If the pattern matches a trailing portion of the
     expanded value of PARAMETER, then the result of the expansion is
     the value of PARAMETER with the shortest matching pattern (the '%'
     case) or the longest matching pattern (the '%%' case) deleted.  If
     PARAMETER is '@' or '*', the pattern removal operation is applied
     to each positional parameter in turn, and the expansion is the
     resultant list.  If PARAMETER is an array variable subscripted with
     '@' or '*', the pattern removal operation is applied to each member
     of the array in turn, and the expansion is the resultant list.
In short, %% removes as much as possible, % as little as possible.
 
# t="hello.world.txt"
# echo ${t%.*}
hello.world
# echo ${t%%.*}
hello
 
From the bash manual:
 
'${PARAMETER%WORD}'
'${PARAMETER%%WORD}'
     The WORD is expanded to produce a pattern just as in filename
     expansion.  If the pattern matches a trailing portion of the
     expanded value of PARAMETER, then the result of the expansion is
     the value of PARAMETER with the shortest matching pattern (the '%'
     case) or the longest matching pattern (the '%%' case) deleted.  If
     PARAMETER is '@' or '*', the pattern removal operation is applied
     to each positional parameter in turn, and the expansion is the
     resultant list.  If PARAMETER is an array variable subscripted with
     '@' or '*', the pattern removal operation is applied to each member
     of the array in turn, and the expansion is the resultant list.