[英]Bash parameter expansion and array index operations
The ${parameter,,pattern}
parameter expansion converts alphabetic characters in parameter to lower-case. ${parameter,,pattern}
参数扩展将参数中的字母字符转换为小写。
On cygwin 1.7.11-1 Bash 4.1.10(4) and also on my debian squeeze Bash 4.1.5(1) ;在cygwin 1.7.11-1 Bash 4.1.10(4)和我的debian squeeze Bash 4.1.5(1)上;
if i run the following, i get a curious result:如果我运行以下命令,我会得到一个奇怪的结果:
$ declare -a a=(Zero One Two Three); n=0; echo "${a[n],,}->${n}"; echo "${a[++n]}->${n}"; echo "${a[++n],,}->${n}"
zero->0
One->1
three->3
$
NB: similar results happen:注意:类似的结果发生:
for ,,*
or ^^
case conversion;用于,,*
或^^
大小写转换;
for some other expansions such as ${parameter##word}
;对于其他一些扩展,例如${parameter##word}
;
for using either prefix/postfix ++
or --
operator;用于使用前缀/后缀++
或--
运算符;
for using $((++n))
instead of just ++n
.使用$((++n))
而不仅仅是++n
。
However, the length expansion ${#parameter}
works as i might expect:但是,长度扩展${#parameter}
的工作方式与我预期的一样:
in the above snippet, echo "${#a[++n]}->${n}"
instead of echo "${a[++n],,}->${n}
would yield 3->2
instead of three->3
~& the length of a[2]="two" is indeed 3 characters.在上面的片段中, echo "${#a[++n]}->${n}"
而不是echo "${a[++n],,}->${n}
会产生3->2
而不是three->3
~& a[2]="two" 的长度确实是 3 个字符。
I imagine that the parameter expansion is happening twice.我想参数扩展发生了两次。 But why is this happening?但是为什么会这样呢?
I grabbed the latest bash source & it appears that this issue is resolved for this version:我抓取了最新的 bash 源代码,看来此版本已解决此问题:
$ ./bash --version
GNU bash, version 4.2.24(1)-release (x86_64-unknown-linux-gnu)
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
$
Note however that on my Debian Squeeze stable , I have:但是请注意,在我的 Debian Squeeze stable上,我有:
$ apt-cache policy bash
bash:
Installed: 4.1-3
Candidate: 4.1-3
Version table:
*** 4.1-3 0
500 http://ftp.uk.debian.org/debian/ squeeze/main amd64 Packages
100 /var/lib/dpkg/status
$
$ bash --version
GNU bash, version 4.1.5(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2009 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
I think this means that currently on Debian Squeeze , the latest stable version of bash is 4.1.5(1) & contains this bug.我认为这意味着目前在Debian Squeeze上,bash 的最新稳定版本是4.1.5(1)并且包含此错误。 Also on Cygwin 1.7.11-1 currently the latest available bash package appears to be version 4.1.10(4) & contains this bug.同样在Cygwin 1.7.11-1上,当前最新可用的 bash package 似乎是版本4.1.10(4)并且包含此错误。 Of course the issue is easy to work around, so no real need to build the 4.2 sources.当然这个问题很容易解决,所以不需要构建4.2源代码。
I got我有
Zero->0
One->1
Two->2
However I think "${a[++n]}->${n}" may not be well defined.但是我认为 "${a[++n]}->${n}" 可能没有明确定义。
try substitute with尝试用
let n=$n+1 ; echo "${a[n],,}->${n}";
To inject the notion of sequence.注入序列的概念。
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