如何在 bash onliner FOR 循环中反转数组？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/13360091/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
How to reverse array in bash onliner FOR loop?
提问by pedrosaurio
How can I reverse the order in which I perform a for loop for a defined array
如何反转对已定义数组执行 for 循环的顺序
To iterate through the array I am doing this:
为了遍历数组，我这样做：
$ export MYARRAY=("one" "two" "three" "four")
$ for i in ${MYARRAY[@]}; do echo $i;done
one
two
three
four
Is there a function where I can reverse the order of the array?
有没有可以颠倒数组顺序的函数？
One thought I had is to generate a sequence of inverted indexes and call the elements by using this reversed index but maybe there is a quicker alternative, or at least easier to read.
我的一个想法是生成一系列倒排索引并使用这个倒排索引调用元素，但也许有一个更快的替代方案，或者至少更容易阅读。
回答by choroba
You can use the C-style for loop:
您可以使用 C 风格的 for 循环：
for (( idx=${#MYARRAY[@]}-1 ; idx>=0 ; idx-- )) ; do
echo "${MYARRAY[idx]}"
done
For an array with "holes", the number of elements ${#arr[@]}doesn't correspond to the index of the last element. You can create another array of indices and walk it backwards in the same way:
对于具有“孔”的数组，元素数量${#arr[@]}与最后一个元素的索引不对应。您可以创建另一个索引数组并以相同的方式向后遍历：
#! /bin/bash
arr[2]=a
arr[7]=b
echo ${#arr[@]} # only 2!!
indices=( ${!arr[@]} )
for ((i=${#indices[@]} - 1; i >= 0; i--)) ; do
echo "${arr[indices[i]]}"
done
回答by bereal
回答by mikeserv
_arr+=( '"${_arrev} is an actual "${array[@]}"' ) ?
_arr+=( '"${_arrev} is created as a result"' )
_arr+=( '"of reversing the key order in"' )
_arr+=( '"this "${_arr}. It handles zsh and"' )
_arr+=( '"bash arrays intelligently by tracking"' )
_arr+=( '"shell "$ENV." quotes=fine ( i hope ) "' )
. <<REVERSE /dev/stdin ?
_arrev=( $(: $((l=${#_arr[@]}${ZSH_VERSION++1})) ; printf '"${_arr[$(('$l'-%d))]}" ' `seq 1 $l`) )
REVERSE
echo ; printf %s\n ${_arrev}
"shell "$ENV." quotes=fine ( i hope ) "
"bash arrays intelligently by tracking"
"this "${_arr}. It handles zsh and"
"of reversing the key order in"
"${_arrev} is created as a result"
"${_arrev} is an actual "${array[@]}"
This should handle any possible array, I think.
我认为这应该处理任何可能的数组。
If you're interested in what's going on up there, I suggest you have a look herefirst. Then maybe here, definitely here, and, if you've got the time, hereand here.
如果你对上面发生的事情感兴趣，我建议你先看看这里。那么也许在这里，绝对在这里，如果你有时间，在这里和这里。
In all of those answers I discuss different
aspects of the here-document (and in many others)which you can use to your advantage. For instance I discuss twice-evaluating variables, which is done above, and in one declare a function that globally declares another function named "_$1"in just 5 or 6 lines - most of which were _$1() { func body ; }.It's pretty handy if you use it correctly.
在所有这些答案中，我讨论了此处文档（以及许多其他文档）的不同方面，您可以利用这些方面的优势。例如，我讨论了两次求值变量，这是在上面完成的，并且在一个函数中声明了一个函数，该函数全局声明了另一个"_$1"仅用 5 或 6 行命名的函数——其中大部分是_$1() { func body ; }. 如果您正确使用它，它会非常方便。
Regarding the auto-switch between bash/zsh,well that's something else, but very simple as well. See here.
关于bash/zsh,井之间的自动切换是另一回事，但也很简单。见这里。
回答by Alek
How about this:
这个怎么样：
for i in `printf '%s\n' "${MYARRAY[@]}"|tac`; do echo $i; done
The output is:
输出是：
four
three
two
one
Limitation: doesn't work if array contain newlines. As a workaround you may implement function:
限制：如果数组包含换行符，则不起作用。作为一种解决方法，您可以实现以下功能：
reverse(){ reversed=();local i;for ((i=$#;i>0;i--)); do reversed+=("${!i}");done; }
and use it this way:
并以这种方式使用它：
reverse "${MYARRAY[@]}" && for i in "${reversed[@]}"; do echo $i; done
回答by Johan Snowgoose
Simple as a string:
简单的字符串：
% unset c; a="1 2 3 4 5"; for b in $a; do c="$b $c"; done; echo $c
5 4 3 2 1
5 4 3 2 1
You sure you want array syntax??:
你确定你想要数组语法？？：
% unset c; declare -a c; a=(1 2 3 4 5); i=0; for b in ${a[*]}; \
do c[$((${#a[@]}-$i))]=$b; i=$(($i+1)); done; echo ${c[*]}
5 4 3 2 1
5 4 3 2 1
回答by Robert
If you are talking about a sequential numerical array (say to delete bash history) you can just list this in reverse order for instance:
如果你在谈论一个连续的数字数组（比如删除 bash 历史），你可以以相反的顺序列出它，例如：
for i in {16..10}; do history -d $i; done
I realize that this is kind of off topic and I apologize for that however I figured that it was probably worth mentioning.
我意识到这有点偏离主题，我为此道歉，但我认为这可能值得一提。
回答by S0AndS0
I'd advise limiting one-liner usage for such things and instead write a function that's sourced into your shell or script. Here's an example for newer versions of Bash where it's possible to pass multiple arrays by reference to a function...
我建议限制此类事情的单行使用，而是编写一个源代码到您的 shell 或脚本中的函数。这是较新版本的 Bash 的示例，其中可以通过引用函数来传递多个数组...
reverse_array(){
local -n _source_array_ref=""
local -n _destination_array_ref=""
for ((_index=${#_source_array_ref[@]}-1; _index>=0; _index--)); do
_destination_array_ref+=("${_source_array_ref[$_index]}")
done
}
_list=(spam ham space)
_new_list=()
reverse_array '_list' '_new_list'
printf '%s\n' "${_new_list[@]}"
#> space
#> ham
#> spam
... however, with this technique do be aware that it makes a function that's impure(has side-effects), meaning that debugging _listor _new_listcan become difficult when in a Bash scripting mindset... also current examples don't take into account that one could re-run reverse_arrayand end up with _new_listappended to multiple times; which may or may not be desired.
...然而，使用这种技术时请注意，它会使函数不纯（有副作用），这意味着在 Bash 脚本思维中调试_list或_new_list可能变得困难......而且当前的例子也没有考虑那个可以重新运行reverse_array并最终_new_list附加到多次；这可能是也可能不是需要的。
回答by M. Modugno
you can also consider using seq
你也可以考虑使用 seq
MYARRAY=("one" "two" "three" "four")
for i in $(seq $((${#MYARRAY[@]} - 1)) -1 0); do
echo ${MYARRAY[$i]}
done
in freebsd you can omit -1 increment parameter:
在 freebsd 中，您可以省略 -1 增量参数：
for i in $(seq $((${#MYARRAY[@]} - 1)) 0); do
echo ${MYARRAY[$i]}
done
