如何使用"$ {@:2}"进行可移植性? [英] How to portability use "${@:2}"?
问题描述
在"${@:2}"，因为它会在不同的外壳之间使事情破裂，而应该使用"${*:2}".
On Allow for ${@:2} syntax in variable assignment they say I should not use "${@:2}" because it breaks things across different shells, and I should use "${*:2}" instead.
但是使用"${*:2}"代替"${@:2}"是无意义的，因为执行"${@:2}"并不等同于"${*:2}"，因为
But using "${*:2}" instead of "${@:2}" is nonsense because doing "${@:2}" is not equivalent to "${*:2}" as the following example:
#!/bin/bash
check_args() {
echo "\$#=$#"
local counter=0
for var in "$@"
do
counter=$((counter+1));
printf "$counter. '$var', ";
done
printf "\\n\\n"
}
# setting arguments
set -- "space1 notspace" "space2 notspace" "lastargument"; counter=1
echo $counter': ---------------- "$*"'; counter=$((counter+1))
check_args "$*"
echo $counter': ---------------- "${*:2}"'; counter=$((counter+1))
check_args "${*:2}"
echo $counter': ---------------- "${@:2}"'; counter=$((counter+1))
check_args "${@:2}"
->
GNU bash, version 4.4.12(1)-release (x86_64-pc-linux-gnu)
1: ---------------- "$*"
$#=1
1. 'space1 notspace space2 notspace lastargument',
2: ---------------- "${*:2}"
$#=1
1. 'space2 notspace lastargument',
3: ---------------- "${@:2}"
$#=2
1. 'space2 notspace', 2. 'lastargument',
如果我不能使用"${@:2}"(如他们所说的)，那是什么等价的我可以代替吗?
If I cannot use "${@:2}" (as they say), what is the equivalent can I use instead?
这是原始问题处理所有参数除了第一个(在bash脚本中)，它们将参数和空格保持在一起的唯一答案是使用"${@:2}"
This is original question Process all arguments except the first one (in a bash script) and their only answer to keep arguments with spaces together is to use "${@:2}"
推荐答案
除非您遵循链接，否则问题中尚不清楚上下文.这涉及到 shellcheck.net的以下建议:
There's context that's not clear in the question unless you follow the links. It's concerning the following recommendation from shellcheck.net:
local _help_text="${@:2}"
^––SC2124 Assigning an array to a string! Assign as array, or use * instead of @ to concatenate.
简短的回答:不要将事物列表(例如参数)分配给简单变量，而应使用数组.
Short answer: Don't assign lists of things (like arguments) to plain variables, use an array instead.
长答案:通常，"${@:2}"将获得除第一个参数以外的所有参数，每个参数都被视为一个单独的项目(单词").另一方面，"${*:2}"会产生单个项目，其中除了第一个参数之外，所有其他参数都粘在一起，并用空格(或$IFS的第一个字符所分隔)分隔.
Long answer: Generally, "${@:2}" will get all but the first argument, with each treated as a separate item ("word"). "${*:2}", on the other hand, produces a single item consisting of all but the first argument stuck together, separated by a space (or whatever the first character of $IFS is).
但是在特定情况下，您要分配给一个普通变量，该变量只能存储单个项目，因此var="${@:2}" 还会将参数折叠为单个项目，但是它的实现方式与"${*:2}"不一致.为了避免这种情况，请使用能够存储多个项目的东西:数组.所以:
But in the specific case where you're assigning to a plain variable, the variable is only capable of storing a single item, so var="${@:2}" also collapses the arguments down to a single item, but it does it in a less consistent way than "${*:2}". In order to avoid this, use something that is capable of storing multiple items: an array. So:
- 真的很糟糕:
var="${@:2}" - 稍差一些:
var="${*:2}" - 更好:
arrayvar=("${@:2}")(括号使它成为数组)
- Really bad:
var="${@:2}" - Slightly less bad:
var="${*:2}" - Much better:
arrayvar=("${@:2}")(the parentheses make this an array)
注意:要取回数组的元素，并将每个元素都正确地视为一个单独的项目，请使用"${arrayvar[@]}".另外，并非所有外壳程序都支持数组(特别是dash不支持它们)，因此，如果使用它们，则应确保使用bash shebang(#!/bin/bash或#!/usr/bin/env bash).如果您确实需要可移植到其他shell，则事情会变得更加复杂.
Note: to get the elements of the array back, with each one treated properly as a separate item, use "${arrayvar[@]}". Also, arrays are not supported by all shells (notably, dash doesn't support them), so if you use them you should be sure to use a bash shebang (#!/bin/bash or #!/usr/bin/env bash). If you really need portability to other shells, things get much more complicated.
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