bash 分配给位置参数
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Assigning to a positional parameter
提问by Freeman
How can I assign a value to a positional parameter in Bash? I want to assign a value to a default parameter:
如何在 Bash 中为位置参数赋值？我想为默认参数赋值：
if [ -z "" ]; then
4=
fi
Indicating that 4 is not a command.
表示 4 不是命令。
回答by msw
The setbuilt-in is the only way to set positional parameters
在set内置的设置位置参数的唯一途径
$ set -- this is a test
$ echo
this
$ echo
test
where the --protects against things that look like options (e.g. -x).
其中--防止看起来像选项的东西（例如-x）。
In your case you might want:
在您的情况下，您可能想要：
if [ -z "" ]; then
set -- "" "" "" ""
fi
but it would probably be more clear as
但它可能会更清楚
if [ -z "" ]; then
# default the fourth option if it is null
fourth=""
set -- "" "" "" "$fourth"
fi
you might also want to look at the parameter count $#instead of testing for -z.
您可能还想查看参数计数$#而不是测试-z.
回答by Nelson
You can do what you want by calling your script again with a fourth parameter:
您可以通过使用第四个参数再次调用脚本来执行您想要的操作：
if [ -z "" ]; then
set a b c "d e f" g h
thisArray=( "$@" )
thisArray[3]=4
set -- "${thisArray[@]}"
echo "$@"
"" "" "" ""
exit $?
fi
echo
Calling above script like ./script.sh one two threewill output:
调用上面的脚本./script.sh one two three会输出：
three
三
回答by Brian Chrisman
This can be done with an assignment directly into an auxiliary array with an export/import type mechanism:
这可以通过直接分配到具有导出/导入类型机制的辅助数组来完成：
##代码##outputs 'a b c 4 g h'
输出“abc 4 gh”
