Bash：如何从参数设置变量，并使用默认值
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Bash: How to set a variable from argument, and with a default value
提问by Steven Lu
It is pretty clear that with shell scripting this sort of thing can be accomplished in a huge number of ways (more than most programming languages) because of all the different variable expansion methods and programs like testand [and [[, etc.
很明显，由于所有不同的变量扩展方法和程序，如testand[和[[等，使用 shell 脚本可以通过多种方式（比大多数编程语言更多）来完成这类事情。
Right now I'm just looking for
现在我只是在寻找
DIR= or .
Meaning, my DIR variable should contain either what is specified in the first arg or the current directory.
意思是，我的 DIR 变量应该包含在第一个 arg 或当前目录中指定的内容。
What is the difference between this and DIR=${1-.}?
这和 有DIR=${1-.}什么区别？
I find the hyphen syntax confusing, and seek more readable syntax.
我发现连字符语法令人困惑，并寻求更具可读性的语法。
Why can't I do this?
为什么我不能这样做？
DIR="" || '.'
I'm guessing this means "if $1 is empty, the assignment still works (DIR becomes empty), so the invalid command '.' never gets executed."
我猜这意味着“如果 $1 为空，则赋值仍然有效（DIR 变为空），因此无效命令 '.' 永远不会被处决。”
回答by rob mayoff
I see several questions here.
我在这里看到几个问题。
“Can I write something that actually reflects this logic”
Yes. There are a few ways you can do it. Here's one:
if [[ "" != "" ]]; then DIR="" else DIR=. fi“What is the difference between this and
DIR=${1-.}?”The syntax
${1-.}expands to.if$1is unset, but expands like$1if$1is set—even if$1is set to the empty string.The syntax
${1:-.}expands to.if$1is unset or is set to the empty string. It expands like$1only if$1is set to something other than the empty string.“Why can't I do this?
DIR="$1" || '.'”Because this is bash, not perl or ruby or some other language. (Pardon my snideness.)
In bash,
||separates entire commands (technically it separates pipelines). It doesn't separate expressions.So
DIR="$1" || '.'means “executeDIR="$1", and if that exits with a non-zero exit code, execute'.'”.
“我能写出真正反映这种逻辑的东西吗？”
是的。有几种方法可以做到。这是一个：
if [[ "" != "" ]]; then DIR="" else DIR=. fi“这和这有什么区别
DIR=${1-.}？”语法
${1-.}扩展为.if$1未设置，但扩展为$1if$1设置 - 即使$1设置为空字符串。语法
${1:-.}扩展为.if$1未设置或设置为空字符串。它$1仅在$1设置为空字符串以外的内容时才会扩展。“为什么我不能这样做？
DIR="$1" || '.'”因为这是 bash，而不是 perl 或 ruby 或其他一些语言。（原谅我的冷漠。）
在 bash 中，
||分隔整个命令（从技术上讲，它分隔管道）。它不分离表达式。所以
DIR="$1" || '.'意思是“执行DIR="$1"，如果以非零退出代码退出，则执行'.'”。
回答by Vaughn Cato
How about this:
这个怎么样：
DIR=.
if [ $# -gt 0 ]; then
DIR=
fi
$#is the number of arguments given to the script, and -gtmeans "greater than", so you basically set DIRto the default value, and if the user has specified an argument, then you set DIRto that instead.
$#是给脚本的参数数量，-gt意思是“大于”，所以你基本上设置DIR为默认值，如果用户指定了一个参数，那么你设置DIR为那个。
回答by icosabit
I use a simple helper function to make such assignments look cleaner. The function below accepts any number of arguments, but returns the first one that's not the empty string.
我使用一个简单的辅助函数来使这些分配看起来更清晰。下面的函数接受任意数量的参数，但返回第一个不是空字符串的参数。
default_value() {
# Return the first non-empty argument
while [[ "" == "" ]] && [[ "$#" -gt "0" ]]; do
shift
done
echo
}
x=$(default_value "" 0)
