bash 中的冒号破折号“:-”是什么意思
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/27445455/
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
What does the colon dash ":-" mean in bash
提问by Stelios
The result is the one desired; after a bit of trial and error. I don't understand what the "2:-" and "3:-" do/mean. Can someone explain.
结果是想要的；经过一些试验和错误。我不明白“2:-”和“3:-”是什么意思。有人可以解释一下。
#!/bin/bash
pid=$(ps -ef | grep java | awk ' NR ==1 {print }')
count=${2:-30} # defaults to 30 times
delay=${3:-10} # defaults to 10 second
mkdir $(date +"%y%m%d")
folder=$(date +"%y%m%d")
while [ $count -gt 0 ]
do
jstack $pid >./"$folder"/jstack.$(date +%H%M%S.%N)
sleep $delay
let count--
echo -n "."
done
回答by Gilles Quenot
It's a parameter expansion, it means if the third argument is null or unset, replace it with what's after:-
这是一个参数扩展，这意味着如果第三个参数为空或未设置，则将其替换为后面的内容：-
$ x=
$ echo ${x:-1}
1
$ echo $x
$
There's also another similar PE that assign the value if the variable is null:
如果变量为空，还有另一个类似的 PE 分配值：
$ x=
$ echo ${x:=1}
1
$ echo $x
1
