bash 初始化变量的不同方式
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Different ways to initialize a variable
提问by Paul Manta
As far as I've seen there are two ways to initialize a variable with the output of a process. Is there any difference between these two?
据我所知，有两种方法可以用进程的输出初始化变量。这两者有什么区别吗？
ex1=`echo 'hello world'`
ex2=$(echo 'hello world')
回答by Micha? ?rajer
You get same effect.
你得到同样的效果。
The $()is recommended since it's more readable and makes it easier to nest one $()into another $().
该$()建议，因为它是更具可读性和更容易窝一个$()到另一个$()。
Update:
更新：
The $()syntax is a POSIX 1003.1 standard (2004 edition). However, on some older UNIX systems (SunOS, HP-UX, etc.) the /bin/shdoes not understand it.
该$()语法是POSIX 1003.1标准（2004年版）。但是，在一些较旧的 UNIX 系统（SunOS、HP-UX 等）上，/bin/sh它不理解。
You might need to use backtick "`" instead or use another shell (usually it's ksh) if you need your script to work on such environment.
如果您需要您的脚本在这样的环境中工作，您可能需要使用反引号 "`" 或使用另一个 shell（通常是 ksh）。
If you don't know which syntax to use - use $(). Backtick syntax is deprecated.
如果您不知道使用哪种语法 - 使用$(). 不推荐使用反引号语法。
回答by Samus_
see http://mywiki.wooledge.org/BashFAQ/082
见http://mywiki.wooledge.org/BashFAQ/082
also notice that $()is POSIX so it does work on sh.
还请注意，这$()是 POSIX，因此它确实适用于 sh。
回答by jaypal singh
There is another way to initialize a variable to a default one if you haven't initialized it yourself.
如果您自己没有初始化变量，还有另一种方法可以将变量初始化为默认变量。
[jaypal:~/Temp] a="I have initialized var a"
[jaypal:~/Temp] echo ${a:="Default value"}
I have initialized var a
[jaypal:~/Temp] a=
[jaypal:~/Temp] echo ${a:="Default value"}
Default value
