bash (=) 符号后有空格的变量赋值？
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Assignment of variables with space after the (=) sign?
提问by kaneda
In Bash, assigning values to variables is done using T=content, with no spaces before or after the equal sign.
在 Bash 中，为变量赋值是使用 完成的T=content，等号前后没有空格。
Despite that I've seen the following in a shell script PWD= /bin/pwdcontaining a space on the right side of the equals sign.
尽管如此，我还是PWD= /bin/pwd在等号右侧包含一个空格的 shell 脚本中看到了以下内容。
What's the purpose of it have a space?
它有一个空间的目的是什么？
采纳答案by Tom Fenech
In the example PWD= /bin/pwd, the variable PWDis set to the empty string before executing the command /bin/pwd. The change only takes effect for that line.
在示例中PWD= /bin/pwd，变量PWD在执行命令之前被设置为空字符串/bin/pwd。更改仅对该行生效。
This can be useful to make a temporary change to a variable for the purposes of running a command, without affecting the original value. Another example of this would be when using read, to set a different IFS:
这对于出于运行命令的目的对变量进行临时更改非常有用，而不会影响原始值。另一个例子是使用read, 设置不同的IFS:
IFS=, read a b c <<<"comma,separated,list"
This sets the field separator to a comma so that a, band care read correctly. After this line, IFSreturns to the default value, so the rest of the script isn't affected.
这将字段分隔符设置为逗号，以便正确读取a,b和c。在此行之后，IFS返回到默认值，因此脚本的其余部分不受影响。
Perhaps on some systems, the output of the command pwdis affected by the value of the variable PWD, so doing this prevents problems caused by PWDbeing overwritten elsewhere.
也许在某些系统上，命令的输出pwd受变量值的影响PWD，因此这样做可以防止因PWD在其他地方被覆盖而导致的问题。
回答by glglgl
We are not talking about two different things here.
我们在这里不是在谈论两种不同的事情。
If we had
如果我们有
PWD=/bin/pwd
we would assign /bin/pwdto PWD.
我们将分配/bin/pwd给PWD.
But
但
PWD= /bin/pwd
means that we call /bin/pwdwith PWDset to the empty string. This assignment only affects the sub process, not the current one.
意味着我们调用/bin/pwdwithPWD设置为空字符串。此分配仅影响子进程，而不影响当前进程。
回答by tripleee
PWD= pwd
This syntax assigns the empty value to the variable PWDfor the duration of the pwdcommand.
此语法PWD在pwd命令的持续时间内将空值分配给变量。
PWD=ick
echo "$PWD"
This assigns PWDfor the remainder of the script.
这分配给PWD脚本的其余部分。
PWD=ick pwd
echo "$PWD"
This assigns PWDonly for the duration of the pwdcommand; the echowill echo the value which was in effect before and after the pwdinvocation.
这PWD仅在pwd命令期间分配；的echo将回声这在效果之前和之后的值pwd调用。
PWD=
This simply assigns the empty value to PWD.
这只是将空值分配给PWD。
Pathologically,
病理上，
PWD = ick
attempts to run the command PWDwith the arguments =and ick
尝试PWD使用参数运行命令=和ick
