重击:在同一行上分配变量不会改变回声行为 [英] Bash: assignment of variable on same line not altering echo behavior

问题描述

a=2
a=3 echo $a     #prints 2

有人可以解释为什么有人在第二行中使用上面的代码吗?a = 3将被忽略，因为其后没有"enter".但是我在上面的脚本中看到了它，并且不确定目的.

can someone explain why would anyone use the above code in line-2. a=3 will be ignored as there is no "enter" after it. But I saw it in script like above and not sure about the purpose.

推荐答案

$ a 在评估 a = 3 之前由外壳程序(Bash)扩展.因此， echo 会将其参数显示为 2 ，这就是它输出的内容.(如果您 set -x ，您会看到执行的是 a = 3 echo 2 .)

$a is expanded by the shell (Bash) before a=3 is evaluated. So echo sees its argument as 2, which is what it prints. (If you set -x you can see that what gets executed is a=3 echo 2.)

var = val命令用于设置环境变量，以便在执行过程中 command 可以看到，但无其他地方.因此，当 command 读取环境变量时(例如，使用 getenv() )， $ var 就是 val .

var=val command is used to set an environment variable to be seen by command during its execution, but nowhere else. So when command reads environment variables (e.g. using getenv()), to it $var is val.

如果 echo 在运行时查找 $ a ，它将具有值 3 .

If echo were to look up $a while running, it would have the value 3.

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