[英]why single quotes and double quotes works differently in “bash -c”?
While using bash -c
option to spawn new shell before running shell script,在运行 shell 脚本之前使用
bash -c
选项生成新的 shell 时,
I encountered unexpected action.我遇到了意想不到的动作。
When I use single quotes
, new shell is spawned to run the script.当我使用
single quotes
时,会生成新的 shell 来运行脚本。
$ bash -c 'echo pid is $$'
pid is 53465
$ bash -c 'echo pid is $$'
pid is 53466
$ bash -c 'echo pid is $$'
pid is 53477
But double quotes
didn't.但
double quotes
没有。
$ bash -c "echo pid is $$"
pid is 2426
$ bash -c "echo pid is $$"
pid is 2426
$ bash -c "echo pid is $$"
pid is 2426
I carefully read similar question and bash manual but could not find why.我仔细阅读了类似的问题和bash 手册,但找不到原因。
Anyone knows the reason why?有人知道原因吗?
So when you execute the command所以当你执行命令时
$ command "echo pid is $$"
The double quotes ensure that the command command
gets a string passed where all substitutions are done.双引号确保命令
command
在所有替换都完成的地方得到一个传递的字符串。 So assuming that the pid of the interactive shell is 1234. You will get the equivalent所以假设交互式 shell 的 pid 是 1234。你会得到等价的
$ command "echo pid is 1234"
When you use single quotes, the command gets passed the string echo pid is $$
where $$
is just like any string.当您使用单引号时,命令会传递字符串
echo pid is $$
其中$$
就像任何字符串一样。 If the command is now bash
, $$ has a special meaning如果命令现在是
bash
, $$ 有特殊含义
$ bash -c 'echo pid is $$'
So now you get the PID returned of the executed command bash
and not of your interactive shell.所以现在你得到了执行命令
bash
而不是你的交互式 shell 返回的 PID。
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