php - awk 单引号或双引号的用法

Question

为什么 awk 命令使用单引号和双引号产生的结果不同？

root@vm90:/root# who | awk "{ print $2 }"
...
root@vm90:/root# who | awk '{ print $2 }'
...

我想在 PHP shell_exec() 函数中使用 awk，它在整个代码中使用单引号。

这...

$output = shell_exec("who | awk '{ print $2 }'");

...有效，但我更喜欢...

$output = shell_exec('who | awk "{ print $2 }"');

...这个，这是行不通的。

Answer 1

来自GNU Awk manual (强调):

Because certain characters within double-quoted text are processed by the shell, they must be escaped within the text. Of note are the characters $, `, \, and ", all of which must be preceded by a backslash within double-quoted text if they are to be passed on literally to the program. (The leading backslash is stripped first.)