awk单引号或双引号用法

Question

Why does the awk command not produce the same results with simple-quotes and double-quotes?

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

I'd like to use awk in a PHP shell_exec() function, which uses simple-quotes in the entire code.

This ...

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

... works, but I prefer ...

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

... this, which doesn't work.

Answer 1

From the GNU Awk manual (emphasis added):

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.)