awk command to find average of a column from a file - php

Question

I have written an awk command to read from a log file and get the average of a specific column.
The command is :
awk '{sum += $NF} END {print sum/NR}' /serach_grater500ms_test.txt
Everything is working fine on terminal but when I try to execute this command in php script as follow :
$no_of_request=exec("awk '{sum += $NF} END {print sum/NR}' /serach_grater500ms_test.txt");
print_r($no_of_request);
It throw me a syntax error
PHP Notice: Undefined variable: NF in /home/javed/Downloads/read_log.php on line 11
awk: line 1: syntax error at or near }
How can I solve it ?

Answer 1

If you want to pass a string into a shell script, you need to quote it properly for the shell. The easiest way to do this is to make it a single-quoted literal; this requires transforming the single quotes inside the string.
exec('awk \'{sum += $NF} END {print sum/NR}\' /serach_grater500ms_test.txt');

Answer 2

In your example the php string is between double-quote ". The character ' inside has no special meaning, you dont need to escape it. On the other hand the $ (like in sum += $NF) is interpreted, so the php replaces $NF with its value (empty!).
With Inian's answer the php string is between single-quote '. So the php does not look for variable inside ($NF is protected); but you have to escape further \' inside.
You can also have it work with the double-quote of your example; but you escape only the \$ of $NF :
$no_of_request=exec("awk '{sum += \$NF} END {print sum/NR}' filee");