Avoid backticks in shell scripts

- $() should be preferred : http://mywiki.wooledge.org/BashFAQ/082
- it makes it easier to discuss these lines on Slack (or any
markdown-compatible thing).

Author: Grégoire Paris

setup/file_permissions.rst

@@ -34,9 +34,9 @@ needed permissions:
     $ rm -rf app/cache/*
     $ rm -rf app/logs/*
 
-    $ HTTPDUSER=`ps axo user,comm | grep -E '[a]pache|[h]ttpd|[_]www|[w]ww-data|[n]ginx' | grep -v root | head -1 | cut -d\  -f1`
+    $ HTTPDUSER=$(ps axo user,comm | grep -E '[a]pache|[h]ttpd|[_]www|[w]ww-data|[n]ginx' | grep -v root | head -1 | cut -d\  -f1)
     $ sudo chmod +a "$HTTPDUSER allow delete,write,append,file_inherit,directory_inherit" app/cache app/logs
-    $ sudo chmod +a "`whoami` allow delete,write,append,file_inherit,directory_inherit" app/cache app/logs
+    $ sudo chmod +a "$(whoami) allow delete,write,append,file_inherit,directory_inherit" app/cache app/logs
 
 3. Using ACL on a System that Supports ``setfacl`` (Linux/BSD)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -48,10 +48,10 @@ following script to determine your web server user and grant the needed permissi
 
 .. code-block:: terminal
 
-    $ HTTPDUSER=`ps axo user,comm | grep -E '[a]pache|[h]ttpd|[_]www|[w]ww-data|[n]ginx' | grep -v root | head -1 | cut -d\  -f1`
+    $ HTTPDUSER=$(ps axo user,comm | grep -E '[a]pache|[h]ttpd|[_]www|[w]ww-data|[n]ginx' | grep -v root | head -1 | cut -d\  -f1)
     # if this doesn't work, try adding `-n` option
-    $ sudo setfacl -R -m u:"$HTTPDUSER":rwX -m u:`whoami`:rwX app/cache app/logs
-    $ sudo setfacl -dR -m u:"$HTTPDUSER":rwX -m u:`whoami`:rwX app/cache app/logs
+    $ sudo setfacl -R -m u:"$HTTPDUSER":rwX -m u:$(whoami):rwX app/cache app/logs
+    $ sudo setfacl -dR -m u:"$HTTPDUSER":rwX -m u:$(whoami):rwX app/cache app/logs
 
 .. note::