Question

PHP has a is_readable function which checks to see if the file is readable by the owner of the script. Is there a corresponding script to see if a file is readable by a specified user, for example

is_readable('Gavrilo Princip', 'black_hand.srj')

Answer 1

Thanks to the help of Chris and AndrewR I have come up with a, as of yet untested, solution. This solution is implemented in shell, and waits for input from standard in (designed to work with Apache RewriteMap). However, it can easily be modified to be called from either the command line or from a PHP script. It is a little bit more complicated than it has to be because we are piping the input of a function (getfacl) to a while loop. When we do this, it starts a new suprocess, so any variables declared or updated inside this loop (ie. result) will not be available to the outside world. Furthermore, I used getfacl as I can later expand it to also work with ACL permissions as well. Finally, for implementation reasons, I already know the owner of the file (user) before calling this script, however, if this is not the case, one can easily find this from the getfacl command.

#!/bin/bash

#USAGE: STDIN viewer:user:file

while read line
do
   viewer=`echo $4 | cut -d ':' -f 1`
   user=`echo $4 | cut -d ':' -f 2`
   file=`echo $4 | cut -d ':' -f 3`

   result=$(
      getfacl $file 2>/dev/null | while read line
      do
         if [[ $user == $viewer ]] && [[ $line =~ ^user: ]]
         then
            permissions=`echo $line | cut -d ':' -f 3`
            if [[ $permissions =~ r ]]
            then
               echo true
               break
            fi
         elif [[ $user == $viewer ]] && [ $line =~ ^group: ]]
         then
            #NOTE: I take advantage of the fact that each user has one single group and that group has the same name as the user's name
            permissions=`echo $line | cut -d ':' -f 3`
            if [[ $permissions =~ r ]]
            then
               echo true
               break
            fi
         elif [[ $line =~ ^other: ]]
         then
            permissions=`echo $line | cut -d ':' -f 3`
            if [[ $permissions =~ r ]]
            then
               echo true
               break
            fi
         fi
      done
   )

   if [[ $result == "true" ]]
   then
      echo true
   else
      echo false
   fi
done