Test if a directory is writable by a given UID?

Question

We can test if a directory is writable by the uid of the current process:

if [ -w $directory ] ; then echo \'Eureka!\' ; fi

But can anyone

Answer 1

I've written a function can_user_write_to_file which will return 1 if the user passed to it either is the owner of the file/directory, or is member of a group which has write access to that file/directory. If not, the method returns 0.

## Method which returns 1 if the user can write to the file or
## directory.
##
## $1 :: user name
## $2 :: file
function can_user_write_to_file() {
  if [[ $# -lt 2 || ! -r $2 ]]; then
    echo 0
    return
  fi

  local user_id=$(id -u ${1} 2>/dev/null)
  local file_owner_id=$(stat -c "%u" $2)
  if [[ ${user_id} == ${file_owner_id} ]]; then
    echo 1
    return
  fi

  local file_access=$(stat -c "%a" $2)
  local file_group_access=${file_access:1:1}
  local file_group_name=$(stat -c "%G" $2)
  local user_group_list=$(groups $1 2>/dev/null)

  if [ ${file_group_access} -ge 6 ]; then
    for el in ${user_group_list-nop}; do
      if [[ "${el}" == ${file_group_name} ]]; then
        echo 1
        return
      fi
    done
  fi

  echo 0
}

To test it, I wrote a wee test function:

function test_can_user_write_to_file() {
  echo "The file is: $(ls -l $2)"
  echo "User is:" $(groups $1 2>/dev/null)
  echo "User" $1 "can write to" $2 ":" $(can_user_write_to_file $1 $2)
  echo ""
}

test_can_user_write_to_file root /etc/fstab
test_can_user_write_to_file invaliduser /etc/motd
test_can_user_write_to_file torstein /home/torstein/.xsession
test_can_user_write_to_file torstein /tmp/file-with-only-group-write-access

At least from these tests, the method works as intended considering file ownership and group write access :-)