How to check linux directory permission for current user which doesn't exist

Question

I have an application where user enters some path which application uses to create directory if doesn't exist and add logging files. Now I want to check if current linux user has permission to create this directory path or not in case it doesn't exist. To give an example suppose /tmp/test is some directory which exists on machine and user enters /tmp/test/apps/log. How can i check write permission to this directory if it doesn't exist? I need to do it via shell command so that I can automate it. Thanks

Answer 1

You need to check if any of the ancestor directories are writable, and if anything would be blocking the creation. There are quite a bit of corner cases, so I might have forgotten some.

#!/bin/bash

# Syntax: can_mkdir_p [-f] <path>
#     -f   Succeed if the directory already exists and is writable
function can_mkdir_p() {
  if [ "$1" == -f ]
  then
    exist_result=0
    shift
  else
    exist_result=66
  fi

  dir="$1"
  if [ -f "$dir" ]
  then
    # the requested path is an existing file, so mkdir is blocked.
    return 65
  elif [ -d "$dir" ]
  then
    # the requested path is an existing directory.
    if [ -w "$dir" ]
    then
      # it is writable; if -f is supplied, it is okay
      # otherwise, report that mkdir is blocked
      return "$exist_result"
    else
      # the directory is not writable; report a permission problem
      return 64
    fi
  fi

  # if the path does not exist, check ancestor directories
  while [ "$dir" != / ]
  do
    parent=$(dirname "$dir")
    if [ -w "$parent" ]
    then
      # the parent is writable, so maybe we can mkdir -p
      if [ -f "$dir" ]
      then
        # nope, a file already exists here that would block mkdir -p
        return 65
      elif [ -d "$dir" ]
      then
        # nope, a directory already exists here;
        # if it was writeable, we would have reported okay
        # in the previous iteration
        return 64
      else
        # we found a writable ancestor that is not blocked, report okay
        return 0
      fi
    fi
    # this level is not okay, try one level up
    dir="$parent"
  done
  # reached the root, still not okay
  return 64
}

# example usage
if can_mkdir_p "$@"
then
  echo Okay
else
  case "$?" in
    64)
      echo "No write permission"
      ;;
    65)
      echo "Something is in the way"
      ;;
    66)
      echo "Already exists"
  esac
fi

But as I said in the comments, you can just try to make a directory and if it fails, complain to the user. This is what most software does.