How to check if a user can access a given file?

Question

*nix user permissions are really simple, but things can get messy when you have to take in account all the parent directory access before reaching a given file. How can I check if the user has enough privileges? If not, then which directory is denying access?

For example, suppose a user joe, and the file /long/path/to/file.txt. Even if file.txt was chmoded to 777, joe still has to be able to access /long/, and then /long/path/ and then /long/path/to/ before. What I need is a way to automatically check this. If joe does not have access, I would also like to know where he has been denied. Maybe he can access /long/, but not /long/path/.

Answer 1

To verify access visually, you can use

namei -m /path/to/really/long/directory/with/file/in

which will output all of the permissions in the path in a vertical list.

or

namei -l /path/to/really/long/directory/with/file/in

to list all owners and the permissions. Other answers explain how to verify this programmatically.

Answer 2

You can use bash to do this.

$ cat check-permissions.sh
#!/bin/bash
file=$1
# Handle non-absolute paths
if ! [[ "$file" == /* ]] ; then
    path=.
fi
dirname "$file" | tr '/' $'\n' | while read part ; do
    path="$path/$part"
    # Check for execute permissions
    if ! [[ -x "$path" ]] ; then
        echo "'$path' is blocking access."
    fi
done
if ! [[ -r "$file" ]] ; then
    echo "'$file' is not readable."
fi
$ ./check-permissions.sh /long/path/to/file.txt

To check this for a specific user, you can use sudo.

sudo -u joe ./check-permissions.sh /long/path/to/file.txt

Answer 3

If you have root access, impersonate the user, then run test -r (read), test -w (write), or test -x (execute) to check whether the user can read/write/execute the given file.

sudo -u otheruser test -w /file/to/test || {
   echo "otheruser cannot write the file"
}

Answer 4

As I got from your question, you should check it for different users (not only joe), so in that case the easiest way is to recursivly check it via sudo like this:

FILE=$1 ; T_USER=$2 ;
if sudo -u $T_USER [ -r "$FILE" ] ; then
    echo "original file $1 is readable for $T_USER"
else
    while sudo -u $T_USER [ ! -x "$FILE" ] ; do FILE=$(dirname "$FILE") ; done
    echo "only $FILE is readable for $T_USER"
fi

usage:

./script.sh /long/path/to/file.txt joe

Answer 5

Here's my attempt at providing this functionality. I've opted to use stat, a while loop, and dirname.

I've created this script, walkdir.bash:

#/bin/bash

cwd="$1"
while [ "x$cwd" != x/ ]; do
  info=`stat "$cwd" |grep "Access: ("`
  printf "%s : %s\n" "$info" "$cwd"

  cwd=`dirname "$cwd"`;
done

You run it like so:

$ walkdir.bash "/home/saml/blog/vmware_networking_tutorial/url.txt"
Access: (0664/-rw-rw-r--)  Uid: (  500/    saml)   Gid: (  501/    saml) : /home/saml/blog/vmware_networking_tutorial/url.txt
Access: (0775/drwxrwxr-x)  Uid: (  500/    saml)   Gid: (  501/    saml) : /home/saml/blog/vmware_networking_tutorial
Access: (0775/drwxrwxr-x)  Uid: (  500/    saml)   Gid: (  501/    saml) : /home/saml/blog
Access: (0700/drwx------)  Uid: (  500/    saml)   Gid: (  501/    saml) : /home/saml
Access: (0755/drwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root) : /home