105

*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/.

slm
  • 369,824
Metalcoder
  • 1,195

5 Answers5

107

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.

Kevin Burke
  • 1,971
exussum
  • 4,023
  • 2
    This should be the right answer. In fact using namei <path> || exit 1 allows you to detect a permission problem much easily in a script. – lorenzog Jun 02 '16 at 10:01
  • 25
    it doesn't answer directly whether joe has access to the file. – jfs Nov 06 '17 at 20:03
  • This also does not elucidate how access control lists might influence things. – Andrew M Sep 15 '21 at 00:41
23

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"
}
Jesse Nickles
  • 185
Kevin Burke
  • 1,971
  • 3
    This is exact answer what I was searching for. Title of this question could be misleading. – Mojtaba Rezaeian Apr 29 '20 at 05:01
  • 3
    a slight embellishment to check if a process running as "USER:GROUP" can access a given file would be the following command: sudo su - USER -g GROUP -s /bin/bash -c "test -r /path/to/file" – MNB Jan 23 '21 at 14:53
21

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
  • sudo -u joe script . Here script is the name of the script file right? so your telling sudo to act like joe was calling the script? – tgkprog Jul 09 '13 at 14:24
  • Precisely. I have modified my answer to clarify that. –  Jul 09 '13 at 14:31
  • I made a slight modification to my script to handle non-absolute paths. –  Jul 09 '13 at 14:47
  • @EvanTeitelman With an absolute path, did you mean to initialize path to be empty? or /? – Gilles 'SO- stop being evil' Jul 09 '13 at 20:31
  • @Gilles: I meant for it to be empty. In the example, path is set to /long the first time around the loop, which is correct. Should I set path to nothing explicitly (path=)? Also, thanks for simplifying out my use of tr. –  Jul 09 '13 at 20:58
  • @EvanTeitelman I tried to test it using the following file: /root/test/test.txt (permissions are 0755, 0700, and 0777). I issued sudo -u joe ./check-permissions.sh /root/test/test.txt, and nothing happened. Did I missed something? – Metalcoder Jul 10 '13 at 17:45
  • @Metalcoder: An edit last night broke it. I've reverted the changes. –  Jul 10 '13 at 18:48
  • btw, if file had permissions like 000, the script wouldn't tell that it is not readable. – rush Jul 10 '13 at 19:32
  • @rush: I have added that functionality to the script. –  Jul 10 '13 at 19:40
  • I wonder whether it is possible to do something like that without being able to log in as joe. It should be possible since groups joe returns all groups the user belongs to. – Ben Usman Apr 30 '18 at 18:15
  • UPVOTE/ACCEPT, my target system doesn't have namei. Also if you check, you usually want to react. I can insert setfacl/chmod and add X to user, while checking access in one step – papo Nov 15 '18 at 04:59
5

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
rush
  • 27,403
  • Joe needs execute permissions on the directories, not read permissions. –  Jul 09 '13 at 14:20
  • @EvanTeitelman yes, you're right. Fixed. – rush Jul 09 '13 at 14:29
  • @rush I tried to test it using the following file: /root/test/test.txt (permissions are 0755, 0700, and 0777). I issued ./script.sh /root/test/test.txt joe and it echoed original file /root/test/test.txt is readable for joe. Also, while trying this I misstyped the test dir: ./script.sh /root/tst/test.txt joe, and it echoed original file /root/tst/test.txt is readable for joe. Did I missed something? – Metalcoder Jul 10 '13 at 17:46
  • @Metalcoder sorry, it's my fault. There was one extra exclamation. It's removed now, you can try it one more time, it should work fine now. – rush Jul 10 '13 at 17:50
  • @rush it worked! That extra exclamation negates the result of -r $FILE, right? – Metalcoder Jul 10 '13 at 18:10
  • @Metalcoder, yes, that's right. – rush Jul 10 '13 at 18:31
2

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
slm
  • 369,824