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If I have a file that I want to make world-readable, but it is deep in several layers of directories that are not world-executable, I have to change the permissions for the whole path and the file.

I can do chmod 755 -R /first/inaccessible/parent/dir but that changes permissions for all the other files in the path directories and makes the file itself executable when I just want it to be readable.

Is there a straightforward way to do this in bash?

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  • 3
    However you do this, consider the security implications. (I am not saying this is bad practice, just always consider the implications, each time) Commented Oct 23, 2017 at 11:22
  • 1
    A directory doesn't have to be executable to read a file in the directory, just (among other things) list all the files in the directory. As long as a potential user knows the full path to the file, they only need read permissions for the intervening directories. Commented Oct 23, 2017 at 15:46
  • @chepner internal tools at my workplace that work on a file (given by full path) fail to read files without executable directories - maybe they are doing something silly and this is specific to my situation Commented Oct 23, 2017 at 16:07
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    @chepner, isn't it the other way round? access(2) says "A file is only accessible if the permissions on each of the directories in the path prefix of pathname grant search (i.e., execute) access." Commented Oct 23, 2017 at 16:40
  • @deltab You're right, I misread the Wikipedia summary. Commented Oct 23, 2017 at 16:58

4 Answers 4

Reset to default
7

One way to do it:

#! /bin/sh
fname=/full/path/to/file
dir=${fname%/*}
while [ x"$dir" != x ]; do
    chmod 0755 "$dir"
    dir=${dir%/*}
done
chmod 0644 "$fname"
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1
  • 1
    dirname is there for exactly this... Commented Oct 23, 2017 at 14:12
1

Integrating chepner's astute comment regarding the directories only really needing execute-permission:

Setup:

$ mkdir -p /tmp/lh/subdir1/subdir2/subdir3
$ touch /tmp/lh/subdir1/subdir2/subdir3/filehere
$ chmod -R 700 /tmp/lh
$ find /tmp/lh -ls
16    4 drwx------   3 user  group        4096 Oct 23 12:01 /tmp/lh
20    4 drwx------   3 user  group        4096 Oct 23 12:01 /tmp/lh/subdir1
21    4 drwx------   3 user  group        4096 Oct 23 12:01 /tmp/lh/subdir1/subdir2
22    4 drwx------   2 user  group        4096 Oct 23 12:01 /tmp/lh/subdir1/subdir2/subdir3
23    0 -rwx------   1 user  group           0 Oct 23 12:01 /tmp/lh/subdir1/subdir2/subdir3/filehere

Prep:

$ f=/tmp/lh/subdir1/subdir2/subdir3/filehere

Do it:

$ chmod o+r "$f"
$ (cd "$(dirname "$f")" && while [ "$PWD" != "/" ]; do chmod o+x .; cd ..; done)
chmod: changing permissions of `.': Operation not permitted
$ find /tmp/lh -ls
16    4 drwx-----x   3 user  group        4096 Oct 23 12:01 /tmp/lh
20    4 drwx-----x   3 user  group        4096 Oct 23 12:01 /tmp/lh/subdir1
21    4 drwx-----x   3 user  group        4096 Oct 23 12:01 /tmp/lh/subdir1/subdir2
22    4 drwx-----x   2 user  group        4096 Oct 23 12:01 /tmp/lh/subdir1/subdir2/subdir3
23    0 -rwx---r--   1 user  group           0 Oct 23 12:01 /tmp/lh/subdir1/subdir2/subdir3/filehere

If you really prefer the intermediate directories to also have other-execute permissions, just change the chmod command to chmod o+rx.

The error message I got from the above results from my non-root userid attempting to change the permissions of the /tmp directory, which I don't own.

The loop runs in a subshell to isolate the changing of directories from your current shell's $PWD. It starts the loop by entering the directory containing the file then loops upwards, chmod'ing along the way, until it lands in the root / directory. The loop exits when it reaches the root directory -- it does not attempt to chmod the root directory.

You could make a script-file or function out of it like so:

function makeitreadable() (
  chmod o+r "$1"
  cd "$(dirname "$1")" &&
    while [ "$PWD" != "/" ]
    do
      chmod o+x .
      cd ..
    done
)
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0

This will on change the sub-folders to 0755

find /first/inaccessible/parent/dir -type d -exec chmod 0755 {} \;

This will search for your file and change its permissions

chmod +r $(find /first/inaccessible/parent/dir -type f -name YourFileName)

Hope that helps. (FYI: by default folders should have 0755)

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-1

Expose the file at a less protected path

You should be able to make a symbolic link to the file, from a different path in your file system -- that should make the original file accessible, with whatever permissions it has, without exposing all the other directories and files surrounding the directory your file is in.

Of course, you need to have/make available a different path that is readable by your target user/system...

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  • 3
    Symlinks do not circumvent permissions. If the file really resides in /unaccessible/file, then ln -s /unaccessible/file /public/file can be done by anyone, but it cannot be accessed nevertheless. Commented Oct 23, 2017 at 12:22
  • I understood that the file itself was (or could be made) accessible, but located in an inaccessible directory. Commented Oct 24, 2017 at 12:44
  • If the directory doesn't have +x, a file inside it cannot be accessed even with symlink. And if all directories in path do have +x, then it can be accessed even without symlink as long as full path is given. Commented Oct 24, 2017 at 19:47

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