Integrating chepner's astute comment regarding the directories only really needing read-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+r .; cd ..; done)
chmod: changing permissions of `.': Operation not permitted
$ find /tmp/lh -ls
16 4 drwx---r-- 3 user group 4096 Oct 23 12:01 /tmp/lh
20 4 drwx---r-- 3 user group 4096 Oct 23 12:01 /tmp/lh/subdir1
21 4 drwx---r-- 3 user group 4096 Oct 23 12:01 /tmp/lh/subdir1/subdir2
22 4 drwx---r-- 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+r .
cd ..
done
)
