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Sudo and Globbing

The question is how we can use the sudo utility to display a list of files in a directory to which we have absolutely no Unix filesystem privileges

Consider the following directory and files contained therein:

$ ls -l
total 4
drwxrwx---. 2 root root 4096 May 22 21:14 demo
$ su
Password: XXXXXXXX
# ls -l demo
total 0
-rw-r--r--. 1 root root 0 May 22 21:14 file1
-rw-r--r--. 1 root root 0 May 22 21:14 file2
-rw-r--r--. 1 root root 0 May 22 21:14 file3
# exit
exit


Note the directory permissions are 770 and the user and group owner is root.

Here is the output when I attempt to list the subdirectory contents:

$ whoami
fpm
$ ls -l demo
ls: cannot open directory demo: Permission denied
$


This is what I expect because I am not root nor a member of the root group. To enable me to view the directory contents, the directory permissions need to be at least 771 (drwxrwx–x) for a non-root user such as myself to list the directory.

So how can I work around this limitation? One workaround is by using sudo. Assuming I have root privledges via sudo either explicitly or as a member of the wheel group, here is what happens when I naively use sudo to attempt to list the subdirectory contents:

$ sudo ls -l demo/file1
-rw-r--r--. 1 root root 0 May 22 21:14 demo/file1
$ sudo ls -l demo/*
ls: cannot access demo/*: No such file or directory
$


I still cannot list the files in the demo subdirectory.

The above does not work because the shell attempts to glob demo/* before passing the result of globing to sudo which then passes it unchanged to ls. Since the shell is not running as root, it is not able to traverse the directory and globbing fails. Thus, ls receives the unexpanded globbing pattern demo/* in its argument list.

Note that ls and other Linux utilities do not perform globbing. The utilities expect a list of filenames as command line arguments.

The most straight forward way to get correct results is to invoke ls -l demo/* in a sub-shell. Quote the command string using either double quotes (“) or single quotes (‘).

$ sudo sh -c "ls -l demo/*"
-rw-r--r--. 1 root root 0 May 22 21:14 demo/file1
-rw-r--r--. 1 root root 0 May 22 21:14 demo/file2
-rw-r--r--. 1 root root 0 May 22 21:14 demo/file3
$ sudo sh -c 'ls -l demo/*'
-rw-r--r--. 1 root root 0 May 22 21:14 demo/file1
-rw-r--r--. 1 root root 0 May 22 21:14 demo/file2
-rw-r--r--. 1 root root 0 May 22 21:14 demo/file3
$ sudo -s ksh -c "ls -l demo/*"
-rw-r--r--. 1 root root 0 May 22 21:14 demo/file1
-rw-r--r--. 1 root root 0 May 22 21:14 demo/file2
-rw-r--r--. 1 root root 0 May 22 21:14 demo/file3
$


As you can see, there is no difference in output.

Note that there may be a difference if you use shell variables. Here is an example where there is a difference.

$ sudo sh -c 'echo $USER'
root
$ sudo sh -c "echo $USER"
fpm


There are other workarounds to this issue besides sudo. For example, I could be added to the root group but that has security implications. Another group of which I am a member could be given group ownership of the subdirectory. POSIX ACLs could be used to grant browsing and other permissions to the subdirectory.

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