SUID doesn't work in Bash

Question

I wrote a simple bash script that makes a backup of the home directory, e puts it into /var/backups. Since that directory is protected, I wrote the script as root, and then set the SUID.

armando@ubuntu:~/scripts/bash $ ll
-rwsr-xr-x 1 root    root    2596 Jul 28 10:43 homebackup.sh*

Even so, I get the "Permission Denied" error when the scripts tries to write into /var/backups. Why?

Answer 1

In Linux and most other modern UNIX-family systems, setuid bits are only recognized for direct binary executables, not scripts.

This is by design, and for security reasons. You can work around it by building a compiled wrapper for your setuid scripts, or using an existing tool (such as sudo with a configuration to avoid needing a password when calling the specific script as the desired user).

See this comprehensive discussion on UNIX StackExchange.