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I have a VMWare VM that directly accesses some disks. In order to run (without running vmware as su which has its own problems) I need to change the ownership of the devices. I've done this manually using Nemo, so I know what needs to be performed.

Writing a shell script to do this, I wrote

chown john:john /dev/disk/by-id/blahblah  /dev/anotherblah  /dev/lastoneblah

I saved it as a file drive-owner, set the execute bit, then in a root shell changed its owner to root and set the +s special mode flag.

The result, according to ls, is -rwsr-sr-x.

If I run that from a normal (non-root) shell, I get errors from chown, Operation not permitted. It runs without complaint from a root shell.

I seem to be missing the part that runs the script as root automatically. I thought that was the purpose and function of the "setuid" permission, and +s is how to specify it.

What am I missing?

3 Answers 3

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Setuid and setgid flags are ignored for shell scripts, for security reasons.

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  • Undoubtedly correct in this case, but not universally true.
    – hildred
    Commented Jul 22, 2015 at 5:48
  • Well that would explain it. So how do I accomplish this? This is a personal computer, so security isn't as much of an issue.
    – JDługosz
    Commented Jul 22, 2015 at 6:41
  • @hildred Yes, that has only been true for most systems released in the past ~20 years. Commented Jul 22, 2015 at 7:12
  • @JDługosz Use sudo instead. Commented Jul 22, 2015 at 7:12
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    @JDługosz You can configure sudo not to ask for a password for chown when run by you. Commented Jul 22, 2015 at 10:39
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Following these steps:

  • Try sudo
  • Try chmod +x script_path
  • If security does not matter, run chmod 777 script_path
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  • You mean the directory that contains the script file? I had left the file in a convenient place related to its purpose, but I suppose it should go in a place where scripts are kept, like a utilities directory. What's the right place for it, esp. If I want to run it automatically when the machine is rebooted?
    – JDługosz
    Commented Jul 22, 2015 at 10:43
  • Hi Sorry for late reply, for the script to execute at the startup, you would have to put your script in /etc/init.d and create a symlink to /etc/rc.d . And of couse you would have to change the file pemission to executable. #chmod +x /etc/init.d/Script_name #ln -s /etc/init.d/script_name /etc/rc.d/ Commented Jul 30, 2015 at 6:59
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@hildred noted this in a comment above. I'll expand it into an Answer as I think it's the correct and highly recommended answer.

The super command

… allows specified users to execute scripts (or other commands) as if they were root; or it can set the uid, gid, and/or supplementary groups on a per-command basis before executing the command. It is intended to be a secure alternative to making scripts setuid root. Super also allows ordinary users to supply commands for execution by others; these execute with the uid, gid, and groups of the user offering the command.

(Emphasis mine).

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