Setuid and setgid
Some programs, such as passwd, need to run as a specific user in order to work properly.
$ which -l passwd
-rwsrwxr-x 1 root root 21544 Mar 30 23:34 /usr/bin/passwd
Notice that the third character in the owner's permissions is s.
This indicates a setuid (set user ID
) command;
when you run passwd, the program runs as the owner of the file
(i.e., root).
An S means that the setuid bit is
set for the file, but the execute bit isn't set.
You might also find some setgid (set group ID
)
commands, which run with the same group ID as the owner
of the file, but not with the owner's user ID.
If setgid is set on a directory, files created in the directory
have the directory's group ID, not that of the file's creator.
This scheme is commonly used for spool areas, such as
/usr/spool/mail, which is setgid and owned by
the mail group, so that programs running as the mail
group can update things there, but the files still belong to their normal
owners.
When running on a Windows host,
mkefs
and
mkifs
can't get the execute (x), setuid (set user ID
), or setgid (set group ID
)
permissions from the file.
Use the perms attribute to specify these permissions explicitly.
You might also have to use the uid and gid
attributes to set the ownership correctly.
To determine whether or not a utility needs to have the setuid or setgid
permission set, see its entry in the Utilities Reference.