Can somebody explain the -R option for userdel command?

Solution 1:

Some userdel versions have this option:

 -R, --root CHROOT_DIR
     Apply changes in the CHROOT_DIR directory and use the
     configuration files from the CHROOT_DIR directory.

In other words, instead of editing /etc/passwd and friends, you're editing CHROOT_DIR/etc/passwd.

For example, you might boot a live CD, mount the hard drive as /mnt, and then use -R /mnt to edit its users.

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