umount Details: Non-Superuser, Loop Devices, External Helpers, and More

--version Print version and exit. NON-SUPERUSER UMOUNTS Normally, only the superuser can umount filesystems. However, when fstab contains the user option on a line, anybody can umount the corresponding filesystem. For more details see mount(8) man page. Since version 2.34 the umount command can be used to perform umount operation also for fuse filesystems if kernel mount table contains user’s ID. In this case fstab user= mount option is not required. Since version 2.35 umount command does not exit when user permissions are inadequate by internal libmount security rules. It drops suid permissions and continue as regular non-root user. This can be used to support use-cases where root permissions are not necessary (e.g., fuse filesystems, user namespaces, etc). LOOP DEVICE The umount command will automatically detach loop device previously initialized by mount(8) command independently of /etc/mtab. In this case the device is initialized with "autoclear" flag (see losetup(8) output for more details), otherwise it’s necessary to use the option --detach-loop or call losetup -d device. The autoclear feature is supported since Linux 2.6.25. EXTERNAL HELPERS The syntax of external unmount helpers is: umount.suffix {directory|device} [-flnrv] [-N namespace] [-t type.subtype] where suffix is the filesystem type (or the value from a uhelper= or helper= marker in the mtab file). The -t option can be used for filesystems that have subtype support. For example: umount.fuse -t fuse.sshfs A uhelper=something marker (unprivileged helper) can appear in the /etc/mtab file when ordinary users need to be able to unmount a mountpoint that is not defined in /etc/fstab (for example for a device that was mounted by udisks(1)). A helper=type marker in the mtab file will redirect all unmount requests to the /sbin/umount.type helper independently of UID. Note that /etc/mtab is currently deprecated and helper= and other userspace mount options are maintained by libmount. ENVIRONMENT LIBMOUNT_FSTAB=<path> overrides the default location of the fstab file (ignored for suid) LIBMOUNT_DEBUG=all enables libmount debug output FILES /etc/mtab table of mounted filesystems (deprecated and usually replaced by symlink to /proc/mounts) /etc/fstab table of known filesystems /proc/self/mountinfo table of mounted filesystems generated by kernel. HISTORY A umount command appeared in Version 6 AT&T UNIX. SEE ALSO umount(2), losetup(8), mount_namespaces(7), mount(8) REPORTING BUGS For bug reports, use the issue tracker at https://github.com/util-linux/util-linux/issues. AVAILABILITY The umount command is part of the util-linux package which can be downloaded from Linux Kernel Archive <https://www.kernel.org/pub/linux/utils/util-linux/>. util-linux 2.39.1 2023‐06‐14 UMOUNT(8)

This section details non-superuser unmount capabilities, the automatic detachment of loop devices initialized by mount, and the use of external unmount helpers. It describes how users can unmount filesystems with the `user` option in `/etc/fstab`, how `umount` handles fuse filesystems, and how it drops suid permissions when unnecessary. It also explains the syntax and purpose of external unmount helpers, including the use of `uhelper=` and `helper=` markers in the deprecated `/etc/mtab` file. Furthermore, it mentions environment variables like `LIBMOUNT_FSTAB` and `LIBMOUNT_DEBUG`, lists relevant files like `/etc/mtab`, `/etc/fstab`, and `/proc/self/mountinfo`, provides historical context, cross-references related documentation (umount(2), losetup(8), mount_namespaces(7), mount(8)), directs users to the bug reporting tracker, and specifies the availability of the `umount` command as part of the util-linux package.