Add section about findmnt, update information about mount info under …

…/proc

modules/storage_mounting/030_mounting_theory.md

@@ -73,24 +73,95 @@ student@linux:~$ mount | grep /dev/sdb
 /dev/sdb1 on /mnt/project42 type ext2 (rw)
 ```
 
-### /proc/mounts
+The output of `mount` used to be simple enough, but these days, with all the pseudo filesystems under `/sys`, `/run` and the like, you have to pipe it through `grep` to find useful information through all the noise.
 
-The kernel provides the info in `/proc/mounts` in file form, but `/proc/mounts` does not exist as a file on any hard disk. Looking at `/proc/mounts` is looking at information that comes directly from the kernel.
+However, as it turns out, using `mount` to *list* mounted filesystems is *deprecated*. From the man page `mount(8)`:
 
-```console
-student@linux:~$ cat /proc/mounts | grep /dev/sdb
-    /dev/sdb1 /mnt/project42 ext2 rw 0 0
+```text
+   Listing the mounts
+       The listing mode is maintained for backward compatibility only.
+
+       For more robust and customizable output use findmnt(8), especially in your scripts.
 ```
 
-### /etc/mtab
+### findmnt
+
+The `findmnt` command is way more useful to show mounted filesystems. Even the default output looks better structured than `mount` and there are a lot of options and arguments to customize the output. Try the default output yourself, as it's quite verbose.
+
+A few examples of useful options:
+
+- `--real`: Only show "real" filesystems (i.e. not pseudo filesystems):
+
+    ```console
+    [vagrant@el ~]$ findmnt --real
+    TARGET       SOURCE    FSTYPE  OPTIONS
+    /            /dev/sda2 xfs     rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota
+    ├─/mnt/part3 /dev/sdc5 xfs     rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota
+    ├─/mnt/part4 /dev/sdc6 fuseblk rw,relatime,user_id=0,group_id=0,allow_other,blksize=4096
+    ├─/mnt/part2 /dev/sdc2 ext4    rw,relatime,seclabel
+    └─/mnt/part1 /dev/sdc1 ext3    rw,relatime,seclabel
+    ```
+
+    Note the tree structure and aligned columns!
+
+- `--fstab`, `-s`: Show only filesystems listed in `/etc/fstab`:
+
+    ```console
+    [vagrant@el ~]$ findmnt --fstab
+    TARGET     SOURCE                                      FSTYPE OPTIONS
+    /          UUID=303db791-9236-4ac4-a176-e2a033576d89   xfs    defaults
+    none       UUID=a4814ebe-c0b2-4819-8129-30f32b3e8772   swap   defaults
+    /mnt/part1 UUID="f89d27bf-94d3-40a4-9fca-2af9cbe94856" ext3   defaults
+    /mnt/part2 UUID="cb44e6c4-a49e-43e1-b9a8-ed5d272b8efb" ext4   defaults
+    /mnt/part3 UUID="6b306b44-23fe-4cef-8c3e-423188e5c088" xfs    defaults
+    /mnt/part4 UUID="2856B0906FE06278"                     ntfs   defaults
+    /vagrant   vagrant                                     vboxsf uid=1000,gid=1000,_netdev
+    ```
 
-The `/etc/mtab` file is not updated by the kernel, but is maintained by the `mount` command. Do not edit `/etc/mtab` manually.
+- `--json`, `-J`: Output in JSON format
+
+- `--pairs`, `-P`: Output in key-value pairs
+
+    ```console
+    [vagrant@el ~]$ findmnt --pairs
+    TARGET="/proc" SOURCE="proc" FSTYPE="proc" OPTIONS="rw,nosuid,nodev,noexec,relatime"
+    TARGET="/sys" SOURCE="sysfs" FSTYPE="sysfs" OPTIONS="rw,nosuid,nodev,noexec,relatime,seclabel"
+    [...]
+    ```
+
+- `--df`, `-D`: imitate the output of `df(1)`
+
+    ```console
+    [vagrant@el ~]$ findmnt  --df
+    SOURCE    FSTYPE     SIZE  USED  AVAIL USE% TARGET
+    devtmpfs  devtmpfs     4M     0     4M   0% /dev
+    tmpfs     tmpfs    384.5M     0 384.5M   0% /dev/shm
+    tmpfs     tmpfs    153.8M  4.4M 149.4M   3% /run
+    /dev/sda2 xfs       61.9G  1.8G    60G   3% /
+    /dev/sdc5 xfs        4.9G 67.5M   4.9G   1% /mnt/part3
+    /dev/sdc6 fuseblk      5G 26.2M     5G   1% /mnt/part4
+    /dev/sdc2 ext4       5.8G   24K   5.5G   0% /mnt/part2
+    /dev/sdc1 ext3       3.9G   96K   3.7G   0% /mnt/part1
+    tmpfs     tmpfs     76.9M     0  76.9M   0% /run/user/1000
+    ```
+
+### Mount info in /proc
+
+The kernel provides the info through the `/proc` pseudo file system (i.e., `/proc` does not exist as a file on any hard disk but provides a file-like interface to information about the kernel).
+
+Several files contain relevant information:
+
+- `/proc/self/mounts`: a list of all mounted file systems, the content is like the output of `mount`. For backwards compatibility, `/etc/mtab` and `/proc/mounts` are symlinks to this file.
+- `/proc/self/mountinfo`: a more detailed list of mounted file systems
+- `/proc/self/mountstats`: statistics about mounted file systems
 
 ```console
-student@linux:~$ cat /etc/mtab | grep /dev/sdb
+student@linux:~$ cat /proc/mounts | grep /dev/sdb
     /dev/sdb1 /mnt/project42 ext2 rw 0 0
 ```
 
+However, `findmnt` is a better way to get and present this information.
+
 ### df
 
 A more user friendly way to look at mounted file systems is `df`. The `df` (*diskfree*) command has the added benefit of showing you the free space on each mounted disk. Like a lot of Linux commands, `df` supports the `-h` switch to make the output more *human readable*.
@@ -136,16 +207,14 @@ student@linux:~$ du -sh /boot /srv/wolf
 
 ## from start to finish
 
-Below is a screenshot that show a summary roadmap starting with
-detection of the hardware (/dev/sdb) up until mounting on `/mnt`.
+Below is a screenshot that show a summary roadmap starting with detection of the hardware (/dev/sdb) up until mounting on `/mnt`.
 
 ```console
 student@linux:~$ sudo dmesg | grep '\[sdb\]'
 sd 3:0:0:0: [sdb] 150994944 512-byte logical blocks: (77.3 GB/72.0 GiB)
 sd 3:0:0:0: [sdb] Write Protect is off
 sd 3:0:0:0: [sdb] Mode Sense: 00 3a 00 00
-sd 3:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support \
-DPO or FUA
+sd 3:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
 sd 3:0:0:0: [sdb] Attached SCSI disk
 
 student@linux:~$ sudo parted /dev/sdb