Feb 082009

I was reading a comment about the df command (in relation to reserved filesystem space) and realised that the clueless newbie was right; it is odd that df does not mention reserved space. Of course it would also be wrong for df to lie about the matter too. I then realised that df is long overdue for a bit of refreshing. If you look at the typical output of the df command, you will find it inconveniently cluttered :-

Filesystem            Size  Used Avail Use% Mounted on
                       12G  7.8G  4.3G  65% /
tmpfs                 2.0G     0  2.0G   0% /lib/init/rw
varrun                2.0G  416K  2.0G   1% /var/run
varlock               2.0G     0  2.0G   0% /var/lock
udev                  2.0G  3.1M  2.0G   1% /dev
tmpfs                 2.0G  344K  2.0G   1% /dev/shm
lrm                   2.0G  2.4M  2.0G   1% /lib/modules/2.6.27-7-generic/volatile
/dev/sdb1             130M   36M   88M  29% /boot
                      2.0G  776M  1.3G  39% /opt
                      5.0G  1.4G  3.7G  28% /var
                      256G  116G  141G  46% /home
                       96G   62G   35G  64% /vmachines
                      256G  6.2G  250G   3% /var/spool/brag
                       16G  4.6G   12G  29% /var/herpes
/dev/sda1             463G  147G  293G  34% /mdata
/dev/scd0             2.4G  2.4G     0 100% /media/CIVCOMPLETEEU
                       32G  1.9G   31G   6% /cdimages
                       16G  498M   16G   4% /sim

Part of the problem is that df does not do quite what it claims to do … to report free space on the mounted filesystems. It also gives some (a very small amount) of additional information about the relevant filesystems … particularly the device the filesystem is mounted on. This “helps” to make the output more cluttered that it needs to be. It is possible that there are those who will argue that the device is the filesystem and not where it is mounted; they are arguably right, but when you use df you are either looking at where in the Unix file hierarchy there are places that have less space than is comfortable, or for places that have enough space to put that big file you are about to download.

Next the command itself has an obscure command to make it easier to type on a slow type-writter like terminal (those who are below a certain age will not realise that we used to comminicate with Unix machines using a terminal that was more like a printer than the screens we use today). It might be better named fsspace with an alias of diskspace for those who want to concentrate on what worries them rather than on what worries the machine.

Next why not take advantage of certain features that have crept almost silently into the command line over the last few decades ? Why not adjust the output to the width of the terminal window (look for the $COLUMNS evironment variable), spacing things out or even adding more information when you have enough space?

Finally if you were to dig around the df command a little bit you will encounter something peculiar called “inodes”. Now I know what an inode is, and I dare say quite a few people reading this will know, but if you do not, knowing how many inodes there are is not very useful information. It is relatively rare (these days) for a filesystem to run out of inodes so this information has a low priority, and why not use a term more understandable than “inodes” ?

Changing a term is something to be avoided in most circumstances which is why we still have “inode” where even the originator of the term has to guess that the “i” means “index”. I would suggest that something like “fileslots”or perhaps “fslots”

We now have the basic specification of something that should look like :-

% diskspace
Filesystem                            Size  %Used %fslots  Avail
/                                      12G    70%      3%   3.6G
/lib/init/rw                          2.0G     0%      0%   2.0G
/var/run                              2.0G     0%      0%   2.0G
/var/lock                             2.0G     0%      0%   2.0G
/dev                                  2.0G     0%      1%   2.0G
/dev/shm                              2.0G     0%      0%   2.0G
/lib/modules/2.6.27-7-generic/vola+   2.0G     0%      0%   2.0G
/boot                                 130M    29%      0%    88M
/opt                                  2.0G    40%      1%   1.2G
/var                                  5.0G    18%      0%   4.1G
/home                                 256G    52%      0%   124G
/cdimages                              32G    65%      0%    12G
/mdata                                463G    36%      1%   280G

This could be improved in some ways – for instance it would be helpful to skip over certain of the filesystems that are not strictly speaking backed by disk. However it is beginning to be useful.

Or would be if the code exists. Fortunately it does.

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