cmdfs - Command File System

Version 0.4

Copyright (C) 2010 Mike Swain

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see



Cmdfs is a FUSE virtual filesystem which applies an arbitary filter command to selected files in a source directory tree to create the destination files. Includes configurable caching and monitoring of the source tree to limit CPU use and prefetch data.



  1. Unpack archive
    tar zxvf cmdfs-<ver>.tar.gz
  2. Configure build
    cd cmdfs-<ver>
  3. Build
  4. Install
    sudo make install


cmdfs <source-dir> <mount-dir> [options]

    -o command=<shell command>
       The command to run to generate the file's content [default: cat]
    -o extension=ext1[;ext2[;...]]
       Specify matching file extension(s) to apply command to
    -o path-re=<regular expression>
       Specify regexp of pathnames to apply command to

    -o mime-re=<regular expression>
       Specify regexp of mime types to apply command to (as returned by file -b --mime-type)

    -o link-thru
       Pass unmatched files through to filesystem as symbolic links [default: files not listed or accessible]
 	-o hide-empty-dirs
      	Don't show directories which have no matching descendant files
    -o monitor
       Watch source directory tree for new files and cache them as they appear. New directories will be monitored [default: not monitored]
    -o cache-dir=<dir>
       Directory to save cache files [default: /usr/local/var/cache/cmdfs/<user>/<source-dir>]
    -o cache-size=<size in Mb>
       Size to attempt to limit cache directory to. Files removed on least-recent access basis.[default: no limit]
    -o cache-entries=<count>
       Number of entries to attempt to limit cache directory to. Files removed on least-recent access basis.[default: no limit]

    -o cache-expiry=<time in secs>
    	Items in cache older than this will be replaced when read
       [default: never expires]

How It Works - An Example

Given a source tree, which includes, say, jpg images, we can generate a view filesystem which contains the same files resized to email size.

The filter command is set up as a fileystem option at mount time. In this case our filter option might be something like (using ImageMagick's convert command):

command=convert - -resize 20% -

Note we follow the UNIX convention of taking the standard input and filtering to standard output. Being a FUSE filesystem, the child process will be run under the group/user of the owner. The command is run within a shell (/bin/sh) process, as if started by /bin/sh -c '<command>'

Obviously there'll be files in the source tree which we dont want to apply the filter to. In this case we identify files of interest by extension:


To mount the filesystem as above for a particular user:

 cmdfs /home/bob/images /mnt/myphotos "-ocommand=convert - -resize 20% -,extension=jpg;gif;png"

Selection Filters

As well as just matching on extension, files of interest can be selected by mime type, matching a regexp (cmdfs uses the file command to determine the mimetype):


Or just a plain path regexp (this allows us to exclude whole directory subtrees etc


These options can be combined to get the required filter selection.

Unmatched Files

By default, files which don't match a filter won't appear in the filesystem view. We may want to see the original files though, so the

option may be specified. This will generate symbolic links to the original files.


The image resize example is a good example of an application where we might want the files in the source filesystem to be filtered as they are added, rather than just on-demand. Cmdfs uses inotify to monitor all the directories in the tree, and automatically monitors new subdirectories as they are added:
When you copy the latest batch of files from your camera the resized versions will automatically be generated.


cmdfs only recreates view files if the modification date changes on the source. All files created using the filter are held in a cache directory, with a name created as a hash from the full path. By default this directory is located in /tmp/cmdfs-cache.<user name>, but can be changed:

Note that some systems will clear the /tmp directory tree between reboots, which may or may not be desired.

The size of the cache directory can be limited using:
cache-size=<size in mb>
cache-entries=<number of files>
The least recently accessed cached files will be removed in the background to maintain these limits.

Expiry on cached files can be specified with:

cache-expiry=<time in secs>
Cached files will be recreated if the are older than this.

Mounting With fstab

To aid in mounting directories for multiple users, cmdfs accepts the source base directory as the first argument so it can be mounted using the conventional mount.fuse script from fstab. This allows supplying cmdfs#<dir> as the fs_spec (see man 5 fstab). The user_allow_other option needs to be enabled in /etc/fuse.conf to allow normal users to mount (see fuse.conf). An example fstab entry:

cmdfs#/media/myphotos /home/bob/images fuse user,allow_other,command=convert\040-\040-resize\04020%\040-,monitor,cache-size=500,extension=jpg;gif;png 0 0

Note that the spaces in the command need to be escaped using \040 for /etc/fstab to parse correctly

Example 2: Dynamically Generated Files

The command used to generate the output does not need to be a simple stdin->stdout filter, it can generate its output from whatever source it chooses. Of course this is of limited utility unless you can specify what command/parameters to run (otherwise every file in the filesystem would have the same content!).

The shell command may therefore contain a substitution token, %f, which will be replaced with the full path/filename of the input file. If, for example, the command is simply '%f', cmdfs will attempt to execute the file (it should have +x permission to allow this)

This effectively gives a 'CGI' type function to the filesytem. e.g. If may contain a script:

$echo 'ps -ef' > /home/bob/showps
ps -ef
$chmod +x /home/bob/showps
$cmdfs /home/bob /tmp/test "-opath-re=.*,command=%f,cache-expiry=5"
$cat /tmp/test/showps
root         1     0  0 Feb13 ?        00:00:01 /sbin/init
root         2     0  0 Feb13 ?        00:00:00 [kthreadd]
root         3     2  0 Feb13 ?        00:00:00 [migration/0]
root         4     2  0 Feb13 ?        00:00:09 [ksoftirqd/0]
root         5     2  0 Feb13 ?        00:00:00 [watchdog/0]
Note that the cache expiry time in the example has been set at 5 secs - this means the ps results will be re-created if older than 5 sesonds.