the file filesystem: mount semi-structured data (like JSON) as a Unix filesystem

exe (macOS) exe (Linux) github

% FFS(1) Version 0.1.2 | File Filesystem Documentation % Michael Greenberg


ffs - the file filesystem


ffs –completions SHELL
ffs [-h|–help]
ffs [-V|–version]


ffs—the file filesystem—lets you mount semi-structured data as a filesystem, allowing you to work with modern formats using familiar shell tools.

ffs uses filesystems in userspace (FUSE); you must have these installed on your system to use ffs.

ffs expects its input to be encoded in UTF-8.


-d, –debug

Give debug output on stderr


Eagerly load all data on startup. ffs’s default behavior is to lazily load data on startup, which avoids preparing data that won’t be read or written.


Don’t add newlines to the end of values that don’t already have them (or strip them when loading)

-i, –in-place

Writes the output back over the input file


Disables output of filesystem (normally on stdout)


Pretty-print output (may increase size)

-q, –quiet

Quiet mode (turns off all errors and warnings, enables –no-output)


Mounted filesystem will be readonly


Emit timing information on stderr in an ‘event,time’ format; time is in nanoseconds


Don’t pad the numeric names of list elements with zeroes; will not sort properly


Don’t use extended attributes to track metadata (see man xattr)


Include ._* extended attribute/resource fork files on macOS.

-h, –help

Prints help information (and exits)

-V, –version

Prints version information (and exits)


–dirmode DIRMODE

Sets the default mode of directories (parsed as octal; if unspecified, directories will have FILEMODE, with execute bits set when read bits are set) [default: 755]


Sets the default mode of files (parsed as octal) [default: 644]

-g, –gid GID

Sets the group id of the generated filesystem (defaults to current effective group id)

-m, –mount MOUNT

Sets the mountpoint; will be inferred when using a file, but must be specified when running on stdin

-o, –output OUTPUT

Sets the output file for saving changes (defaults to stdout)

–munge MUNGE

Set the name munging policy; applies to ‘.’, ‘..’, and files with NUL and ‘/’ in them [default: rename] [possible values: filter, rename]

  • Under –munge rename (the default), fields named ‘.’ and ‘..’ will be renamed to ‘_.’ and ‘_..’, respectively. Every NUL byte will be replaced with the text ‘_NUL_’ and every forward slash will be replaced with the text ‘_SLASH_’. Unless you manually change the name of these renamed files, they will be saved back with their original names, i.e., ‘_..’ will turn back into a field called ‘..’, and ‘and_SLASH_or’ will be turned back into ‘and/or’. New files created with such names will not be converted back.
  • Under –munge filter, fields named ‘.’, ‘..’, or with NUL or ‘/’ in them will simply be dropped (with a warning).
–new NEW

Mounts an empty filesystem, inferring a mountpoint and output format. Running –new FILE.EXT is morally equivalent to running:

echo '{}' | ffs --source json -o *FILE*.*EXT* --target *EXT* -m *FILE*

where the mountpoint FILE will be created (and removed) by ffs.

–completions SHELL

Generate shell completions (and exits) [possible values: bash, fish, zsh]

-s, –source SOURCE_FORMAT

Specify the source format explicitly (by default, automatically inferred from filename extension) [possible values: json, toml, yaml]

-t, –target TARGET_FORMAT

Specify the target format explicitly (by default, automatically inferred from filename extension) [possible values: json, toml, yaml]

-u, –uid UID

Sets the user id of the generated filesystem (defaults to current effective user id)



Sets the input file (use ‘-‘ for stdin) [default: -]

Data model

The data model for ffs is a superset of that of its supported formats (currently, JSON, TOML, and YAML); ffs maps values in these formats to filesystems. Here are the different types and how they’re mapped to a filesystem:


Automatically detected. The following order is used for UTF-8 encodable data: null, boolean, integer, float, datetime, string. If data can’t be encoded in UTF-8, it will always be bytes.


Mapped to a file. Either true or false.


Mapped to a file. When saving, bytes are typically encoded in base64.


Mapped to a file. Some portion of an RFC 3339 date/time.


Mapped to a file. No larger than 64 bits.


Mapped to a file. No larger than 64 bits.


Mapped to a directory. List directories will have numerically named elements, starting from 0. Filenames will be padded with zeros to ensure proper sorting; use –unpadded to disable padding. While mounted, you are free to use whatever filenames you like in a list directory. When list directories are saved, filenames are ignored and the sorted order of the files (in the current locale) will be used to determine the list order.


Mapped to a directory. Named directories (also known as maps, objects, hashes, or dictionaries) will use field names as the file/directory names for their contents. Some renaming may occur if fields have special characters in them; see –munge above.


Mapped to a file. The file will be empty.


Mapped to a file. The file will be encoded in UTF-8 holding the string.

By default every file will have a newline appended to its contents; this newline will be removed when the filesystem is dumped back to a file. To disable these newlines, use –exact.

You can inspect and alter the types of files and directories using the extended attribute user.type (use xattr on macOS and attr/getfattr/setfattr on Linux). The names given here are the valid values for the user.type attribute.



Configures tracing output. Use the format key=level, where key should probably be ffs and level should be one of error, warn, info, debug, or trace. The default is ffs=warn. Setting -q turns off all output; setting -d sets ffs=debug. To get more information from FUSE bindings, add fuser, e.g., ffs=debug,fuser=info.



Successfully unmounted.


A FUSE or other filesystem error occurred.


Command-line argument parsing error.


The general workflow is to run ffs, do some work, and then unmount the mountpoint using umount. It’s typical to run ffs in the background, since the program will not terminate until the userspace filesystem is unmounted.

By default, ffs will work off of stdin, in which case you must specify a mountpoint with -m. If you have a mountpoint/directory mnt, you can download information on GitHub commits, work with them, and save the modified output to commits.json by running:

curl https://api.github.com/repos/mgree/ffs/commits | ffs -m mnt -o commits.json 

If you had already downloaded the file to commits.json, you could simply run:

ffs -i commits.json
# do edits in commits directory
umount commits
# changes are written back to commits.json (-i is in-place mode)

If you want to create a new file wholesale, the –new flag is helpful.

ffs --new file.json
# do edits in file directory
umount file
# corresponding json is in file.json

To mount a JSON file and write back out a YAML file, you could run:

ffs -o output_data.yaml input_data.json
# do edits in the input_data directory ffs created
umount input_data

When filenames are present, extensions will be used to infer the format being used. You can specify the source and target formats explicitly with –source and –target, respectively.

You can use extended attributes to change a list directory to a named one (or vice versa); this example uses macOS, with Linux alternatives in comments.

$ ffs -i list.json &
[1] 41361
$ cat list.json
$ cd list
$ mv 0 loneliest_number
$ mv 1 to_tango
$ mv 2 three
$ mv 3 not_true
$ xattr -l .                    # Linux: getattr --match=- .
user.type: list
$ xattr -w user.type named .    # Linux: setattr -n user.type -v named .
$ ls
loneliest_number not_true         three            to_tango
$ cd ..
$ umount list
[1]+  Done                    target/debug/ffs -i list.json
$ cat list.json


attr(1), fuse(4), fusermount(3), getfattr(1), mount(8), setfattr(1), umount(8), xattr(1)

To learn more about FUSE (Filesystem in Userspace), check out libfuse (Linux) https://github.com/libfuse/libfuse and macFUSE (macOS) https://osxfuse.github.io/.


See https://github.com/mgree/ffs/issues.


Copyright 2021 (c) Michael Greenberg. GPL-3.0 licensed.