# DATA and persistent files

FDS keeps SYSTEM read-only. Store documents, source code and other persistent
files on a DATA cartridge. With one healthy DATA cartridge inserted, FDS mounts
it at `/data` and gives the `fds` user access to its files.

```sh
fds bays
fds bay 02
mkdir -p /data/projects
printf 'My first FDS file\n' >/data/projects/hello.txt
cat /data/projects/hello.txt
```

Confirm that the bay reports `MOUNTED READ WRITE` before writing. Only one DATA
cartridge is active at a time. To select DATA explicitly:

```sh
fds data use 02
```

If another DATA is active, eject it first. In recovery, DATA starts read-only and
requires this explicit activation before it can be used for ordinary writes.

## Programs using DATA

Run a system command under the DATA bay's process tracking when it should stop
as part of ejecting that DATA:

```sh
fds run 02 -- /usr/bin/tail -f /data/application.log
```

This background command uses `/data` as its working directory. Its output goes
to the cartridge service log. A directly launched PROGRAM is tracked under its
PROGRAM bay; close it if it keeps files open on a different DATA cartridge.

## Finish a session

```sh
cd "$HOME"
fds eject 02
```

Wait for SAFE, then remove the cartridge. `fds poweroff` performs the storage
shutdown sequence for all active bays. Unexpected power loss or pulling a drive
during a write can damage files; [recovery](recovery.md) explains checking DATA.

In the emulator, writes go to a separate session overlay. Eject preserves that
overlay, and reinserting the original image creates a new overlay. See
[exporting emulator DATA](workstation.md#save-emulator-data) before moving or
deleting a session.