Flags:

-f

Only do the work if the map is floating-point

-i

Only do the work if the map is integer

-n

Only do the work if the map doesn’t have a NULL-value bitmap file

-c

Create NULL-value bitmap file validating all data cells

-r

Remove NULL-value bitmap file

-z

Re-create NULL-value bitmap file (to compress or uncompress)

--help

Print usage summary

--verbose

Verbose module output

--quiet

Quiet module output

--ui

Force launching GUI dialog

Parameters:

map=name [required]

Name of raster map for which to edit null values

setnull=val[-val][,val[-val],...]

List of cell values to be set to NULL

null=float

The value to replace the null value by

r.null and reclassified maps

r.null does not support reclassified maps because, if r.null was run on the reclass raster it would alter the original and any other reclass rasters of the original. Therefore r.null does not allow recoding reclassified maps (products of r.reclass).
As a workaround, the way to recode such a map is: The user creates a raster map out of the reclassified map by copying it:

r.mapcalc "newmap = reclass"

NULL data compression

By default no data files (i.e., NULL files) are not compressed unless a specific environment variable is set. The NULL file compression must be explicitly turned on with export GRASS_COMPRESS_NULLS=1.
Warning: such raster maps can then only be opened with GRASS GIS 7.2.0 or later. NULL file compression can be managed with r.null -z.

External maps

From the r.external documentation: GDAL-linked (r.external) maps do not have or use a NULL bitmap, hence r.null cannot manipulate them directly. Here NULL cells are those whose value matches the value reported by the GDALGetRasterNoDataValue() function. To apply the GDAL-linked the user need to either create a MASK (e.g. with r.mask) and then "apply" it using e.g. r.resample, or use r.mapcalc to create a copy with the appropriate categories changed to NULL (if() condition).