Convert Sun Rasterfiles to PNM Safely with rasttopnm

You will convert a Sun rasterfile into a Netpbm image, check whether the result is PBM, PGM or PPM, and keep the original safe if a replacement fails. The examples use rasttopnm from Netpbm 11.5.2, installed here as Debian package netpbm 2:11.05.02-1.1build1.

Allow about ten minutes. You need a shell, a readable Sun rasterfile, and a directory where you can create the output. The conversion is an ordinary unprivileged operation. Do not use sudo unless filesystem permissions genuinely require it.

1. Check the installed command

Confirm that the executable is the one you expect and inspect its local version:

$ command -v rasttopnm
/usr/bin/rasttopnm
$ rasttopnm --version
rasttopnm: Using libnetpbm from Netpbm Version: Netpbm 11.5.2
...

The version output contains build details as well as the Netpbm version, so the final lines can differ between distributions. The useful checkpoint is that the command resolves and reports Netpbm 11.5.2 on this machine.

Its basic interface is rasttopnm [options] [rastfile]. With a file name, it reads that file. With no file name, it reads a Sun rasterfile from standard input. The converted image is written to standard output, which is why a redirection or a pipeline is normally part of the command.

2. Convert to a new output file

Replace the two placeholder paths with your own files:

$ rasttopnm /path/to/input.ras > /path/to/output.pnm
rasttopnm: writing PGM file

The status line identifies the output family selected from the input. A black-and-white input produces PBM, a greyscale input produces PGM, and a colour input produces PPM. The file name does not control that choice, so .pnm is a safer extension than claiming that every result is a PPM.

Check both the exit status and the file itself:

$ printf 'exit status: %s\n' "$?"
exit status: 0
$ file /path/to/output.pnm
/path/to/output.pnm: Netpbm image data, size = 4 x 2, rawbits, greymap

Your file description will reflect the actual dimensions and image type. For a more direct header check, use head only on the text header, not on the whole binary image:

$ head -n 1 /path/to/output.pnm
P5

P4, P5 and P6 are the binary PBM, PGM and PPM signatures commonly produced by Netpbm. If the output is going into another Netpbm command, pass the file on unchanged and let that command read the signature.

3. Keep a useful output while testing

Shell redirection with > truncates an existing destination before rasttopnm has finished. That is a destructive overwrite of the old output. Use a temporary name in the same directory, then replace the destination only after verification:

$ rasttopnm /path/to/input.ras > /path/to/output.pnm.new
$ status=$?
$ if [ "$status" -eq 0 ] && [ -s /path/to/output.pnm.new ]; then
>     mv -- /path/to/output.pnm.new /path/to/output.pnm
> else
>     printf 'conversion failed; old output was kept\n' >&2
>     rm -f -- /path/to/output.pnm.new
>     exit "$status"
> fi

The temporary file is deliberately removed on failure. The original rasterfile is only read, and the previous PNM remains in place unless the conversion succeeds and the final mv runs. If you stop before that point, remove the .new file later after checking that it is not needed.

Checkpoint: run file /path/to/output.pnm after the move and confirm the dimensions and type are what you expected. If the converter prints an error, keep the original rasterfile and investigate the input path or format rather than retrying over a good destination.

4. Use standard input and quiet mode

The optional input argument makes pipelines possible. This example first reads a rasterfile and then sends it to rasttopnm through standard input:

$ cat /path/to/input.ras | rasttopnm -quiet > /path/to/output.pnm
$ printf 'exit status: %s\n' "$?"
exit status: 0

-quiet suppresses the normal message that says which PNM type is being written. It does not suppress the image data on standard output, and it does not turn errors into success. In scripts, keep standard output reserved for the image and use the exit status for control flow.

For a simple file conversion, naming the input directly is easier to review than the cat pipeline. Use standard input when another program is already producing a raster stream or when a pipeline is part of the design.

5. Understand the -index exception

Most users should omit -index. A colormapped Sun rasterfile contains a colour map and pixel data represented by map indices. Without this option, rasttopnm uses the colours described by that map.

-index tells the converter to use the map indices as the pixel values instead. This is useful when a greyscale raster originally had matching values and its colour map was later changed, for example to equalise or otherwise alter displayed colours. Without -index, you get the colours from the modified map. With it, you recover the image represented by the original indices.

$ rasttopnm -index /path/to/colormapped.ras > /path/to/index-values.pnm
rasttopnm: writing PGM file

This option changes how the input colour map is interpreted; it does not edit the rasterfile and it does not modify the colours stored on disk. It was added in Netpbm 10.56, so it is present in the installed 11.5.2 release. If a script must run against older Netpbm installations, check the target package before depending on it.

6. Diagnose the common traps

If the command cannot open the input, check the path and read permission without changing the file:

$ ls -l /path/to/input.ras
$ test -r /path/to/input.ras && printf '%s\n' 'input is readable'

If the command succeeds but the result looks wrong, verify the dimensions and PNM type before blaming the viewer. The output type follows the input: a grayscale image will not become a colour PPM merely because you named the destination picture.ppm. Rename it to match the detected type if another tool depends on the extension, or keep the neutral .pnm name.

If a pipeline fails, do not treat an empty or partial destination as a valid image. Check the producer's status as well as rasttopnm's status, and use the temporary-file pattern from step 3. Keep the original until the new image opens correctly in the next tool in your workflow.

No service restart, configuration edit or elevated privilege is needed for this conversion. The only state-changing examples above create, replace or remove image files. Review the paths before running them, especially the final mv and the cleanup rm -f.