Convert a 12-Row PPM to an Apple Volume Label

You will finish with an Apple volume label image produced from a PPM file, plus a repeatable check that the source has the dimensions ppmtoapplevol requires. The command writes a binary format to standard output, so the result should go to a named file or another program, never directly to your terminal.

Allow about fifteen minutes. You need Netpbm, a PPM image, and a shell. This guide uses Netpbm 11.5.2 from the installed Ubuntu package netpbm 2:11.05.02-1.1build1. The command was added in Netpbm 10.54, so older installations may not contain it.

1. Check the installed command

Start with read-only checks. No elevated privileges are needed:

$ command -v ppmtoapplevol
/usr/bin/ppmtoapplevol
$ ppmtoapplevol -version
ppmtoapplevol: Using libnetpbm from Netpbm Version: Netpbm 11.5.2
ppmtoapplevol: Built from source dated 2024-03-31 09:09:47
ppmtoapplevol: Built by Debian

The exact build details can differ. The useful line is the Netpbm library version. -version is a common Netpbm option, not an option unique to this program. There are no command-line options specific to ppmtoapplevol.

2. Check the PPM dimensions

The input must be exactly 12 rows tall and no more than 255 columns wide. Its height cannot be 11 or 13, and a width of 256 is already too large. Inspect the file before conversion:

$ pamfile volume-label.ppm
volume-label.ppm: PPM raw, 160 by 12  maxval 255

Your format description may say PPM plain instead of PPM raw; both are PPM input. The dimensions are the part that matters here. A file that looks like a narrow banner can still fail if it has the wrong row count.

If pamfile reports more than 255 columns, reduce the width. If it reports the wrong height, make it 12 rows. The manpage points to pamscale and pamcut for this preparation, but the right operation depends on whether preserving the whole image or cropping it matters.

For example, this scales an input to exactly 12 rows while preserving its aspect ratio. The resulting width is calculated by pamscale, so check it afterwards:

$ pamscale -ysize 12 source.ppm > scaled.ppm
$ pamfile scaled.ppm
scaled.ppm: PPM raw, 213 by 12  maxval 255

Scaling can make text or small features less legible. It also might produce a width above 255. If so, choose a smaller width or crop deliberately. Do not overwrite the original while experimenting; the redirection above creates a separate file.

3. Convert the image to a named output file

Pass the PPM path as the optional argument and redirect standard output to a new file:

$ ppmtoapplevol scaled.ppm > volume-label.applevol
$ printf 'exit status: %s\n' "$?"
exit status: 0
$ wc -c volume-label.applevol
536 volume-label.applevol

The byte count depends on the source width. It is not a text file and should not be opened in a terminal or edited with a text editor. The successful exit status means the conversion completed; it does not mean the label is visually suitable for a particular Apple boot screen.

When the input is valid, the output begins with a small binary header followed by image data. For a 4 by 12 test image, the installed program produced 54 bytes, including the header. Treat that as an observed implementation detail rather than a file-format parser contract. Let the Apple consumer interpret the result.

To keep informational Netpbm messages off standard error in scripts, add the common -quiet option:

$ ppmtoapplevol -quiet scaled.ppm > volume-label.applevol
$ test -s volume-label.applevol
$ printf 'non-empty output: %s\n' "$?"
non-empty output: 0

-quiet does not make an invalid image valid and does not turn the binary output into text. Keep the redirection in place.

4. Use standard input in a pipeline

The optional filename can be omitted. In that form, the command reads the PPM from standard input, which is useful when another Netpbm command prepares it:

$ pamscale -ysize 12 source.ppm | ppmtoapplevol -quiet > volume-label.applevol
$ test -s volume-label.applevol
$ printf 'pipeline output is present: %s\n' "$?"
pipeline output is present: 0

There is a practical trap here: pamscale -ysize 12 controls the height but not the maximum width. Always inspect the intermediate result when the source dimensions are uncertain, or give pamscale a width constraint as well and verify the final dimensions with a saved intermediate file.

For a script, use a temporary file when you need to inspect dimensions before committing the result:

$ tmp_ppm=$(mktemp --suffix=.ppm)
$ trap 'rm -f "$tmp_ppm"' EXIT
$ pamscale -ysize 12 source.ppm > "$tmp_ppm"
$ pamfile "$tmp_ppm"
$ ppmtoapplevol -quiet "$tmp_ppm" > volume-label.applevol

This removes only the temporary intermediate when the shell exits. The output file remains. If you need to keep the intermediate for diagnosis, copy it somewhere intentional before ending the shell.

5. Diagnose a rejected image

A wrong height or width is a normal input error. The converter exits non-zero and explains the measured dimension:

$ ppmtoapplevol image-11-rows.ppm > volume-label.applevol
ppmtoapplevol: Input image must be 12 rows tall.  Yours is 11
$ printf 'exit status: %s\n' "$?"
exit status: 1

Because shell redirection opens the destination before the program runs, do not use a valuable existing output path while testing failures. Prefer a new path, or write to a temporary file and move it into place only after a zero exit status. A failed run may leave an empty or partial destination, depending on where the failure occurs.

For an oversized image, the diagnostic is similar:

$ ppmtoapplevol image-256-columns.ppm > /tmp/volume-label.applevol
ppmtoapplevol: Input image must not be more than 255 columns wide.  Yours is 256
$ printf 'exit status: %s\n' "$?"
exit status: 1

Use pamcut to crop a known region when that is the intended result, or use pamscale to reduce the complete image. Neither operation needs sudo. If a file is owned by another account, fix the input and output locations or permissions through your normal change process instead of granting the converter unnecessary privileges.

6. Keep the format boundary clear

ppmtoapplevol converts in one direction: PPM input to an Apple volume label image. Netpbm does not provide a reverse converter. Keep the original PPM if you may need to revise the label; the generated file is not a convenient editable source.

The command does not mount a volume, change firmware settings, write to an EFI system partition or install a label. Those actions would be separate, system-specific workflows. This guide stops at producing the output file. Before copying it into a boot-related location, confirm the destination, preserve a backup and follow the documentation for the Apple system that will consume it.

No elevated privilege is required for the checks or conversion shown here. Only a later copy into a protected system location might need administrative access, and that is intentionally outside these examples.