Create an Apple Mac .disk_label with grub-render-label

Apple's firmware is fussy about disk labels, and grub-render-label is the GRUB tool that renders one. Feed it a short string or a text file and it writes out the binary .disk_label the firmware expects. Allow about ten minutes. You need a Linux shell, the grub-render-label command from grub-common, and a GRUB PF2 font. The examples use GRUB 2.12-1ubuntu7.3, installed here on Ubuntu.

• What it does. Produces an Apple-specific GRUB label image, nothing more.
• What it will not touch. It does not install GRUB, alter boot entries, write a disk, or change firmware settings.
• Where it writes. The examples write under /tmp or your working directory, so none of them need sudo.

1. Check the installed command and font

First confirm which program will run and record its version:

$ command -v grub-render-label
/usr/bin/grub-render-label
$ grub-render-label --version
grub-render-label (GRUB) 2.12-1ubuntu7.3

The installed manual documents -f or --font as accepting a PF2 font file. Common GRUB installations provide unicode.pf2 under /usr/share/grub. Locate one rather than guessing a path:

$ find /usr/share/grub -type f -name '*.pf2' -print
/usr/share/grub/unicode.pf2
/usr/share/grub/euro.pf2
/usr/share/grub/ascii.pf2

Use a font that contains the characters in your label. If no PF2 file is found, stop here and install the GRUB package through your normal distribution process. Do not copy a font from an arbitrary website into a boot directory.

2. Render a label from a string

Supply the label with -t and the destination with -o. This example keeps the generated file in a temporary directory while you check it:

$ workdir=$(mktemp -d /tmp/grub-label-XXXXXX)
$ grub-render-label \
    --font=/usr/share/grub/unicode.pf2 \
    --text='Example Mac' \
    --output="$workdir/example.disk_label"
$ printf 'status=%s\n' "$?"
status=0

There is no text preview in the terminal: a successful run just produces binary data. Check its type and size instead of eyeballing it:

$ file "$workdir/example.disk_label"
/tmp/grub-label-XXXXXX/example.disk_label: data
$ wc -c < "$workdir/example.disk_label"
1671

3. Read the label from a file

Use -i or --input when the label is maintained in a file, or handed over by another step in a build. Keep the input as plain text:

$ printf '%s\n' 'Recovery Mac' > "$workdir/label.txt"
$ grub-render-label \
    --font=/usr/share/grub/unicode.pf2 \
    --input="$workdir/label.txt" \
    --output="$workdir/recovery.disk_label"
$ test -s "$workdir/recovery.disk_label" && echo 'label created'
label created

The input file is read, not replaced. Keep it under version control or in your build inputs if the label must be reproducible. A trailing newline can end up as part of the input, so write the file deliberately and check the rendered result on the target Mac.

4. Keep binary output away from the terminal

The manual says the default output is standard output. That is fine in a pipeline, but running an unredirected render in an interactive terminal is asking for trouble: the result is binary, not readable text.

$ grub-render-label \
    --font=/usr/share/grub/unicode.pf2 \
    --text='Example Mac' \
    > "$workdir/piped.disk_label"
$ test -s "$workdir/piped.disk_label" && echo 'standard output captured'
standard output captured

Use --output when the destination is a named file, and shell redirection only when the command is one stage in a pipeline. A zero exit status only tells you this render completed; it is not proof that a bootloader will accept the label.

5. Choose colours or verbose diagnostics carefully

The command exposes separate options for background and text colour, but the installed manpage does not define a colour vocabulary. Do not assume a value accepted by another GRUB tool is valid here: test any chosen values in a temporary output file before they go anywhere near an image-building script.

Add --verbose when a render fails and you need to know why. Keep the output file separate while testing:

$ grub-render-label --verbose \
    --font=/usr/share/grub/unicode.pf2 \
    --text='Example Mac' \
    --output="$workdir/verbose.disk_label"
$ printf 'status=%s, bytes=%s\n' "$?" "$(wc -c < "$workdir/verbose.disk_label")"
status=0, bytes=1671

That byte count is an example from this installed version, not a required constant. A different label, font or package build produces a different size.

6. Avoid overwriting a useful label

$ new_label="$workdir/example.disk_label.new"
$ grub-render-label \
    --font=/usr/share/grub/unicode.pf2 \
    --text='Example Mac' \
    --output="$new_label"
$ test -s "$new_label"
$ mv -- "$new_label" "$workdir/example.disk_label"
$ test -s "$workdir/example.disk_label" && echo 'replacement installed'
replacement installed

The mv only changes the destination within the temporary directory, so if rendering fails, the old file is untouched because the new path was separate all along. If you are replacing a label inside a boot image or a mounted system, follow that image's backup and recovery procedure first: this command itself has no undo.

7. Diagnose the common failures

Missing arguments usually means the command was not given enough to render. Check that you supplied a text or input file, an output destination or redirection, and a usable font:

$ test -r /usr/share/grub/unicode.pf2 && echo 'font readable'
font readable
$ test -r "$workdir/label.txt" && echo 'input readable'
input readable
$ grub-render-label --help

• Input path wrong? Check it with ls -l and test -r without changing anything.
• Font path wrong? Rerun find /usr/share/grub -type f -name '*.pf2'.
• Output empty? Inspect the exit status and stderr before reusing the destination.
• Do not reach for sudo as a generic repair. It fixes none of the above: not a missing argument, an invalid font, or a bad path.

For a final check, compare the output size and, where possible, inspect the label through the Apple Mac boot workflow that will actually consume it. Keep the original input and any previous boot image until that check succeeds.