Replace bmptoppm with bmptopnm

Your old script still calls bmptoppm, and it still works, but only because the name is now a link to bmptopnm. This guide converts a BMP with the current command, checks the output type instead of guessing it, and keeps an existing output safe while you test. Allow about ten minutes. You need a shell, a readable BMP file and the netpbm package.

It describes Netpbm 11.5.2 from Debian package 2:11.05.02-1.1build1. The installed bmptoppm is a symbolic link to bmptopnm, so new scripts should use bmptopnm directly.

1. Confirm which command you have

Check both names before changing a script or troubleshooting an image:

$ command -v bmptoppm
/usr/bin/bmptoppm
$ command -v bmptopnm
/usr/bin/bmptopnm
$ ls -l "$(command -v bmptoppm)"
lrwxrwxrwx ... /usr/bin/bmptoppm -> bmptopnm

Permissions, date and exact link formatting can differ. What matters is that both commands resolve and the old name points at the replacement. Check the implementation version without converting an image:

$ bmptopnm --version
bmptopnm: Using libnetpbm from Netpbm Version: 11.5.2
...

The remaining version lines are build details.

2. Convert a BMP to a Netpbm image

Pass the input filename to bmptopnm and redirect standard output to a new destination:

$ bmptopnm /path/to/input.bmp > /path/to/output.pnm
$ printf 'converter status: %s\n' "$?"
converter status: 0

A status of 0 means the converter completed. The output may be PBM, PGM or PPM, and that is the main difference from the historical name: bmptopnm picks a more specific format when the BMP data allows it. A black-and-white image can produce PBM, and a grayscale image can produce PGM.

Inspect the result before passing it to another program:

$ file /path/to/output.pnm
/path/to/output.pnm: Netpbm image data, size = 32 x 32, rawbits, bitmap
$ head -c 2 /path/to/output.pnm
P4

The exact file wording varies. The magic bytes identify the format:

• P4: PBM.
• P5: PGM.
• P6: PPM.

3. Protect an existing output while testing

Shell redirection with > truncates its destination before the converter starts. That is destructive if the destination already holds a useful image. Use a temporary name in the same directory, then replace the old file only after checking the new one:

$ bmptopnm /path/to/input.bmp > /path/to/output.pnm.new
$ status=$?
$ if [ "$status" -eq 0 ] && [ -s /path/to/output.pnm.new ]; then
>     file /path/to/output.pnm.new
>     mv /path/to/output.pnm.new /path/to/output.pnm
> else
>     printf 'conversion failed; original output was left alone\n' >&2
>     rm -f /path/to/output.pnm.new
>     exit "$status"
> fi
/path/to/output.pnm.new: Netpbm image data, ...

$ cp --preserve=all /path/to/output.pnm /path/to/output.pnm.bak
$ mv /path/to/output.pnm.bak /path/to/output.pnm

4. Read the BMP header diagnostics

Use -verbose to see what the converter read from the header:

$ bmptopnm -verbose /path/to/input.bmp > /path/to/output.pnm
bmptopnm: BMP image header says:
bmptopnm:   Width: 32 pixels
bmptopnm:   Height: 32 pixels
bmptopnm:   Bits per pixel in raster: 1
bmptopnm: WRITING PBM IMAGE

Header details and wording depend on the input. The messages go to standard error, while the converted image still goes to standard output. Keep the redirection in place or binary image data will spill into your terminal.

5. Handle failures without guessing

A missing or unreadable input is a path or permission problem, not a reason to run the converter as root:

$ test -r /path/to/input.bmp && echo readable
$ ls -l /path/to/input.bmp
$ bmptopnm /path/to/input.bmp > /tmp/output.pnm
$ printf 'converter status: %s\n' "$?"

• Restricted files: use sudo only if the file or destination is deliberately restricted and your system policy permits that access. Ordinary image conversion should run as your normal user.
• Unsupported compression: if the command reports a BMP compression mode it cannot convert, keep the original file and use a tool that supports that encoding.
• Exit status 0 is not proof of a good image. Check the reported dimensions and inspect the result with a trusted viewer or a later converter.
• Old scripts: replace bmptoppm with bmptopnm so the intended command is clear. Review any downstream code that assumes a PPM magic number, because the replacement can deliberately emit PBM or PGM.