Convert an Img-Whatnot File to PPM with imgtoppm

imgtoppm reads an Img-whatnot file and writes a plain PPM image, and the whole thing is one short command. The installed build is Netpbm 11.5.2, from Debian package netpbm 2:11.05.02-1.1build1.

Budget ten minutes. You need a shell, an Img-whatnot input file, enough disk space for the output, and the Netpbm package. This is an ordinary user-level conversion: nothing here needs sudo.

1. Check the installed command

Confirm the shell runs the expected binary and note the local Netpbm build:

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

The final diagnostic lines vary; the Netpbm version is the part worth checking. If command -v comes back empty, install the package through your normal system administration process. Do not grab a binary from some other host just to make this example run.

2. Convert a named file to a new PPM file

Input path first, standard output redirected to the destination:

$ imgtoppm /path/to/input.img > converted.ppm

Swap in your actual Img-whatnot file for /path/to/input.img. The .img suffix is only a reminder to you; imgtoppm reads the file contents and writes PPM to standard output. A clean conversion is quiet, since the image stream is redirected away from the terminal.

$ test -s converted.ppm && printf '%s\n' 'PPM output was written'
PPM output was written

A zero-byte file is not a usable conversion. The shell creates the redirection target before the program even starts, so a failed run can still leave an empty or partial file sitting there.

3. Protect an existing destination

Redirecting with > truncates an existing file before imgtoppm does anything. If the destination matters, pick a new name or make the shell refuse an existing one:

$ set -o noclobber
$ imgtoppm /path/to/input.img > converted.ppm
$ set +o noclobber

With noclobber on, the shell will not replace an existing regular file, and the setting only affects the current shell. If the command fails because converted.ppm already exists, use another output path, or remove the old file only once you have confirmed it is disposable: deleting an image is irreversible without a backup.

In a script, write to a temporary file in the same directory, check the exit status and size, then rename it into place. Skip broad temporary paths, and never replace a file another process is actively reading without a plan for that.

4. Stream an input file through standard input

The input argument is optional. Leave it off and imgtoppm reads standard input instead, which is handy in a pipeline:

$ cat /path/to/input.img | imgtoppm > converted.ppm

You do not need the pipe for a normal file, but it shows the input contract. Prefer feeding the file directly when there is no reason to add another process:

$ imgtoppm /path/to/input.img > converted.ppm

Keep diagnostic output out of the image stream: redirect standard output to the PPM file and let standard error stay on the terminal, or route it to its own log.

5. Handle bad input and exit status

Feed it a deliberately empty file to see the failure shape without touching a real image:

$ imgtoppm /dev/null > failed.ppm
imgtoppm: missing pixel data header
$ printf 'exit status: %s\n' "$?"
exit status: 1

That diagnostic means the input was not a valid Img-whatnot stream, not that failed.ppm is safe to use. Check the status straight after imgtoppm runs, before any other command, and discard or quarantine failed.ppm once you have looked at its size.

A missing path fails differently. Check spelling, permissions and file type before reaching for elevated privileges:

$ test -r /path/to/input.img || printf '%s\n' 'Input is missing or unreadable'
$ imgtoppm /path/to/input.img > converted.ppm
$ status=$?
$ printf 'imgtoppm status: %s\n' "$status"
imgtoppm status: 1

Only bring in an administrator when the file is deliberately restricted and your local policy allows access. sudo will not repair an invalid image format.

6. Avoid invented options

The imgtoppm manpage defines no options of its own. Its synopsis is just imgtoppm [imgfile], so do not assume --help, an output filename flag or a resize switch belongs to this program:

$ imgtoppm --help
imgtoppm: Use 'man imgtoppm' for help.

On this build, that argument is treated as an input filename rather than a portable help option. Read the local imgtoppm(1) page and the common Netpbm options it references before you wire one into automation. Output goes wherever shell redirection sends it, not to an imgtoppm-specific destination flag.