Convert CMU Window Manager Bitmaps to PBM with cmuwmtopbm

You will turn a CMU window manager bitmap into a PBM image file, verify the result, and keep failed conversions from replacing a good output. The installed command is Netpbm 11.5.2. Allow about ten minutes if the source bitmap is already available.

1. Check the installed command

This guide assumes a Linux system with the netpbm package installed and a readable CMU window manager bitmap. The command has one positional input argument and no cmuwmtopbm-specific switches. It writes the PBM image to standard output, so you normally redirect that output to a new file.

$ command -v cmuwmtopbm
/usr/bin/cmuwmtopbm
$ dpkg-query -W -f='${Package} ${Version}\n' netpbm
netpbm 2:11.05.02-1.1build1
$ cmuwmtopbm --version
cmuwmtopbm: Using libnetpbm from Netpbm Version: Netpbm 11.5.2

The version output includes build details after the Netpbm version. Exact packaging and diagnostic text can differ on another distribution, but the installed package and library version are useful when comparing results.

2. Convert a bitmap to PBM

Give the input file as the optional cmuwmfile argument. Replace the example paths with paths you control:

$ cmuwmtopbm /path/to/window-manager-bitmap.cmuwm > /path/to/window-manager-bitmap.pbm

This is an ordinary, unprivileged conversion. Do not use sudo merely because the file has an unfamiliar format. You need read access to the input and write access to the destination directory.

Checkpoint: inspect the output without opening the binary pixel data in a text editor:

$ file /path/to/window-manager-bitmap.pbm
/path/to/window-manager-bitmap.pbm: Netpbm image data, size = WIDTH x HEIGHT, rawbits, bitmap
$ head -c 2 /path/to/window-manager-bitmap.pbm
P4

A raw PBM file starts with the magic number P4. The dimensions in the file output should match the bitmap you intended to convert. The remaining bytes are binary image data, so terminal output after the header may look like noise.

3. Use standard input when that fits the workflow

The filename argument is optional. With no argument, the installed program reads the CMU bitmap from standard input and still writes PBM to standard output:

$ cmuwmtopbm < /path/to/window-manager-bitmap.cmuwm > /path/to/window-manager-bitmap.pbm
$ test -s /path/to/window-manager-bitmap.pbm && echo 'PBM output is non-empty'
PBM output is non-empty

This form is useful in a pipeline, but it can make the input less obvious when you are returning to a script later. Prefer the named argument in a short-lived command and standard input when another tool is already producing the CMU bitmap.

4. Protect an existing output file

Shell redirection with > truncates the destination before cmuwmtopbm starts. That is a destructive overwrite if the destination already contains a useful PBM. Convert to a temporary name in the same directory, check it, then replace the old file only when the check succeeds:

$ cmuwmtopbm /path/to/window-manager-bitmap.cmuwm > /path/to/window-manager-bitmap.pbm.new
$ test -s /path/to/window-manager-bitmap.pbm.new
$ file /path/to/window-manager-bitmap.pbm.new
/path/to/window-manager-bitmap.pbm.new: Netpbm image data, size = WIDTH x HEIGHT, rawbits, bitmap
$ mv /path/to/window-manager-bitmap.pbm.new /path/to/window-manager-bitmap.pbm

The final mv changes the destination name, so run it only after confirming the temporary file is the expected image. If conversion fails, the original destination remains untouched. Remove an incomplete temporary file with rm -- /path/to/window-manager-bitmap.pbm.new after checking its contents and path. That removal is irreversible, so do not add it to an automatic recovery command unless you have identified the exact file.

5. Diagnose the common failures

If the input path is wrong or unreadable, the command exits non-zero and reports the operating system error. Check the path without changing anything:

$ ls -l -- /path/to/window-manager-bitmap.cmuwm
$ test -r /path/to/window-manager-bitmap.cmuwm && echo readable
readable

A missing file produces a failure such as Unable to open file ... for reading. Fix the path or permissions, then rerun the conversion. Do not interpret a created but empty output as a successful image; redirection can create that file before the converter reports an error.

If the PBM dimensions are unexpected, first confirm that the input is really a CMU window manager bitmap and that the file was not truncated during a previous transfer. cmuwmtopbm has no option to resize or repair an input. A successful exit status means that the parser completed; it does not tell you whether the source was the intended bitmap. Keep the original input until the PBM opens correctly in a suitable image viewer or converts successfully with another Netpbm tool.

6. Know what this command does not change

cmuwmtopbm reads the source and emits a new PBM stream. It does not edit the CMU bitmap, install anything, change system configuration or contact a service. No elevated privilege is required for files in your own working directory. If the source is protected and you need administrative access, copy it to a working directory using your normal access procedure, then perform the conversion as your regular user where possible.