Convert a MGR Bitmap to PBM with mgrtopbm

You will convert a MGR bitmap into a PBM image, either from a named file or from standard input, then verify the output without changing the source. Allow about ten minutes if the MGR file is already available. The examples use Netpbm 11.5.2, installed here as package version 2:11.05.02-1.1build1.

You need a Linux shell, the netpbm package and a readable MGR bitmap. The command writes PBM data to standard output. It does not edit the input file and normally needs no elevated privileges.

1. Check the installed command

Confirm the executable and package version before putting it in a script:

$ command -v mgrtopbm
/usr/bin/mgrtopbm
$ dpkg-query -W -f='${Package} ${Version}\n' netpbm
netpbm 2:11.05.02-1.1build1
$ mgrtopbm --help
mgrtopbm: Use 'man mgrtopbm' for help.

The last message is the installed program's help response. The manual defines one positional argument, mgrfile, and no options specific to mgrtopbm. It also says that the command recognises options common to libnetpbm programs. Do not assume that a flag from another image converter belongs here.

2. Convert a named MGR file

Choose a new destination and redirect the PBM output into it:

$ mgrtopbm /path/to/input.mgr > /path/to/output.pbm

A successful conversion normally prints no progress message because the image itself is going to standard output. A zero exit status means the command completed; it is still worth checking that the output exists and describes the dimensions you expect.

$ file /path/to/output.pbm
/path/to/output.pbm: Netpbm image data, size = 640 x 480, rawbits, bitmap
$ head -c 2 /path/to/output.pbm
P4

The wording from file can vary. The useful checks are a non-empty output, the expected width and height, and the PBM binary magic number P4. PBM is a black-and-white bitmap format, not a greyscale or colour format.

3. Test the standard-input form

Omit the filename when another command is supplying the MGR bitmap. This is useful in a pipeline and is the form shown by the manual's optional argument:

$ cat /path/to/input.mgr | mgrtopbm > /path/to/output.pbm

A shell pipeline can hide the exit status of the first command. In Bash, use pipefail when the input command matters:

$ set -o pipefail
$ cat /path/to/input.mgr | mgrtopbm > /path/to/output.pbm
$ printf 'pipeline status: %s\n' "$?"
pipeline status: 0

For a regular file, passing the filename is easier to diagnose because an input-open error is attached directly to mgrtopbm. Use standard input when the bitmap is genuinely produced by an earlier command or arrives from a redirected file descriptor.

4. Keep replacement output recoverable

$ mgrtopbm /path/to/input.mgr > /path/to/output.pbm.new
$ test -s /path/to/output.pbm.new
$ file /path/to/output.pbm.new
/path/to/output.pbm.new: Netpbm image data, size = 640 x 480, rawbits, bitmap
$ mv /path/to/output.pbm.new /path/to/output.pbm

The mv step replaces the destination only after the converter has finished and the temporary file is non-empty. If checking fails, leave the existing output alone and remove only the incomplete temporary file:

$ rm -- /path/to/output.pbm.new

That removal is irreversible, so check the path before running it. There is no need to delete the original MGR bitmap after conversion. Keeping it gives you a clean recovery path if the PBM later proves unusable.

5. Diagnose the common failures

If the input cannot be opened, inspect the path and permissions without changing anything:

$ ls -l /path/to/input.mgr
$ test -r /path/to/input.mgr && echo readable

If the second command prints nothing, fix the path or access permissions through the owner or administrator responsible for the file. Do not make a whole directory world-readable just to get one conversion to work.

If the output is empty or file does not recognise it as PBM, keep the input and capture the command's diagnostic on standard error:

$ mgrtopbm /path/to/input.mgr > /tmp/mgrtopbm-output.pbm 2> /tmp/mgrtopbm-error.txt
$ status=$?
$ printf 'exit status: %s\n' "$status"
$ sed -n '1,20p' /tmp/mgrtopbm-error.txt

Do not treat a file with the right name as a valid image. Check its size, PBM header and dimensions. If a later viewer rejects it, preserve the original MGR file and investigate the source format rather than repeatedly overwriting the same destination.

6. Hand the PBM to the next tool

Once the PBM has passed the checks, pass it to another Netpbm program or image utility that accepts PBM. For example, if pnmtopng is installed, this creates a PNG without touching the MGR source:

$ pnmtopng /path/to/output.pbm > /path/to/output.png
$ file /path/to/output.png

pnmtopng is a separate program and is not an option to mgrtopbm. Keep each conversion as a separate checked step so a failure is easy to locate.