Convert a MGR Bitmap to PBM with mgrtopbm

An old MGR bitmap turns up and nothing modern opens it, so mgrtopbm converts it to a PBM image any Netpbm tool can read. Allow about ten minutes if the MGR file is already to hand. 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, never edits the input file, and normally needs no elevated privileges.

1. Check the installed command

Confirm the executable and package version before you put 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.

That last line is the installed program's actual help response. The manual defines one positional argument, mgrfile, and no options specific to mgrtopbm itself, though it does recognise options common to libnetpbm programs. Don't assume 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 what's going to standard output. A zero exit status means the command completed, but it's still worth checking that the output exists and matches 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 checks that matter 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 greyscale or colour.

3. Test the standard-input form

Skip the filename when another command is supplying the MGR bitmap. This is handy in a pipeline, and it's 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 it. In Bash, use pipefail when the input command's success actually 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 directly is easier to diagnose, because an input-open error is then attached straight to mgrtopbm. Use standard input only when the bitmap genuinely comes from an earlier command or 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 only replaces the destination once the converter has finished and the temporary file is non-empty. If a check 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 you run it. There's no need to delete the original MGR bitmap after conversion: keeping it gives you a clean way back if the PBM later turns out unusable.

5. Diagnose the common failures

If the input can't 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 that second command prints nothing, fix the path or access permissions through the file's owner or administrator. Don't make a whole directory world-readable just to get one conversion working.

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

$ 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

Don't treat a file with the right name as a valid image. Check its size, its PBM header and its 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 on 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 at all:

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

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