munin-run is a command line tool to run a munin plugin manually. The script must be run with sufficient privileges, usually as root, as it switches to either munin-node's default user (often "nobody") or any user specifically configured to run the given plugin.

Running munin-run with the --debug flag will often give plenty of hints on what might be wrong.

# munin-run --debug df
# file: 'df'
# Want to run as euid/egid 65534/108
# Running as uid/gid/euid/egid 65534/108 108/65534/108 108
DEBUG: About to exec "/etc/munin/plugins/df".
_dev_hda1.value 83