out

root@lethe # mandb --debug
From the config file /nix/store/rwlpxkjpmrvh5d478brpk8hr3s7j1yy8-man-db-2.7.5/etc/man_db.conf:

Mandatory mandir `/usr/man'.
Mandatory mandir `/run/current-system/sw/share/man'.
Mandatory mandir `/run/current-system/sw/share/man'.
Path `/bin' mapped to mandir `/run/current-system/sw/share/man'.
Path `/usr/bin' mapped to mandir `/run/current-system/sw/share/man'.
Path `/sbin' mapped to mandir `/run/current-system/sw/share/man'.
Path `/usr/sbin' mapped to mandir `/run/current-system/sw/share/man'.
Path `/usr/local/bin' mapped to mandir `/usr/local/man'.
Path `/usr/local/bin' mapped to mandir `/run/current-system/sw/share/man'.
Path `/usr/local/sbin' mapped to mandir `/usr/local/man'.
Path `/usr/local/sbin' mapped to mandir `/run/current-system/sw/share/man'.
Path `/usr/X11R6/bin' mapped to mandir `/usr/X11R6/man'.
Path `/usr/bin/X11' mapped to mandir `/usr/X11R6/man'.
Path `/usr/games' mapped to mandir `/run/current-system/sw/share/man'.
Path `/opt/bin' mapped to mandir `/opt/man'.
Path `/opt/sbin' mapped to mandir `/opt/man'.
Global mandir `/usr/man', catdir `/var/cache/man/fsstnd'.
Global mandir `/run/current-system/sw/share/man', catdir `/var/cache/man'.
Global mandir `/usr/local/man', catdir `/var/cache/man/oldlocal'.
Global mandir `/run/current-system/sw/share/man', catdir `/var/cache/man/local'.
Global mandir `/usr/X11R6/man', catdir `/var/cache/man/X11R6'.
Global mandir `/opt/man', catdir `/var/cache/man/opt'.
Added section `1'.
Added section `n'.
Added section `l'.
Added section `8'.
Added section `3'.
Added section `2'.
Added section `5'.
Added section `4'.
Added section `9'.
Added section `6'.
Added section `7'.
`/usr/man'	`'	`1'
`/run/current-system/sw/share/man'	`'	`1'
`/run/current-system/sw/share/man'	`'	`1'
`/bin'	`/run/current-system/sw/share/man'	`0'
`/usr/bin'	`/run/current-system/sw/share/man'	`0'
`/sbin'	`/run/current-system/sw/share/man'	`0'
`/usr/sbin'	`/run/current-system/sw/share/man'	`0'
`/usr/local/bin'	`/usr/local/man'	`0'
`/usr/local/bin'	`/run/current-system/sw/share/man'	`0'
`/usr/local/sbin'	`/usr/local/man'	`0'
`/usr/local/sbin'	`/run/current-system/sw/share/man'	`0'
`/usr/X11R6/bin'	`/usr/X11R6/man'	`0'
`/usr/bin/X11'	`/usr/X11R6/man'	`0'
`/usr/games'	`/run/current-system/sw/share/man'	`0'
`/opt/bin'	`/opt/man'	`0'
`/opt/sbin'	`/opt/man'	`0'
`/usr/man'	`/var/cache/man/fsstnd'	`-1'
`/run/current-system/sw/share/man'	`/var/cache/man'	`-1'
`/usr/local/man'	`/var/cache/man/oldlocal'	`-1'
`/run/current-system/sw/share/man'	`/var/cache/man/local'	`-1'
`/usr/X11R6/man'	`/var/cache/man/X11R6'	`-1'
`/opt/man'	`/var/cache/man/opt'	`-1'
`1'	`'	`-5'
`n'	`'	`-5'
`l'	`'	`-5'
`8'	`'	`-5'
`3'	`'	`-5'
`2'	`'	`-5'
`5'	`'	`-5'
`4'	`'	`-5'
`9'	`'	`-5'
`6'	`'	`-5'
`7'	`'	`-5'
/run/current-system/sw/bin/mandb: warning: $MANPATH set, prepending /nix/store/rwlpxkjpmrvh5d478brpk8hr3s7j1yy8-man-db-2.7.5/etc/man_db.conf

path directory /nix/store/b3ysnlwxcacrf427w1v34h6gmhjb9f2a-groff-1.22.3/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /root/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /opt/bin is in the config file
warning: /opt/man: No such file or directory

path directory /root/.local/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /usr/lib/ccache/bin/ is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /root/.cargo/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /home/phg/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /opt/bin is in the config file
warning: /opt/man: No such file or directory

path directory /home/phg/.local/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /usr/lib/ccache/bin/ is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /home/phg/.cargo/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /nix/store/vlbqv3g7978z55smh8nnj2f4fbk16hi8-compton-0.1_beta2.5/bin is not in the config file
but does have a ../man, man, ../share/man, or share/man subdirectory
adding /nix/store/vlbqv3g7978z55smh8nnj2f4fbk16hi8-compton-0.1_beta2.5/share/man to manpath

path directory /nix/store/cyzwflbwhdzy6c621jk9vg43x93dr98l-unclutter-8/bin is not in the config file
but does have a ../man, man, ../share/man, or share/man subdirectory
adding /nix/store/cyzwflbwhdzy6c621jk9vg43x93dr98l-unclutter-8/share/man to manpath

path directory /nix/store/zgvyiv6mhxxc1r0ynf54mcx2112j9j7k-procps-3.3.12/bin is not in the config file
but does have a ../man, man, ../share/man, or share/man subdirectory
adding /nix/store/zgvyiv6mhxxc1r0ynf54mcx2112j9j7k-procps-3.3.12/share/man to manpath

path directory /nix/store/yjd47a4rv7mbpvj6x75hpyssi60hhcn0-iproute2-4.13.0/bin is not in the config file
but does have a ../man, man, ../share/man, or share/man subdirectory
adding /nix/store/yjd47a4rv7mbpvj6x75hpyssi60hhcn0-iproute2-4.13.0/share/man to manpath

path directory /nix/store/mhhb2r53yln5hqgphg7l0n90676nzzpx-coreutils-8.28/bin is not in the config file
but does have a ../man, man, ../share/man, or share/man subdirectory
adding /nix/store/mhhb2r53yln5hqgphg7l0n90676nzzpx-coreutils-8.28/share/man to manpath

path directory /nix/store/qzrwcsnrhlqcqf6zan5f2w16x8141hkd-curl-7.56.1-bin/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /nix/store/rdcl0gyajxavgxhfznsm0kvvb4nw4mgj-alsa-utils-1.1.4/bin is not in the config file
but does have a ../man, man, ../share/man, or share/man subdirectory
adding /nix/store/rdcl0gyajxavgxhfznsm0kvvb4nw4mgj-alsa-utils-1.1.4/share/man to manpath

path directory /nix/store/v2sa14rgs4ac86vqzcsvdg63cs4kzbv1-findutils-4.6.0/bin is not in the config file
but does have a ../man, man, ../share/man, or share/man subdirectory
adding /nix/store/v2sa14rgs4ac86vqzcsvdg63cs4kzbv1-findutils-4.6.0/share/man to manpath

path directory /nix/store/ghpl76prlfyy8slmj90znx22yckdxr7f-xterm-330/bin is not in the config file
but does have a ../man, man, ../share/man, or share/man subdirectory
adding /nix/store/ghpl76prlfyy8slmj90znx22yckdxr7f-xterm-330/share/man to manpath

path directory /home/phg/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /opt/bin is in the config file
warning: /opt/man: No such file or directory

path directory /home/phg/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /run/wrappers/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /etc/per-user-pkgs/phg/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /home/phg/.nix-profile/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /nix/var/nix/profiles/default/bin is not in the config file
and doesn't have ../man, man, ../share/man, or share/man subdirectories

path directory /run/current-system/sw/bin is not in the config file
but does have a ../man, man, ../share/man, or share/man subdirectory
adding /run/current-system/sw/share/man to manpath

path directory /usr/local/bin is in the config file
warning: /usr/local/man: No such file or directory
/run/current-system/sw/share/man is already in the manpath

adding mandatory man directories

warning: /usr/man: No such file or directory
/run/current-system/sw/share/man is already in the manpath
/run/current-system/sw/share/man is already in the manpath
/usr/man:/run/current-system/sw/share/man:/usr/local/man:/run/current-system/sw/share/man reduced to /usr/man:/run/current-system/sw/share/man:/usr/local/man
manpath=/usr/man:/run/current-system/sw/share/man:/usr/local/man:/usr/X11R6/man:/opt/man
warning: /usr/man: No such file or directory
adding /run/current-system/sw/share/man to manpathlist
warning: /usr/local/man: No such file or directory
warning: /usr/X11R6/man: No such file or directory
warning: /opt/man: No such file or directory
final search path = /run/current-system/sw/share/man
failed to open /var/cache/man/13238 O_RDONLY
create_db(/run/current-system/sw/share/man): /var/cache/man/13238
Testing /run/current-system/sw/share/man for new files
Examining man0p
	subdirectory man0p has been 'modified'
Processing manual pages under /run/current-system/sw/share/man...
/run/current-system/sw/bin/mandb: warning: cannot create catdir /var/cache/man
warning: cannot create catdir /var/cache/man
/run/current-system/sw/bin/mandb: can't create index cache /var/cache/man/13238: No such file or directory
/run/current-system/sw/bin/mandb: can't update index cache /var/cache/man/13238: No such file or directory
done.