I am logged under 'achille' achille belongs to 2 groups:

groups achille 
achille: achille users 

I want to change the group owner of file 'tree.test' to group 'users'

ls -l tree.test
-rw-r--r--. 1 achille achille 2512881 Dec  1 11:53 tree.test

I have got full control over my home dir:

ls -ld $HOME
drwx------. 15 achille achille 4096 Dec 10 11:51 /home/achille

I try:

chown :users tree.test
chown: changing group of 'tree.test': Operation not permitted

I try:

chgrp users tree.test
chgrp: changing group of 'tree.test': Operation not permitted

ok, so I decide to have as primary group the group 'users'

usermod command is available to achille, so I use it (right?):

usermod -g users achille

usermod: Permission denied.

usermod: cannot lock /etc/passwd; try again later

I log as root to perform this operation (changing primary group)

su - 

password:

then

usermod -g users achille

and check it out:

id -gn achille 
users

then ctrl + D to log back to achille

the file finally belongs to groups 'users' but the result is bad because my home dir and all files/dirs inside also belong to group users.

I finally found a way (without beeing logged as root) with newgrp command:

newgrp users

id -gn 

users

touch file0

file0 will have users as group owner

but it looks like newgrp works as long as achille belongs to the group you want to change to otherwise you 've got this error message:

newgrp games 

Password: (what password to put here?)

newgrp: failed to crypt password with previous salt: Invalid argument ????

so now I am just wondering (and asking), if chgrp,chown,usermod are worth keeping in tools bag beeing an unprivileged user. thanx folks!