useradd -m -G users,wheel,audio -s /bin/bash <username> creates a user account (default uid=1000)

or if a <uid> and <gid> is known and desired:

useradd -m -G users,wheel,audio -s /bin/bash -u <uid> -g <gid><username>

Creates the user account and its home directory where the files .bash_logout .bash_profile .bashrc and the empty directory .ssh get created.

It uses the file /etc/default/useradd for the defaults. However it also uses settings in /etc/login.defs that contains PASS_MAX_DAYS the maximum number of days a password may be used.

It also assigns a unique number to the user and assigns numbers for its belonging groups.

passwd <username> sets a password

Over time a lot of other directories and files are created in the user account. Most of them are hidden and start therefore with a . character. Some of them can be considered as garbage from no more installed programs or can even contain outdated incompatible data that might cause problems.

To delete a user userdel <username> or including deleting the users data userdel -r <username>

/etc/passwd contains list of users

<user>:<password>:<UID>:<GID>:<comment>:<Home directory>:<Shell>

GID Group ID is the primary group of the user, if the user creates a file, then this is the group id given to the file.

UID User ID

A file belongs to an owner and a group.

In the past the passwords were in this file but now it contains just an x since /etc/passwd is to easy accessible and creates therefore a security risk, so the passwords got moved to /etc/shadow accessible just by root. See man 5 shadow. The file contains additional data defining as expiration date of a password.

Password * means nobody can log in. Nothing means no password and you will get prompted for one when you log in next time.

chown -R <my name> /home/<username> to fix the user name.

groups or groups <username> shows where a user belongs to.

usermod -a -G <group> <username> adds a user to a group.

groupadd -g <gid> <groupname> creates a new group with a give gid

For the groups /etc/group contains the configuration:

<group name>:<password>:<gid>:<list of users>

groups have a password and a group id. The password is usually not used it allowed users to add themselves to other groups knowing the password. This now commonly done by the administrator having root privileges.

Users belong to primary group but can also belong to a supplementary group. Users using this group as supplementary group are added here as well.

The supplementary groups are where the user has access rights, but just the primary group /etc/passwd is the group where files and directories are created. There are different philosophies how groups are assigned:

The first method is more open. Sensitive data should be kept in encrypted directories (as encfs).

The second method that has become the default is more restrictive and can block easily file read access between the users. /etc/login.defs sets this behavior when it contains

USERGROUPS_ENAB yes

Manually changing the user number and group number afterwards is possible but obviously not the standard way to go. usermod is the way.

chgrp -R <primary group name = often username> /home/<username> will assign to all files in the user accounts the group ownership

Passwords can be reset by using a liveCD or mount the physical hard disk on an other computer and delete the passwords in /etc/shadow

Just make the password field empty since this means no password and next time you will be prompted to add a new password.

So change

root:<Some sting>:::::::

to

root::::::::

Keep the other numbers as they are. See man shadow for what they are

Maybe this is not necessary since Linux can read the Window disk (if not encrypted).

fdisk -l shows the disks

The disk must be writable so ntfs-3g /dev/sd<nm> /mnt/windows and repeat this for all the partitions.

cd <...>/Windows/System32/config

chntpw -l SAM shows all Windows users

chntpw -u <username> SAM modifies the user information as clearing the password and unlock the account

Multiple computers on a network exchange usually files between them. To keep it simple make sure that:

The numbers are more important than the names, since they are stored with the individual files. The names are just defined in /etc/passwd and /etc/group.

A new user can therefore be added with the same uid and gid as on an other computer using: useradd -u <uid> -g <gid><username>

Fixing an inconsistency among computer is more work. The user can be assigned to a new uid and gid with:

usermod -u <uid><username>

and

usermod -g <gid><username>

but its files and directories will be keep the old ids and need to be fixed in additional steps.

/etc/login.defs contains behavior data of login as timeouts, retries, …

passwd is the command to change the password. passwd<username> can be used by root to reset/set a user password.

groups show groups where I’m member

groupadd creates new group

useradd -m -G users<username> Adds a new user

usrmod modifies a user

userdel deletes a user

grpmodmodifies a group

groupdel deletes a group

id<username> shows UID and to what group <username> belongs. id does the same with the current user

chown change file owner. The following command sets the <username> to all files in the users home directory: chown -R<username> /home/<username>

chgrp change the primary group ownership. The following command sets the <primarygroup> to all files in the home directory: chgrp -R <primarygroup> /home/<username>. In case <primarygroup> is the same string as <username> the command is chgrp -R <username> /home/<username>

When modifying the important user related files using the propitiate commands creates backup files. Those backup files appear as /etc/<original file>~