CrazyRails ~ Flex on Ruby on Rails

Here’s a quick cheat-sheet for Unix user administration from the command line terminal. Make sure you are logged in as “root” user for these, or enter “sudo -i” for root user access.

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useradd - Add a new user

Options:

• -d home directory
• -s starting program (shell)
• -p password
• -g (primary group assigned to the users)
• -G (Other groups the user belongs to)
• -m (Create the user’s home directory

Example: To add a new user with

• a primary group of users
• a second group mgmt
• starting shell /bin/bash
• password of xxxx
• home directory of super
• create home directory
• a login name of super

useradd -gusers -Gmgmt -s/bin/shell -pxxxx -d/home/super -m super

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usermod - Modify an existing user

Options:

• -d home directory
• -s starting program (shell)
• -p password
• -g (primary group assigned to the users)
• -G (Other groups the user belongs to)

Example: To add the group ‘others’ to the user super

usermod -Gothers super

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userdel - Delete a user

Options:

• -r (remove home directory)

Example: To remove the user ’super’ and his home directory

userdel -r super

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passwd - (Re)set user’s password

Options:

• username (Only required if you are root and want to change another user’s password)

Example: To change the password for the account you are currently logged in as…

passwd
Enter existing password
Enter new password
Enter new password again (to validate)

Example: To change the password for ’super’ user while logged in as ‘root’:

passwd super
Enter existing password (can be either super’s or root’s password)
Enter new password
Enter new password again (to validate)

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su - Switch user

To switch to another user, use the su command. This is most commonly used to switch to the root account.

Example: To switch to root account:
su
Enter root’s passwd

Example: To switch to the user ’super’:
su super
Enter super’s or root’s passwd

To return to original user, enter exit

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Where is user and group information stored on Linux?

Usernames and primary groups are stored in /etc/passwd. This file can be directly edited using the “nano” editor, although this is not recommended. The format for the file is:

• Username (normally all lower case)
• Password (encrypted - only contains the letter ‘x’)
• User ID (a unique number of each user)
• Primary Group ID
• Comment (Normally the person’s full name)
• Home directory (normally /home/<username>
• Default shell (normally /bin/bash)

Each field is separated by a colon.

Passwords for each user are stored in /etc/shadow. This file should only be changed using the passwd command.

Group information is stored in /etc/group. This file can be directly edited using the “nano” editor. The format for the file is:

• Group name
• Group password (hardly ever used)
• Group ID
• Usernames (separated by commas)

Each field is separated by a colon.

Default files

When a new user is created, the default files and directories that are created are stored in /etc/skel.

This directory can be modified to fit your needs. Modifications only effect new users and does not change anything for existing users.