CNT-2311-Chapter 7 Notes

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Linux Usernames

Consist of any combination of upper and lowercase letters, numbers and many punctuation symbols.

User names must begin with a letter and are case sensitive

Usernames may consist of 32 characters but may truncate a username to only 8 characters.


Users are defined not by their name but by a UID (User Id).

First hundred IDs saved for system use.

To remove user accounts use the following userdel commands

userdel -r causes system to remove all files from user home directory

userdel -f -r forces the system to remove all files

To find files owned by a certain User use the find command

find / -uid 529, this should list all files owned by that UID.


useradd username is the simplest command to add a user.

-c comment parameter passes public information like phone number or real name

-d home-dir specify an accounts home directory

-e YYYY-MM-DD expiration date for the account to be disabled

-f inactive days set the number of day an account is disabled after a password expires

-g default group Set the default group

-G group Adds additional groups, multiple groups can be add with commas.

-s Sets default shell login for the user.

-p encrypted-password uses a pre-encrypted password for the user to the system


  1. useradd -d /home4/roger -g project4 -G project1, project4 -s /bin/tcsh sally
  2. passwd pwnd


Passwd is much easier to use to set or change a passwd for a user.

passwd commands

-k indicates system should update an expired account

-l locks an account by prefixing the encrypted password with an exclamation mark

-u unlocks an account by removing the prefixed exclamation mark

-d removes the password from an account

-S displays account information about the password

Many passwd parameters can only be run in root.


Usermod and useradd are very similar, a very noteable difference is that Usermod allows you to edit existing accounts.

usermod allows -m parameter to be run with -d. This changes a user directory and moves the files to that new directory.

usermod allows you to change a users login name with the -l parameter, IE usermod bilbo -l bbaggins. bilbo > bbaggins


Groups are a collection of accounts mainly for security reasons.

Every file on a Linux system is associated with a specific group and different permissions can be assigned to various members of that group.

Group membership is controlled through /etc/group file.

Each user has a primary group that is set in the /etc/passwd file.

To change from one group to another type newgrp (desired group).

Adding Groups

The groupadd command is used to add new groups

group [-g GID [ -o]] [-r] [-f]

-g specifies desired GID

-r Creates a sub-500 GID

-f Force creation


  1. groupadd project3

To delete groups use the groupdel command.

System Log Files

The traditional system logger is syslogd, often installed from a package called sysklogd.

To manually log data you would use the logger command the following are syntax used with logger

-i Records the loggers process ID number

-s Echos data to a standard error

-d Causes the logger to use datagrams instead of stream

-f filename command to log the contents of a file.

-p pri Speciffy prority.

System Time

To set the system time you will have to use the data parameter with a combination of the following syntax

date [-u|--utc|--universal] [MMDDhhmm[[CC]YY}[.ss]]

By default date assumes you are trying to set local time. If you want to set the universal include the -u or -utc.

-r or --show will display the set time for the hardware clock

--set and --date=newdate sets the hardware clock manually

--systohc sets the hardware clock based on the software clock

--hctosys sets the software clock based on the hardware clock

--UTC or --localtime will tell the hardware clock hold that specific time.


  1. date 01011630

This will set the date to January 1st at 4:30pm