Linux CLI

Linux is case-sensitive

3 main streams utilized in CLI:

• stdin = 0
• stdout = 1
• stderr = 2

Text Manipulation
grep
cut

• cut -d = delimiter
• cut -f#,#,# = fields to print

sed
awk

Shells

• bash: /bin/bash /usr/bin/bash
• cshell
• shell: /bin/sh
• tshell: /bin/tsh
• zshell: /bin/zsh

Declaring Variables
NOW=$(date)

• to call this variable, $NOW

Double quotes expand special characters

• echo "The date is $NOW" will produce the variable
  Single quotes do not expand special characters
• echo 'The date is $NOW' will produce $NOW

\ = one literal character

history
!# reruns # command
!! rerun last command

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.bashrc

• Can store custom functions

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Managing Users and Groups

su - Log in as specific user

• "-" changes environmental variables to the new account

sudo - run command with elevated privileges

• /etc/sudoers file controls sudo rights

UID - Account identifier that is unique to the system

• UID can be changed however you want
• UID of root is 0
  wheel - A group that is allowed to utilize sudo

3 Types of user accounts:

• root
• standard
  • Can assume root privileges if required (wheel group/sudoer) or explicit sudo commands
  • Least privilege applies
• service (daemon accounts)
  • Can authenticate but not get shell
    • set shell to /sbin/nologin or /bin/false
  • Usually owns configuration files and or executables

/etc/passwd contains account information

• 'x' in field 2 = use the shadow password
• User:PasswordHash:UID:GID:FullName:HomeDir:Shell

/etc/shadow requires root privileges to read

/etc/login.defs declares default configurations for new user accounts

• Password policies
• UID/GID declarations
• Password encryption

/etc/skel "Default User" profile copied at account creation only.

chfn (change finger/change full name)

useradd -p WILL NOT encrypt the user password
passwd is used to set a users password
passwd -u = unlock
passwd -l = lock

usermod -U = unlock
usermod -L = lock

chage (change aging)

• E = Set expiration date
• l = Password aging information
• M = Max password age
• m = Minimum password age
• W = Change password expiration warning

usermod (Add users to groups):

• e = expiration date
• aG = Add user to group and keeps them in their existing groups
• l = change user login name

userdel -r user1 = remove profile and directories associated

/etc/group contains groups, ids, and members
group user1 = view user1 groups
groupadd
groupmod = can set password on groups for temporary membership
groupdel

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User Profiles

Created from /etc/skel and read at login (can force rereade with . .bashrc):
~/.bash_logout
~/.bash_profile = Only read with the first login. Good for environmental variables.
~/.bashrc = Custom functions per profile

/etc/profile
/etc/bashrc = Custom functions globally defined

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LAB

useradd -D user1 = View default settings for new users

useradd -c "Comment" user1 = Add comment to user account

useradd -e 2025/12/31 user1 = Add expiration date to user account

passwd user1 = Add password to user account

usermod -c "Comment" user1 = Add comment to user account

chage -l user1 = List password information for user account

chage -E 2026/12/31 user1 = Change password expiration for user account

passwd -l user1 = Lock user account

passwd -u user1 = Unlock user account

usermod -L user1 = Lock user account

usermod -U user1 = Unlock user account

userdel user1 = Delete user account

userdel -r user1 = Delete user account and associated directories

groupadd Group1 = Add group

groupmod -n NewName Group1 = Add new group name to exisiting group

usermod -aG Group1 user1 = Add user account to group

groupdel Group1 = Delete group but not users within group

who = Display logged in users

w = Display logged in users

last = Display recent logins