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System Administration

📖 Ubuntu Server Guide - Server administration documentation 📖 RHEL System Admin Guide - RHEL documentation

User and Group Management

User Management Commands

# Create user
useradd username                    # create user (minimal)
useradd -m -s /bin/bash username    # create with home dir and shell
useradd -m -G sudo,developers user # create with groups

# Modify user
usermod -aG groupname username      # add user to group (-a = append)
usermod -s /bin/zsh username        # change shell
usermod -L username                 # lock account
usermod -U username                 # unlock account

# Delete user
userdel username                    # delete user only
userdel -r username                 # delete user + home directory

# Password
passwd username                     # set/change password
passwd -l username                  # lock password
passwd -u username                  # unlock password

# User information
id username                         # show UID, GID, groups
whoami                              # current username
who                                 # logged-in users
last                                # login history

Group Management

groupadd groupname                  # create group
groupmod -n newname oldname         # rename group
groupdel groupname                  # delete group
groups username                     # show user's groups

Important User Files

File Purpose Format
/etc/passwd User accounts username❌UID:GID:comment:home:shell
/etc/shadow Encrypted passwords username#⃣...
/etc/group Group definitions groupname❌GID:members
/etc/sudoers Sudo configuration Edit with visudo only 
# /etc/passwd example:
alice:x:1001:1001:Alice Smith:/home/alice:/bin/bash

# Fields: username:password(x=in shadow):UID:GID:comment:home:shell
# UID 0 = root, 1-999 = system, 1000+ = regular users

sudo

sudo command                        # run as root
sudo -u user command                # run as specific user
sudo -i                             # start root shell
visudo                              # safely edit /etc/sudoers

sudoers entry: 

# User privilege specification
alice ALL=(ALL:ALL) ALL             # full sudo access
bob ALL=(ALL) /usr/bin/systemctl    # only systemctl
%admin ALL=(ALL) ALL                # admin group gets full access

File Permissions and Ownership

Understanding Permissions

-rwxr-xr-x  1  alice  staff  4096  Jan 15 10:30  myfile.txt
|type|owner|group|other| links|owner|group|size|  date     | name

Type: - (file), d (directory), l (symlink)
r = read (4)
w = write (2)
x = execute (1)

chmod - Change Permissions

Numeric (Octal): 

chmod 755 file     # rwxr-xr-x (owner: rwx, group: r-x, others: r-x)
chmod 644 file     # rw-r--r-- (owner: rw-, group: r--, others: r--)
chmod 600 file     # rw------- (owner: rw-, group: ---, others: ---)
chmod 700 dir      # rwx------ (owner: rwx)
chmod 777 file     # rwxrwxrwx (everyone: rwx) - avoid in production

Common Permission Patterns: | Octal | Symbolic | Use Case | |-------|----------|----------| | 755 | rwxr-xr-x | Executables, directories | | 644 | rw-r--r-- | Regular files | | 600 | rw------- | Private files (SSH keys) | | 700 | rwx------ | Private directories (.ssh) | | 664 | rw-rw-r-- | Shared group files | | 775 | rwxrwxr-x | Shared group directories |

Symbolic: 

chmod u+x file     # add execute for owner
chmod g-w file     # remove write for group
chmod o=r file     # set others to read only
chmod a+r file     # add read for all (a = all)
chmod -R 755 dir/  # recursive

chown - Change Ownership

chown user file              # change owner
chown user:group file        # change owner and group
chown :group file            # change group only
chown -R user:group dir/     # recursive

Special Permissions

Permission Octal Effect on Files Effect on Directories
SUID 4000 Run as file owner -
SGID 2000 Run as file group New files inherit group
Sticky 1000 - Only owner can delete files 
chmod 4755 file    # SUID + rwxr-xr-x
chmod 2775 dir     # SGID + rwxrwxr-x
chmod 1777 /tmp    # Sticky + rwxrwxrwx (like /tmp)

Process Management

Viewing Processes

ps aux                      # all processes (BSD format)
ps -ef                      # all processes (System V format)
top                         # interactive process viewer
htop                        # enhanced interactive viewer (if installed)
pgrep processname           # find PID by name

ps aux output columns: | Column | Description | |--------|-------------| | USER | Process owner | | PID | Process ID | | %CPU | CPU usage | | %MEM | Memory usage | | STAT | Process state (R=running, S=sleeping, Z=zombie) | | COMMAND | Command that started the process |

Managing Processes

kill PID                    # send SIGTERM (graceful stop)
kill -9 PID                 # send SIGKILL (force stop)
kill -HUP PID               # send SIGHUP (reload config)
killall processname         # kill all processes by name

Key Signals: | Signal | Number | Action | |--------|--------|--------| | SIGHUP | 1 | Reload configuration | | SIGINT | 2 | Interrupt (Ctrl+C) | | SIGKILL | 9 | Force kill (cannot be caught) | | SIGTERM | 15 | Graceful termination (default) |

Background and Foreground

command &                   # run in background
Ctrl+Z                      # suspend foreground process
bg                          # resume in background
fg                          # bring to foreground
jobs                        # list background jobs

Service Management (systemd)

systemctl Commands

systemctl start service     # start service
systemctl stop service      # stop service
systemctl restart service   # restart service
systemctl reload service    # reload config (no restart)
systemctl enable service    # start at boot
systemctl disable service   # do not start at boot
systemctl enable --now svc  # enable AND start
systemctl status service    # show status
systemctl is-active service # check if running
systemctl is-enabled svc    # check if enabled at boot
systemctl list-units --type=service  # list services

Targets (Runlevels)

Target Description Old Runlevel
poweroff.target Shutdown 0
rescue.target Single user 1
multi-user.target Text mode, network 3
graphical.target GUI 5
reboot.target Reboot 6 
systemctl get-default               # show boot target
systemctl set-default multi-user.target  # set default

Log Management

System Logs

journalctl                  # all system logs
journalctl -u service       # logs for specific service
journalctl -f               # follow (live tail)
journalctl -p err           # errors and above
journalctl --since today    # today's logs

Key Log Files: | File | Content | |------|---------| | /var/log/syslog | General system log (Debian) | | /var/log/messages | General system log (RHEL) | | /var/log/auth.log | Authentication events | | /var/log/kern.log | Kernel messages | | /var/log/dmesg | Boot and hardware messages |

System Monitoring

df -h                       # disk space usage
du -sh /path                # directory size
free -h                     # memory usage
uptime                      # system uptime and load
uname -a                    # system information
hostname                    # system hostname
lscpu                       # CPU information
lsblk                       # block devices (disks)

Key Facts for the Exam

  1. useradd -m creates the home directory, without -m it may not
  2. usermod -aG group user - the -a flag is critical (append, not replace)
  3. Permission 755 = rwxr-xr-x, 644 = rw-r--r--, 600 = rw-------
  4. Root user UID = 0, regular users start at 1000
  5. systemctl enable --now enables and starts in one command
  6. SIGTERM (15) is graceful, SIGKILL (9) is forced
  7. /var/log contains system logs
  8. visudo is the safe way to edit sudoers (syntax checking)