# Display messages in kernel ring buffer
dmesg
# Display CPU information
cat /proc/cpuinfo
# Display memory information
cat /proc/meminfo
# Display free and used memory ( -h for human readable, -m for MB, -g for GB.)
free -h
# Display PCI devices
lspci -tv
# Display USB devices
lsusb -tv
# Display DMI/SMBIOS (hardware info) from the BIOS
dmidecode
# Show info about disk sda
hdparm -i /dev/sda
# Perform a read speed test on disk sda
hdparm -tT /dev/sda
# Test for unreadable blocks on disk sda
badblocks -s /dev/sda
# Display the user and group ids of your current user.
id
# Display the last users who have logged onto the system.
last
# Show who is logged into the system.
who
# Show who is logged in and what they are doing.
w
# Create a group named "test".
groupadd test
# Create an account named john, with a comment of "John Smith" and create the user's home directory.
useradd -c "John Smith" -m john
# Delete the john account.
userdel john
# Add the john account to the sales group
usermod -aG sales john
# Search for pattern in file
grep pattern file
# Search recursively for pattern in directory
grep -r pattern directory
# Find files and directories by name
locate name
# Find files in /home/john that start with "prefix".
find /home/john -name 'prefix*'
# Find files larger than 100MB in /home
find /home -size +100M