Some important commands

# Find CPU specifications
cat /proc/cpuinfo

# System information via uname:
uname -r [Kernel release]
uname -a [All information about the kernal]
uname -s [Kernel name]
uname -m [Machine hw name]
uname -p [Processor type]
uname -o [OS]
uname -n [Node name]

# Running kernel and compiler:
cat /proc/version

# X server version:
X -showconfig or Xorg -version

# List of Kernel modules loaded:
lsmod

# Memory and swap information
cat /proc/meminfo
free
free -m [in MB]

# Hard drives information:
fdisk -l

# Free/used drive space or mount points:
df -h

# Distribution ??
cat /etc/issue
cat /etc/issue.net

# Finding or locating files
find
locate
which
whereis

# Log messages as they happen:
tail -f /var/log/messages (shows last 10 lines).

# What processes are running [concise information]:
ps -A
ps -A | grep <PROCESS_NAME/PID> [For specific process]

# What process are running [Brief information]:
ps -ef
ps -ef | grep -i <PROCESS_NAME/PID> [For specific process]

# Environment list:
env | more

# Current userid and assigned groups:
id

# All command aliases for the current user:
alias

# Packages installed:
rpmquery --all | less
rpmquery --all  <NAME>
rpmquery --all | grep -i <NAME>
rpm -qa | grep <NAME>

# Present working directory
pwd

# Compiler version
gcc -v
gcc --version

# More information about the commands and sub options:
man <COMMAND_NAME>
man sudo