List applications listening on ports

netstat -lnp

List running processes

ps aux

List running processes and search for a particular process name

ps aux | grep <process name>

Search for text within a set of files

grep "text string" *.*

Search for text within a set of files, case insensitive

grep -i "text string" *.*

Search for text within a set of files and display X lines before and after match

grep -C 2 "text string" *.*

Check the current directory

pwd

Get BIOS system information

sudo dmidecode

On Debian distros, you can use apt-get to find available package versions using:

apt-cache madison <package name>

To install a specific version of a package, use the above command to get version info then:

sudo apt install <package name>=<version>

And finally using apt-mark, you can hold onto a specific package version:

sudo apt-mark hold <package name>

To remove old kernel packages from the boot partion on Ubuntu you can use the following command:

sudo apt remove --purge $(dpkg -l 'linux-image-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d')