Linux Important commands

check disk space in human readable format:

sudo df -h

subdirectory using the most space:

sudo du -h -d 1 /var/

how your process are consuming memory, cpu and which cores are under pressure:

sudo  htop

see process id :

sudo ps aux

    a = show processes for all users
    u = display the process's user/owner
    x = also show processes not attached to a terminal

Use of sed:

sed 's/mikesh/mukesh/'  test.txt

netstat | wc -l