source:
https://www.man7.org/linux/man-pages/man1/intro.1.html
https://access.redhat.com/solutions/505983












// check top10 command you typed
history | awk '{print $2}' | sort | uniq -c | sort -nr | head 
     82 ls
     48 sudo
     23 cd
     16 history
     14 oneko
     12 vi
     10 ps
     10 curl
      8 uname
      8 make















// check system environment variable
env
















// check the default value of PS1
echo $PS1
${PROMPT_START@P}\[\e[${PROMPT_COLOR}m\]${PROMPT_USERHOST@P}\[\e[0m\]${PROMPT_SEPARATOR@P}\[\e[${PROMPT_DIR_COLOR:-${PROMPT_COLOR}}m\]${PROMPT_DIRECTORY@P}\[\e[0m\]${PROMPT_END@P}\$\[\e[0m\]










// change PS1 
//An assignment PS1="What next, master? " would change the prompt as indicated.
export PS1="What next, master? "



/*******************
\u = username
\h = hostname
\W = current working directory
export PS1='[[home]\u@\h \W]\$ '   
*******************/















// mount & umount
Disks and filesystems
       The command mount will attach the filesystem found on some disk
       (or floppy, or CDROM or so) to the big filesystem hierarchy.  And
       umount detaches it again.  The command df will tell you how much
       of your disk is still free.






// To kill a process forcefully:
// kill -9 PID
Processes
       On a UNIX system many user and system processes run
       simultaneously.  The one you are talking to runs in the
       foreground, the others in the background.  The command ps will
       show you which processes are active and what numbers these
       processes have.  The command kill allows you to get rid of them.
       Without option this is a friendly request: please go away.  And
       "kill -9" followed by the number of the process is an immediate
       kill.  Foreground processes can often be killed by typing
       Control-C.






// get more intro:
whatis kill
curl cheat.sh/kill