操作方法
###how to use emacs ctrl+b move back on character ctrl+f move forward one character del delete the character to the left of the cursor
ctrl+d delete the character underneath the cursor ctrl+a move to the start of the line ctrl+e move to the end of the line
esc-f move forward a word esc-b move backward a word ctrl+l clear the screen,reprint the current line at the top ctrl+k kill or delete the text from the current cursor position to the end of the line
esc-d kill from te cursor to the end of the current word,or if between words,to the end of the next word esc-del kill from the cursor the start of the previous word,or if between words,to the start of the previous word ctrl+w kill from the cursor to the previous white space.This differs from esc-del because the boundaries separating words differ.
### fc -l(low case letter L) history-number to edit the history command
##to display the environment variables echo $PATH echo $PS1