Often Used

tail -f file check files that change often(log file)
head -2 file.txt first 2 lines
tail -1 file.txt last line
find /sbin -name *v find a file in sbin which ends in v. -iname means case insensitive
rmdir [-p] directory remove a directory
rm -rf directory recursively removes directory
mkdir -p dir1/dir2/dir3 -p represents creating parents
sudo -i login as root
echo "cd ~/Desktop/Java\ Files" >> ~/.bashrc set default directory in ubuntu
ps -fA | grep python 查看所有python进程
killall -9 python kill所有python进程，9代表force
cd change to HOME directory
ls -a list all hidden files
man ls display the manual page for the item
ls -l shows file or directory, size, long listing
ls -al long listing including hidden files
echo $PATH show path variables
!$ the last argument of the previous command
!! the last command
history print history command
![command id] or !-2(last last) execute command id
echo *.jpeg print files of a specific kind
file [filename] what is the type of this file
less [filename] see what is in filename = more [filename]
cat [filename] see the content of the file
cat是一次性显示整个文件的内容，还可以将多个文件连接起来显示，它常与重定向符号配合使用，适用于文件内容少的情况；
more和less一般用于显示文件内容超过一屏的内容，并且提供翻页的功能。more比cat强大，提供分页显示的功能，less比more更强大，提供翻页，跳转，查找等命令。而且more和less都支持：用空格显示下一页，按键b显示上一页。
ctrl + L = clear can be used when typing
ctrl + A move cursor to the command beginning
ctrl + E move cursor to the command end
echo "Test Page" > testpage type into testpage file

curl
curl --user name:password http://somesite.com -v login
curl --data "Username=xx&Password=xx&Login=Login" [url that your form submits] login
curl --user user:pass --cookie-jar ./somefile https://xyz.com/a save cookie to file
curl --cookie ./somefile https://xyz.com/b read cookie from file

ls -latr long listing including all files reverse sorted by time
ls -t list files by time, most recent first
ls -r reverse order
ls -R list directories recursively
tree list tree structure of the directory and files
tree -d only directory structure

ls -l "my notes.txt" Use double or single quotes to avoid space confusion

• ls -F list files by type:
  / is a directory
  @ is a symbolic link
  * is an executable

Permissions

- regular file
d directory
l symbolic link

-rw-r--r--
first - is permission type
next three for user
next three for group
next three for other

chmod change mode
ugoa user category: user, group, other, all
+-= add, substract or set permissions
rwx read, write, execute

chmod g+rwx sales.data add rwx permissions to sales.data
chmod g+wx, u+rwx file, o= add to multiple groups, here o is ---

Permission order is in rwx.
Avoid 777(rwx) and 666(rw-).
chmod 400 my-cat400 only read permission for user, others no permissions
500 read + execute

chgrp sales sales.data change group to sales for this sales.data file

VIM

vi [file]
vim [file] strengthed vi
view [file] read mode

command mode
Use Esc to enter command mode
k,j,h,l up, down, left, right
^ go to the beginning
$ go to the end

insert mode
i insert at the cursor position
I insert at the beginning of the line
a append after the cursor position
A append at the end of the line

line mode
start with :
:w saves the file
:w! forces the file to be saved
q! quit without saving
wq! write and quit
:15 go to the 15th line
:$ position the cursor on the last line
:set nu turn on line numbering
:set nonu turn off line numbering

repeating commands
8k move up 8 lines

deleting text
x delete a character
dw delete a word
dd delete a line
D delete from the current position

changing text
r replace the current character
cw change the current word
cc change the current line
c$=C change the text from the current position
~ reverse the case of a character

copy & paste
yy copy the current line
y3w copy 3 words from the current position
p paste the most recent deleted or copied text

undo/redo
u undo
Ctrl+R redo

search
/the forward searching the word "the", n to go to the next item, N to go to the previous item

Manipulate files

rm file remove file
rm -r dir remove the directory and its contents recursively
rm -f file force removal
cp src_file dest_file
cp src_file [src_files] dest_dir
cp -r src_dir des_dir recursively copy from directory
mv src_file des_dir
mv src_file des_file
mv -i, cp -i interactive mode

tar archive
tar cxtvzf tarfile
c: create a tar archive
x: extract files from the archive
t: display the table of content
v: be verbose
z: use compression
f file use this file
tar cf target.tar [dir] - create a target.tar with dir content
tar tf tps.tar list content
tar xf target.tar extract content
tar xvf target.tar verbose version(list what is being extracted)
tar zcf target.tar.gz(tgz) directory create compressed directory

compression
gzip compress files
gunzip uncompress files
du -k file display sizes in kilobytes
du -h file display sizes in human readable format