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Essential unix:
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Unix regards all data as "files". Files consists of records
which terminate with a "Line Feed" (\n or \012).
The end of the file is "control D".

[Windows uses "\r\n" to mark the end of the line.
To convert Unix files to DOS files use dos2unix or unix2dos]

# Moving around directories 
$ cd ~	#go to the top level
$ pwd

$ tree 

$ mkdir NewDirectory
$ cd  NewDirectory

$ rmdir NewDirectory #delete NewDirectory (must be empty)
$ rm b               #remove file "b"
$ rm -i b            #ask ask question and if yes then remove file "b"

$ date
$ cal
$ cal 2017
$ xcalc &      #this means put job in "background"
$ fortune
$ xeyes &
# Creating text files

$ cat > Rabbits.txt
 %
 Rabbits are the best pets 
 They are superior to cats
 They are nicer to hold than snakes.
 They are not as jumpy as dogs.
 Everyone should own a rabbit
 It is fun to own a buck (male rabbit) & a doe (female rabbit)
 Over time you will have lots of rabbits to pet
 %
 (control D)

# At the shell level, "#" is the comment character
# Within a program it is common to use "%" as a comment character

$ more Rabbits.txt   #has lots of features (see "man" page)

$ clear      #clear screen

$ less Rabbits.txt

#going up and down the directory tree
$ cd ..
$ ls -l 
$ cd NewDirectory
$ ls -l 

$ which cd
$ info cd
$ man cd
$ file Rabbits.txt

$ cp Rabbits.txt Pets.txt   #copies input file to output file
$ compress Pets.txt
$ ls -l  *.txt

Unix is really amazing (>,<,|)

# * is wildcard 
# < is input to the program (usually from another file)
#   (can be dropped if the context is clear)
# > is out from the program (usually directed to a file)
# | is "pipe from this program to next program"
# Comment line
#
$ cat < b.txt  #formal way of saying Rabbits.txt is input file
$ cat   b.txt   #for most (but not all) untilities you can skip "<"

$ wc < Rabbits.txt   
$ wc Rabbits.txt    

$ head  Rabbits.txt
$ head -n 3  Rabbits.txt    #this is called as flag or option 
$ tail  Rabbits.txt         #sometimes there are default values

#pipes allow you take input of one program and feed it to another program

$ head -n 10 Rabbits.txt | wc    #provides word count for first ten lines

           # "tee" is like a plumbers "T" pipe. 
           # it takes input stream and writes to file and also sends it to next
           # program!
$ head -n 10 Rabbits.txt | tee RabbitsHeader.txt wc 

 
$ yes | head -n 10                        #create a file with identical lines
$ yes | head -n 10  | tee OneThroughTen.txt  #write to both screen & file
$ yes | head -n 10 | nl | tee OneThroughTen.txt  #number the lines 

$ du  #disk usage
$ df  #file store usage

Other useful tools

	#Create files "numbers" and "alphabets"

$more numbers         #file1
	1
	2
	3
$more alphabets       #file 2
	a
	b
	c
$ paste a b       #side by side
$ paste -s a b    #top & botom

$ grep "dog" Rabbits.txt
$ grep -v "dog" Rabbits.txt  #try it out

$ more d
rabbits 1
cat  6
eog  2
lion  3
donkey  5
zebra 4



$ sort   d           #default sort is column1 and ascii sequence
$ sort -k2 -n d      #sort on column2 and numerically
$ sort -k2 -nr d     #reverse sort 


$ expand < Infile        #expands tabs to spaces


---- power tools ---- 

$ cut -d" " -f1 < RRLyrae.dat 
$ cut -d" " -f1,2 < RRLyrae.dat > c.dat


$ sed '1d' < b.txt    #delete first line 
$ sed '1,3d' < b.txt  #delete line 1 through 3 (useful to delete header lines)
$ sed '$d' < b.txt    #delete last line
$ sed '/%/d' < Rabbits.txt   #delete lines containing "%" character (comment)

$ uniq 
$ join
$ wget URL 
$ curl URL