More about ls command
-	ls command allows you to specify wildcards
-	everything matches *
-	* can be put as many times as you want in an expression
-	* matches any sequence of characters of any length
-	? any single character
-	a set of characters can be specified using [....], where
	.... is a set of characters. [....] matches any characters
	from the set
-	to get a list of all files ending with net
ls *net
-	to get a list of all files starting with form followed by any (single)
	character followed by .doc
ls form?.doc
-	Files starting with an uppercase alphabet
ls [A-Z]*
-	Files starting with a digit
ls [0-9]*
-	Files starting with digits in the range 1 to 4
ls [1234]*
ls [1-4]*
-	Files starting with digits in the range 1 to 4 and 6 to 9
ls [1-46-9]*
-	Files starting with a digit followed by a lower case alphabet
	followed by a digit followed by any sequence
ls [0-9][a-b][0-9]*
-	it is geberally not desirable to have filenames with unprintable
	characters. But if you have them by accident, you should use the
	od command to find out what the filename looks like
-	don't use filenames such as -g
-	command du can be used to look at the disk space taken up by a
	particular branch in a directory structure
				Regular expression
-	Similar to wild cards
-	If you are searching for a pattern, you may want to specify things like
	at the beginning of the line there is a string From
-	Any string is a regular expression itself
-	grep From mail/sent-mail
-	From is a regular expression
-	To get lines that start with From you type
-	grep "^From" mail/sent-mail
-	^ indicates beginning of line
-	$ stands for the end of the line
-	The following expression look for blank lines
-	grep "^$" mail/sent-mail
-	. matches a single character similar to ? in ls
-	Let us store ls in a file so that we can try grep on that file
ls > templist
-	The following command is looking for an expression which has
	the string form followed by any character followed by . followed
	by doc
grep form.\.doc templist
-	\. to suppress special meaning of .
-	similarly special meaning of $ can be suppressed by \$ and so on
-	we can use a set of characters just like ls
-	[A-Z] means uppercase characters between 
-	r* means zero or more occurrences of r
-	r+  means one or more occurrences of r
-	r?  means zero or one occurrences of r
-	Other filters that use regular expressions are: sed and awk
sed 's/^[a-z]*/xxx/' /etc/passwd | more
-	The above command is going to modify the passwd file at
	command line. Note that the passwd file doesn't get modified
	but the modified version is printed on the screen
-	sed command accepts a single line program enclosed in single quotes
	The program here is trying to substitute a string at the beginning
	of the file (^) which starts with zero or more occurrences of
	lowercase characters ([a-z]*) by xxx in the passwd file
-	sed utility is good for some quick modifications
-	instead of putting the program on the command line you can also
	put it in a file and use the file as the program
sed -f program.sed input-file
-	Let us look at a possible use of sed in the following script

sed 's/xxx/4jul220dhv/g' < 22aadt.template.cfg > 22dhv.cfg
snnsbat 22dhv.cfg 22dhv.log >& 22junk
rm 22dhv.cfg
rm 22junk
-	There is a template file with xxx as a dummy string
-	We want to replace xxx with name of a data set to create
	a cfg file
-	s/xxx/datasetname/g allows us to replace xxx by the dataset
	name. The g at the end stands for global substitution
-	sed goes through the input file line by line and applies
	the program to each line
-	sed is useful for editing large files
-	or editing during a batch program
-	awk allows you to 
-	1.txt has an example of how awk can be useful
ls -1 | awk '{print "chown",$1,$1}' | sh
-	we were in the dir /extra/city
-	ls -1 gave list of all the user dir in single column
-	awk program printed the string chown followed by first field
	followed by first field one more time
-	sample output is: chown brenda brenda
-	this output was directly sent to sh for execution
-	awk also executes the given program for each line
-	Each line is separated into fields. By default, the field separator
	is space and tab characters
-	common variables are:
	$0 - entire line
	$1,$2,$3,.... first,second,third field, ..
 	NF number of fields on the current line
	$NF is last field on the line
	NR is the line number
-	field separator can be changed using -F option
wascana/extra/pawan 128 >awk -F: '{print NR,$1}' /etc/passwd | more
1 root
2 bin
3 daemon
4 adm
5 lp
6 sync
7 shutdown