base on http://www.etalabs.net/sh_tricks.html

#Printing the value of a variable
1. use printf not echo
 printf %s\\n "$var" | cut -d' ' -f1

#Reading input line-by-line
2. Reading input line-by-line
 IFS= read -r var	# note: IFS=read does NOT work!! after = there MUST be a space
 			# This command reads a line of input, terminated by a newline or end of file or error condition, 
			# from stdin and stores the result in var. Exit status will be 0 (success) if a newline is reached, and nonzero (failure) 
			# if a read error or end of file terminates the line.
			# -r Do not treat a <backslash> character in any special way.

3. Reading input from pipe: use HERE doc, old way(#2) does not work
 $(echo '1 2 3' | IFS= read var1)	#FAIL!

 IFS= read var2 << EOF			#ok
 $(echo '1 2 3')
 EOF

also check
./shell/cp.xargs.notes.txt

#Using find with +
4. find with +
 Of course the much smarter way to use find to efficiently apply commands to files is with -exec and a + replacing the ;
 $find path -exec command '{}' +

#Getting non-clobbered output from command substitution

f='/usr/local/bin/perl'
var=$(dirname "$f"; echo x)  #note: there is a \n before x
var=${var%??}	#get rid of x and \n
var=${f%/*}	#suffix substitude 

Q: why eval?
A:
eval : concatenate command parameter and then execute in shell
 1 eval just like $()
 2 eval command par1 par2 ...	# the string right after eval must be a command
 3 eval will interpreate $var as its value

cmd='echo'
eval $cmd i love my country	#just like $(echo i love my country)


for i in `seq 0 100` # assign an array value
do
 eval arr$i=$i
done

sum=0
for i in `seq 0 100`
do
 sum=$(expr $sum + $arr$i) #expr to add two nums
done

#Returning strings from a shell function
a='string'
b=a
eval y=\$$b #var y now 'string'
eval y=$$b #var y now an address