performance shell scripting

high_perfomance_while.sh

# The first method opens file.txt, writes one line, then closes file.txt.  Then repeats those three steps 100 times
# The second method opens file.txt, writes 100 lines and closes file.txt

#!/usr/bin/sh

# typical use of a while loop - 
# while $count is less than or equal to 100. 
# append the value of $count to file.txt

count=1
while [ $count -le 100 ]
do
        echo $count >> file.txt
        let count=count+1
done


#!/usr/bin/sh

# better
# instead of opening a file and writing the value of $count at each loop iteration ...
# write the results of each iteration once when the loop completes

count=1
while [ $count -le 100 ]
do
        echo $count
        let count=count+1
done > file.txt

# best

n=0
substnum=
while read grp
do
	echo "^git-..*[ 	]$grp"
	substnum="$substnum${substnum:+;}s/[ 	]$grp/$n/"
	n=$(($n+1))
done <"$grps" >"$match"