compare string in two files

Shell Programming and Scripting
1

i have two files:

file1.txt
AA
BB
DD
EE

file2.txt
AA,AAA
BB,BBB
CC,CCC
DD,DDD
EE,EEE
FF,FFF

i want an output file:
file3.txt
AA,AAA
BB,BBB
DD,DDD
EE,EEE

how can i do this in shell script?

2

Try searching the forum the next time,
there are many similar threads:

awk 'NR==FNR{x[$1];next}$1 in x' FS=',' file1.txt file2.txt>file3.txt

Use nawk or /usr/xpg4/bin/awk on Solaris.

3 
join -t"," -j1 1 -j2 1 -o 1.1 2.2 file1 file2