Summary of command usage or shell programing
a. test -e demo.sh && echo "exist" || echo "not exist" #test the file demo.sh exist or not.
b. test -z $filename && echo "you must input a filename"&& exit 0
c. test ! -e $filename && echo "the filename does not exist" && exit 0
d. test -f $filename && echo filetype="file is a filename"&& exit 0
e. test -d $filename && echo filetype="file is a directory "&& exit 0
f. test -r $filename && perm="readable"
g. test -w $filename && perm="$perm writable"
h. test -x $filename && perm="$perm executable" 1
hello kevin hello Friday