Simple Perl Script: Two questions - perl

Question

I have a small program:
#!/user/bin/perl
use strict;
system ("clear");
my($option, $path);
do
{
print "\tEnter the number of your chosen option:\n";
print "\n";
print "\tOption\t\tCommand\n";
print "\t======\t\t=======\n";
print "\t1\t\tDate\n";
print "\t2\t\tDirectory Listing\n";
print "\t3\t\tCalendar\n";
print "\t4\t\tVi Editor\n";
print "\t5\t\tCalculator\n";
print "\t6\t\tExit\n\n";
chomp($option=<STDIN>);
SWITCH:
{
($option =="1") and do
{
system(date);
last;
};
($option =="2") and do
{
print "Enter the path:"; ############################
chomp($path=<STDIN>); #This is giving me an error#
system(ls $path); ############################
last;
};
($option =="3") and do
{
system(cal);
last;
};
($option =="4") and do
{
system(vi);
last;
};
($option =="5") and do
{
system(bc);
last;
};
}
}while ($option!=6);
print "Goodbye!\n";
sleep 2;
First question: Can anyone help me how to write the proper command to create a directory listing in case 2.
Second Question: Why do I get a loop if I use
$date = `date`;
print "$date";
instead of
system(date);

Answer 1

You should be able to solve a lot of your problems by remembering to put quotes around literal arguments to system():
system("date");
system("ls $path");
and the same for most other places you call system() (your first call to system("clear") is correct).
It is a quirk of Perl that calling something like system(cal) works at all, because the unquoted cal is treated as a "bareword" by Perl, which happens to be roughly equivalent to a string when passed to a function such as system(). Relying on this behaviour would be terribly bad practice, and so you should always quote literal strings.

Answer 2

You could read the path like:
chomp($path=<STDIN>);
system("ls $path");
Not sure why you'd get the loop for $date =date;print "$date";. But I don't think there's a date function unless you're using a package for it. You can show a time like:
my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime;
$year += 1900;
$mon += 1;
printf "%04d-%02d-%02d %02d:%02d:%02d",
$year, $mday, $mon,
$hour, $min, $sec;
On most unix systems perl resides in /usr/bin, without the e in user, so you might consider double-checking the first line of your script.

Answer 3

Your immediate problems were caused by quoting issues and the lack of use warnings in your script.
It's also worth noting that menu-driven scripts like yours are ideal candidates for dispatch tables. A dispatch table is a technique for defining actions as data. The actions are Perl subroutines. The data is usually a set of key-value pairs that end up getting stored in a hash.
The keys to the hash are the choices made by the user (menu items 1-6 in your case).
The values in the hash are called code references. There are two ways to set up these code references: (1) Directly in the dispatch table, using anonymous subroutines; or (2) using the &\foo syntax, which would create a reference to a subroutine named foo.
The handy thing about this approach is that your menu() method can be reused -- simply with a different dispatch table and a different usage message.
This example is so small that the benefit of reuse might not seem compelling, but the general technique of having data -- in the form of a dispatch table -- control program behavior is powerful in many contexts.
# Always use both of these.
use strict;
use warnings;
sub dispatch_table {
return
1 => sub { system 'date' },
2 => \&ls_path,
3 => sub { system 'cal' },
4 => sub { system 'vi' },
5 => sub { system 'bc' },
6 => sub { print "Goodbye!\n"; sleep 2 },
;
}
sub ls_path {
print "\nEnter the path: ";
chomp(my $path=<STDIN>);
# Note quoting. To be super robust, you would
# need to escape apostrophes in the path.
system "ls '$path'";
}
sub usage_message {
return "Choose wisely:
Option Command
====== =======
1 Date
2 Directory Listing
3 Calendar
4 Vi Editor
5 Calculator
6 Exit
";
}
sub menu {
system 'clear';
my %dt = dispatch_table();
my $option;
print usage_message();
while (1){
print "> ";
chomp($option = <STDIN>);
last if exists $dt{$option};
}
$dt{$option}->();
}
menu();

Answer 4

I can not reproduce your loop with:
$date =date;print "$date";
I doubt that is exactly how you coded it since I get a compile error
with use strict;. If you can show a reduced code example which still illustrates the problem, we could help debug it further.
If you are trying to capture the output of an external command into a variable, you could use backticks or qx:
my $date = qx(date);
print "$date";
On a side note, whenever I see a series of print statements, I think here-doc:
print <<"EOF";
Enter the number of your chosen option:
Option Command
====== =======
1 Date
2 Directory Listing
etc...
EOF
A little easier to read and maintain, no?
Finally, it is also a good idea to use warnings;.

Answer 5

The first couple of suggests I have are, first like others have already suggested, use warnings is strongly encouraged. Older Perl interpreters may require you use the older form #!/usr/bin/perl -w as the first line of your Perl script. Second, there is a Switch module available, to make the switch statement look less ugly. I've also shown usage of subroutines to clean up the appearance of the program.
I've attached a alternative version of your script with some potential suggestions. Note it uses a slightly different alternative for switch. If available, I'd recommend using the Switch module. It includes a different way of printing the time, and of course fixes your problem with the system calls.
I hope that helps.
#!/usr/bin/perl
use strict;
use warnings; # otherwise /usr/bin/perl -w in first line
sub menu() {
print <<EOM;
Enter the number of your chosen option:
Option Command
====== =======
1 Date
2 Directory Listing
3 Calendar
4 Vi Editor
5 Calculator
6 Exit
EOM
}
sub showtime() {
my $time = localtime;
print $time,"\n";
}
sub listdir() {
my $path;
print "Enter the path: ";
chomp($path = <STDIN>);
system("ls $path");
print "\n";
}
system("clear");
my $option;
do {
menu();
chomp($option = <STDIN>);
# SWITCH:
for ($option) {
/1/ and do {
showtime();
};
/2/ and do {
listdir();
};
/3/ and do {
system("cal");
};
/4/ and do {
system("vi");
};
/5/ and do {
system("bc");
};
last;
}
} while ($option != 6);
print "Goodbye!\n";
sleep 2;