Perl Learning - 9 (printf, filehandle, open, close)

printf is powerer to control the outputs,it's used with a format string,and a list of elements.

 

printf "Hello,$s; your password expires in %d days!\n",$user,$days_to_die;
printf "in %d days!\n",17.85; # in 17 days!

 

Format string has some kind of 'conversion',each format string starts with '%' and ends with a character.
The number of elements and format strings must be the same,otherwise printf doesn't work.

 

%d gets integer number,like %d ... 17.85 is 17.
%s gets characters,-15 means align by left.
%f gets fload number.

 

printf "%6d\n",42;
printf "%2d\n",2e3+1.95;
printf "%15s\n","wilma";
printf "%-15s\n","flintstone";

printf "%12f\n",6*7+2/3;
printf "%12.3f\n",6*7+2/3;
printf "%12.0f\n",6*7+2/3;

 

$ ./printf.pl
Hello,Larry; your password expires in 5 days!
in 17 days!
    42
2001
          wilma
flintstone
   42.666667
      42.667
          43

 

If you want output a '%',use '%%','\%' don't work.

printf "Monthly interest rate: %.2f%%\n",5.25/12; # Monthly interest rate: 0.44%

 

Filehandle is the name of I/O connection in Perl program,is the bridge between perl and outside world.
It's name of connection,but not name of file.

 

Perl has six buildin filehandle: STDIN,STDOUT,STDERR,DATA,ARGV,ARGVOUT
STDIN=standard input
STDOUT=standard output
STDERR=standard error

 

If you need other filehandles,use operater 'open'.

open CONfig,"dino"; # input from dino
open CONfig,"<dino"; # input from dino
open bedROCK,">fred"; # output to fred,cover the original contents of fred
open LOG,">>logile"; # append to fred

 

The upper-case are filehandles,the lower-case are filenames.
When opening a filehandle,the default action is get input from file,so the first two has same results.

We can use expressions with filenames.

 

my $selected_output="my_output";
open LOG,"> $selected_output"; # note the space after '>'

 

Use 'close' to close a filehandle.
close bedROCK;

 

if(!open LOG,">>logfile"){
 die "Cannot create logfile:$!";
 }
 
'die' will end program when 'open' fails,and tell you it cannot open logfile.
$! is the system error messages that shell throws out.

 

$ ./die_open.pl
Cannot create logfile:Permission denied at ./die_open.pl line 4.

 

If you don't want the program name,just put a "\n" at the end.

$ cat die_open.pl
#!/usr/bin/perl

if(!open LOG,">>logfile"){
        die "Cannot create logfile:$!\n";
        }
  
$ ./die_open.pl
Cannot create logfile:Permission denied

 

After a filehandler opens,it can be used as STDIN to get input lines.

 

if(!open PASSWD,"/etc/passwd"){
 die "How did you get logged in?($!)";
 }
while(<PASSWD>){
 chomp;
 print "$_\n";
 }
 
$ ./die_open.pl
root:x:0:0:RHEL5:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
......

Exercises:

1. Write a program,print the reversed lines from input.

#!/usr/bin/perl

while(<>){
    push @lines,$_;
}

@lines=reverse @lines;
foreach(@lines){
    print $_;
}
###########################################

2. Write a program,user input lines of characters,then print the formatted lines that right align by 20 bits.

#!/usr/bin/perl

print "1234567890" x 7 . "\n";

while(<>){
    chomp;
    printf "%20s\n",$_;
}
###########################################

3. Modify the above program,let user input the align width.

#!/usr/bin/perl

chomp($width=<STDIN>);
print "1234567890" x $width . "\n";
while(<>){
    chomp;
    printf "%${width}s\n",$_;
}