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

amy_888

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, "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?($!)";
　　}
　　while(){
　　chomp;
　　print "$_\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>　　#!/usr/bin/perl
　　chomp($width=);
　　print "1234567890" x $width . "\n";
　　while(){
　　chomp;
　　printf "%${width}s\n",$_;
　　}