How does the '\b' character in perl work? - linux

Question

According to my research the '\b' character used in perl print statements should act like a "backspace", that is, moving the cursor one character back, and deleting the current character. For this reason, I had planned to use this operation to print operational status on a single line, updating as it progressed. However, I noticed that while the cursor does indeed move back, the characters underfoot are not deleted, and therefore, longer messages remain after shorter print statements. I have compiled the following sample code to explain my findings:
#!/usr/bin/perl
use strict;
use warnings;
my $m;
#set to nonzero so that the screen will update before \n
local $| = 1;
print "Current number shown: ";
$m = "LONG MESSAGE TEMP";
print $m;
print "\b" x length($m);
foreach(1..22) {
$m = $_;
print $m;
print "\b" x length($m);
#sleep 1; #Uncomment to see updates
}
print "\n";
And this was the output:
Current number shown: 22NG MESSAGE TEMP
If this is indeed the correct operation of '\b', is there another escape that deletes the character as well as moving the cursor back? I would like to avoid using '\r' which starts at the beginning of the current line. Otherwise, how am I using the escapes incorrectly?

Answer 1

"\b" is just a fancy way of writing chr(0x08). Your terminal will likely move the cursor rather than display anything, but that's entirely up to it.
If you can rely on it, then you can achieve what you want by overwriting with spaces.
my $last_length = 0;
sub update {
my ($s) = #_;
print("\b" x $last_length);
print(" " x $last_length);
print("\b" x $last_length);
print($s);
$last_length = length($s);
}
Or with less flicker:
my $last_length = 0;
sub update {
my ($s) = #_;
my $diff = $last_length - length($s);
print("\b" x $last_length);
print($s);
print(" " x $diff);
print("\b" x $diff);
$last_length = length($s);
}

Answer 2

Just output some extra space characters to overwrite what you need to overwrite.
#!/usr/bin/env perl
use strict;
use warnings;
use Time::HiRes qw(sleep);
sub backspace {
print "\b" x $_[0];
print " " x $_[0];
print "\b" x $_[0];
}
local $| = 1;
my $m = "LONG MESSAGE TEMP";
print "Current number shown: ", $m;
sleep 1;
for (1..22) {
backspace( length($m) );
$m = $_;
print $m;
sleep 0.2;
}
print "\n";

Answer 3

Depending on how it is used, \b can have a special meaning within a Perl command:
\b is the backspace character only inside a character class. Outside a character class, \b alone is a word-character/non-word-character boundary.
To substitute "def" for each occurrence of the word "ABC" within a file, use the Perl command:
perl -pi -e 's/\bABC\b/def/g' file
which will leave strings such as "ZABCD" unchanged.