sed, awk, Perl

---

sed

---

sed execution model: for each line in the input file,

1. read input line into "pattern space",
2. apply commands to pattern space,
3. send pattern space to stdout.

sed -e '1,/Subject:/d
s/^[<tab> ]*//
s/[<tab> ]*$//
s/et al\./<I>et al.<\/I>/g
/et$/{
$!N
s/\n[<tab> ]*/\n/
s/et\nal\./<I>&<\/I>/
P
D
}' file.html >newfile.html

---

awk

---

awk execution model:

BEGIN {commands executed once before any input is read}
{main input loop executed for each line of input}
END {commands executed once after all input is read}

awk 'BEGIN { FS = " "; ns = 0; total = 0 }
{sum = $2 + $3 + $4
avg = sum / 3
ns++
total += avg
print ns ": " $1, avg }
END { print ns, "students: " total / ns }' scores

Peter 85 90 95
Paul 25 25 50
Mary 100  80   60

1: Peter 90
2: Paul 33.3333
3: Mary 80
3 students: 67.7778

---

Perl

---

Perl programs are very similar to UNIX shell scripts, except that the entire Perl program is compiled before it is executed.

#!/usr/bin/perl
%words = ("fred","camel","barney","llama",
   "betty","oyster","wilma","oyster");
print "What is your name? ";
$name = <STDIN>;
chop($name);
$original_name = $name; #save for greeting
$name =~ s/\W.*//; # get rid of everything after first word
$name =~ tr/A-Z/a-z/; # convert everything to lowercase
if ($name eq "randal") {
    print "Hello, Randal!  How good of you to be here!\n";
} else {
    print "Hello, $original_name!\n"; # generic greeting
    $secretword = $words{$name}; # get the secret word
    if ($secretword eq "") { # oops, not found
	$secretword = "groucho"; # sure, why a duck?
    }
    print "What is the secret word? ";
    $guess = <STDIN>;
    chop($guess);
    while ($guess ne $secretword) {
	print "Wrong, try again.  What is the secret word? ";
	$guess = <STDIN>;
	chop($guess);
    }
}