Shell Programming and Scripting

a 1
a 2
a 3
a 4
b 1
b 2
b 3
b 4
c 1
c 2
c 3
c 4

a 3 
a 4
b 3
b 4
c 3
c 4

grep "[34]" $filename

awk 'NR>1 && $1!=x{print a ORS b}{a=b;b=$0;x=$1}' file

awk 'NR>1 && ($1 != p) {print a[c-1]"\n"a[c] ;c=0} {p=$1 ; a[++c]=$0 }' infile

$ awk 'NR>1 && $1!=x{print a ORS b}{a=b;b=$0;x=$1}' file
a 3
a 4
b 3
b 4

$  awk 'NR>1 && ($1 != p) {print a[c-1]"\n"a[c] ;c=0} {p=$1 ; a[++c]=$0 }' infile
a 3
a 4
b 3
b 4

my %hash;

while(<DATA>)  {
    # get only the first field ( a b c d )
    $firstfield = (split / /,$_)[0];

    # if already it arrived, move that to index 1, 
    if ( defined $hash{$firstfield}->[0] )  {
        $hash{$firstfield}->[1] = $hash{$firstfield}->[0];
    }    
    # get the current line to 0th index.
    $hash{$firstfield}->[0] = $_;
}

# print the two elements.
foreach ( keys %hash )  {
    print $hash{$_}->[0];
    print $hash{$_}->[1] if defined ( $hash{$_}->[1] );
}

__DATA__
a 1
a 2
a 3
a 4
b 1
b 2
b 3
b 4
c 1
c 2
c 3
c 4

perl t1.pl
c 4
c 3
a 4
a 3
b 4
b 3