Scrutinizer,
I think the OP is saying the input file has lines with 4 fields using <tab> as a separator and some fields are empty. Unfortunately, when IFS is set to <tab> (or <space>) groups of <tab>s (or <space>s and <tab>s) are treated as a single field separator. I don't believe that this behavior for IFS set to a single <tab> character conforms to the standards, but since this seems to be existing practice in both bash and ksh, this may be considered a bug in the standard.

benalt,
If I understand what you're trying to do, the following should work with ksh (or any shell that performs the parameter expansions required by the POSIX standards):

Code:

#!/bin/ksh
while IFS='' read -r line
do	printf 'input:"%s"\n' "$line"
	d=${line##*	}
	line=${line%	*}
	c=${line##*	}
	line=${line%	*}
	b=${line##*	}
	line=${line%	*}
	a=${line##*	}
	printf 'a=%s,b=%s,c=%s,d=%s\n' "$a" "$b" "$c" "$d"
done < file.txt

(where the whitespace in % * and in * } is a single literal <tab> character in all of the above parameter expansions.

If file.txt contains:

Code:

a	b	all fields present	d
a		field 2 empty	d
	b	field 1 empty	d
		fields 1 & 2 empty	d
		fields 1, 2, & 4 empty	
a	b	c	d \
e	f	g	h

the output produced is:

Code:

input:"a	b	all fields present	d"
a=a,b=b,c=all fields present,d=d
input:"a		field 2 empty	d"
a=a,b=,c=field 2 empty,d=d
input:"	b	field 1 empty	d"
a=,b=b,c=field 1 empty,d=d
input:"		fields 1 & 2 empty	d"
a=,b=,c=fields 1 & 2 empty,d=d
input:"		fields 1, 2, & 4 empty	"
a=,b=,c=fields 1, 2, & 4 empty,d=
input:"a	b	c	d \"
a=a,b=b,c=c,d=d \
input:"e	f	g	h"
a=e,b=f,c=g,d=h