null string matching in sed?

[Allasso]

Hello,

I would assume the expression ^$ should match a null string. Yet when I run:

echo -n | sed 's/^$/nullstring/'

I get no output.

Can anyone tell me why?

[wje_lf]

I would assume the expression ^$ should match a null string.

It doesn't. It matches an empty line.

when I run:

echo -n | sed 's/^$/nullstring/'

I get no output.

Can anyone tell me why?

Yes. The output is not an empty line. The output is completely empty: that is, no lines at all.

Hope this helps.

[Allasso]

thanks, yes it helps, sort of.

yes, I see that I can type:

echo | sed 's/^$/nullstring/' (newline not suppressed in echo command)

and I get a substitution and output.

It doesn't make sense to me though, since the doc for GNU sed (which is what I am using) states that when a line is read into the pattern space, the trailing newline is removed.

My "problem" is this: I am using sed in a shell script to process text that is generated from fields in a document. Each field is processed separately, using a while loop and sequentially going from one field to the next with each pass. When there are no more fields, ie, no more output form my awk '{ print $'$var' } command, I want to exit the loop. Since the output of the awk command is piped to sed anyway, I was trying to use

sed 's/^$/output/w file'
test -s file

to flag the loop to exit.

So two questions,

1) is there a way to get sed to flag when there is no input
2) do you see another way(s) to accomplish this simply.

I am able to get the job done just by using some reverse logic, (sed 's/.*/output/' - flagging everytime there IS a string, and then clearing the file each time at the top of the loop) but it is a bit clumsier, and more important, I would like have more light on the subject.

Appreciate your answers, thanks again.

[wje_lf]

the doc for GNU sed (which is what I am using) states that when a line is read into the pattern space, the trailing newline is removed.

That statement is correct. But sed puts the newline back on when sending a line to standard output.

1) is there a way to get sed to flag when there is no input

There's no way that I can see.

2) do you see another way(s) to accomplish this simply.

Probably. I know as little about awk as I possibly can, but doesn't it have a way to tell you how many fields are on a line? If so, make that many passes. Maybe something like this:

#!/bin/bash

# ...

field_number=0

while true
do
  field_number=$(( $field_number + 1 ))
 
  if [[ $field_number -gt $number_of_fields ]]
  then
    break
  fi

  # ...

done

# ...

[Allasso]

Thanks, Bill. Sounds like a good solution. Don't know if it is more effecient than the one I am using, but I appreciate the alternative.

[scm]

I know as little about awk as I possibly can, but doesn't it have a way to tell you how many fields are on a line?

awk's NF variable:

echo | awk '{print NF}'

Note that echo -n won't give you zero since it doesn't yield an input line to awk.