sed, insert text in a zero-sized file

losko · 2015-08-24 09:29:59

Hi,
I'm just figuring out if there's a workaround for this behavior using sed:

Creating or resetting 'myfile' (0 byte file):

$ > myfile

using:

$ sed -i '1i text' myfile

or even:

$ sed -i '1s/^/text\n/' myfile

Doesn't work, 'myfile' is still empty.

If I create or reset 'myfile' this way (1 byte file):

$ echo > myfile

sed is working properly, but counting lines with 'cat myfile | wc -l' returns always actual lines of text +1.

Is this the normal behavior of sed?
I cant't find it nowhere...

Raynman · 2015-08-24 09:49:27

For a file that has no first line, the address "1" won't match anything, so the insert or substitute command is never executed. With the echo, you write a newline to the file, giving the sed expression a line to match on. Then you insert another newline, increasing the line count (which you can check with `wc -l myfile` btw, no need for cat).

sed is line oriented but you want to use it on a file without any lines for some reason. There might be a way I don't know of, but it's still a strange thing to do. Why not simply use echo for this? Or what are you really trying to do?

losko · 2015-08-24 12:48:53

@Raynman, thank you for the extremely clear answer.

The correct question should have been:
"is there a way to insert a line at the beginning of a zero-sized file using sed?"
Answer: no.

My goal is to find an universal way to insert a line at the beginning of a file, no matter its size.

Yes, right now I'm using 'echo' to do the job, appending new lines at the bottom of the file, and then using 'tac' instead of 'cat'.

Raynman · 2015-08-24 12:59:52

You could also handle an empty file as a special case in your script (then echo else sed) or look at answers to similar questions such as http://superuser.com/q/246837

losko · 2015-08-25 08:56:51

Yes, I know.
At the moment the upside-down solution is the simplest, by far.

$ > myfile

0 byte!

$ echo "three" >> myfile
$ echo "two" >> myfile
$ echo "one" >> myfile

Using 'tac' instead of 'cat':

$ tac myfile
one
two
three

$ tac myfile | wc -l
3

Technically the topic is not solved until (if ever) sed can handle that...
Bye

dice · 2015-08-25 17:35:59

I think it would help if you told us what you are trying to accomplish. Maybe someone knows another way/different tool to do just what you want.
If I understand your 'upside' down with tac correctly you try to insert a line at the top of the file even if it is an empty file.

JFTR:

$ tac myfile | wc -l

No piping needed here.

frostschutz · 2015-08-25 17:50:30

( echo insert a line ; cat file ) > file.tmp && mv file.tmp file || rm file.tmp

Appending at the bottom is nicer, does not require entire file to be re-written. Makes reading and printing out backwards more expensive though.

mykhal · 2016-10-08 10:25:17

AWK solution, for the pipe filter scenario instead of editing in place. Let's suppose you want to prefix the lines with "x" (even if there is only a "non-line"):

$ code='BEGIN{t="x";printf(t)}{if(NR>1){sub(/^/,t)}print}'

on real lines:

$ seq 3 | awk "$code"
x1
x2
x3

on empty file:

$ cat /dev/null | awk "$code"; echo
x

(Note that no newline is appended, so the echo command was added for clarity)

If you really need in-place editing, there's GNU awk (gawk) and has a module for that (-i inplace), however printing from the BEGIN section does write to stdout instead to the file (version 4.1.4, not sure if it is a bug).. so i could not get it working correctly.

If you really have to use sed, i think it's impossible - its universe is non-empty sequence of lines.

And i wonder if the awk code can be made yet shorter - i use Perl much more, but it is not present on the platform i was solving the problem on: Plan 9.

Last edited by mykhal (2016-10-08 10:59:28)

graysky · 2016-10-08 11:05:26

Is touch not an option?

% touch myfile
% wc -l myfile 
0 myfile

Trilby · 2016-10-08 11:56:59

Ed will do exactly what you ask for:

echo -e "0a\nYour line here\n.\nw" | ed -s myfile

However, I'd like to echo Raynman's earlier question "What are you really trying to do?"

I sense a big X Y question here, and there's likely a far better approach to your end goal.

adam_danischewski · 2017-05-25 11:16:02

If you would really like to use sed, you can without the -i option:

sed : <(echo '#!/bin/bash') myfile

Trilby · 2017-05-25 11:36:29

Do not necrobump old threads - especially for "solutions" that will not work on arch linux.

Closed.

Arch Linux

#1 2015-08-24 09:29:59

sed, insert text in a zero-sized file

#2 2015-08-24 09:49:27

Re: sed, insert text in a zero-sized file

#3 2015-08-24 12:48:53

Re: sed, insert text in a zero-sized file

#4 2015-08-24 12:59:52

Re: sed, insert text in a zero-sized file

#5 2015-08-25 08:56:51

Re: sed, insert text in a zero-sized file

#6 2015-08-25 17:35:59

Re: sed, insert text in a zero-sized file

#7 2015-08-25 17:50:30

Re: sed, insert text in a zero-sized file

#8 2016-10-08 10:25:17

Re: sed, insert text in a zero-sized file

#9 2016-10-08 11:05:26

Re: sed, insert text in a zero-sized file

#10 2016-10-08 11:56:59

Re: sed, insert text in a zero-sized file

#11 2017-05-25 11:16:02

Re: sed, insert text in a zero-sized file

#12 2017-05-25 11:36:29

Re: sed, insert text in a zero-sized file

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