Solved How to add a blank line before the first line?

[nobody]

In linux it goes as

sed -i '1i\\' file.txt

In FreeBSD ... how?

 

[gpw928]

This works on FreeBSD:

sed  -i "" -e '1i\
' file.txt

 

[Kai Burghardt]

In linux […]

There is no “Linux sed”. You probably meant GNU sed but it isn’t part of Linux. Another method is the line‑based editor ed(1):​

ed -s file.txt << EOT
1i

.
wq
EOT

Since you need to rewrite the entire file anyway (I don’t know of any file system that supports prepending data blocks), the naive solution is of course:​

cat - file.txt > file.txt~ << EOT

EOT
# cat instead of mv to preserve file attributes of original file.
cat file.txt~ > file.txt
rm $_

This needs some protection against errors, though, e. g. if the first cat(1) fails because the disk is full you do not want to continue. For a non‐interactive sh(1)ell script you can specify the -e option.​

 

[gpw928]

I don't think that there is a common syntax that will get the job done with both FreeBSD sed(1) and GNU sed using an identical command.

Hence the alternative solutions offered above by Kai Burghardt above offer better portability.

 

[Erichans]

In FreeBSD ... how?

I don't think that there is a common syntax that will get the job done with both FreeBSD sed(1) and GNU sed using an identical command.

Leverage this:

[0-0] # which sed gsed
/usr/bin/sed
/usr/local/bin/gsed
[1-0] # cat i.txt
L 1
L 2
L last
[2-0] # sed -e '1H;1g' i.txt

L 1
L 2
L last
[3-0] # gsed -e '1H;1g' i.txt

L 1
L 2
L last
[4-0] #

Of course, more compact would be sed -e '1{H;g;}' i.txt* but, that only works with gsed(1).

___
* edit: for a one-digit line number, actually requiring more characters ....

 

[gpw928]

Leverage this:

An elegant solution. For in-place editing, the processing of the "I" (or "i") option is problematic with gsed:

$ gsed  -i "" -e '1H;1g' i.txt
gsed: can't read : No such file or directory
$ echo $?
2

It's possible to work around by removing the space between the "-i" and its (empty) argument, which works for both sed and gsed.

 

[nobody]

Command

sed -i -e '1{H;g;}' file.txt

works in FreeBSD and linux. Great thanks for everyone.

 

[Maturin]

echo > empty_line.txt
cat empty_line.txt file.txt > new_file_with_added_empty_line.txt
Adds a blank line before the first line in all shells on all unixlike systems, right?
Sorry, couldn't resist.

 

[eternal_noob]

I find the question of "why" more interesting than the question of "how".

 

[nobody]

echo > empty_line.txt
cat empty_line.txt file.txt > new_file_with_added_empty_line.txt
Adds a blank line before the first line in all shells on all unixlike systems, right?
Sorry, couldn't resist.

If you use > , you will delete original contents of the file.

The purpose is to add a blank row in the first line to the file which has some contents.

 

[Maturin]

If you use > , you will delete original contents of the file.

Maybe the filenames I used were too long to see what I've done:
first line produces a new file containing an empty line, only.
second line concatenates this file with the file in question a empty line has to be added to, and creates a new file again, which then is the original file with an added line before its context.

I spared the parts needed to rename the new one to the name of the original file, and delete the spare file, because I did not really meant this seriously, didn't really meant to discuss this very basic shell usage.
Clearly this thread was about sed/ed usage, but since the "specifications" let that part out I simply wanted to make a joke by fulfilling the asked goal with another way.


Edit:
I didn't see Kai Burghardt already posted that "naive solution", even more elegant than my approach.