Simple bash + sed s command to join six lines

Given the following file:

temp.txt

line1
line2
line3
line4
line5
line6
line7

And the following bash/sed commands:

for (( i=0; i<6; i++ ))
do
sed "N;1s/\\(.\*\\)\n\\(.\*\\)/ /" temp.txt
done

doesn't produce the desired stout:

line1 line2 line3 line4 line5 line6
line7

Thanks in advance for any insight you can provide.

Your code is:

for (( i=0; i<6; i++ ))
do
sed "N;1s/\(.*\)\n\(.*\)/ /" temp.txt
done

• N tells sed to read the next line. By doing so the current line number is incremented. So testing for line 1 can never succeed. (You should test for line 2.)
• You have not used -i and so the file will not be changed.
• It is rather inefficient to call sed 6 times instead of just doing everything in a single invocation. (And you should only call it 5 times to join 6 lines.)

A simpler alternative:

sed -i '1{N;N;N;N;N;s/\n/ /g;}' temp.txt

In the case that the input could have fewer than 6 lines and up to 6 lines should be joined, an alternative script is:

sed -i ':a;$q;N;1,6s/\n/ /;ta' temp.txt

Note that BSD-ish sed requires a literal newline after any label.

Behaviour of N when there is no next line varies between implementations. Some output the existing contents of the pattern space, others follow posix and discard it. (GNU sed will do either: seq 3 | sed N vs seq 3 | sed --posix N.) Prefixing $q sidesteps this ambiguity and forces output.

One way:

$ sed -n -e '1h; 2,6H; 6{g; s/\n/ /g}; 6,$p' temp.txt
line1 line2 line3 line4 line5 line6
line7

The first 6 lines get added to the hold space, then on the 6th line, the hold space is moved into the pattern space, and newlines are replaced with spaces. The 6th and following lines are printed out (The -n option turns off printing by default).

(Might be GNU sed specific because of the \n in the regular expression, but since this is tagged linux I assume that's the version you're using)

You can also use ed instead, which has an actual join command:

$ printf '%s\n' '1,5s/$/ /' 1,6j ,p Q | ed -s temp.txt                         
line1 line2 line3 line4 line5 line6
line7

Add a space to the end of the first 5 lines, join the first 6 lines, and print out the changed file. If you wanted to modify the file directly, just change ,p Q to w to write it out to disk.

$ awk -v max=6 '{printf("%s%s", $0, (NR<max ? " " : "\n"))}' file
line1 line2 line3 line4 line5 line6
line7

This might work for you (GNU sed):

sed ':a;N;s/\n/&/5;Ta;s// /g' file

Or if you prefer:

sed ':a;N;s/[^\n]*/&/6;Ta;s/\n/ /g' file

Or with paste:

paste -sd'     \n' file

echo '
line1
line2
line3
line4
line5
line6
line7' | 

 mawk '!__ < (_ = NR % 6) || ORS = _ ? OFS : RS' ORS=' '    

line1 line2 line3 line4 line5 line6
line7  

another way to be a tiny bit more efficient (124 micro-ops instead of 153) :

gawk 'ORS < FS ? ORS = FS : NR % 6 || ORS = RS' ORS=' '