Copy previous line to current line from text file

Question

I have a file with lines, just like this:

A
B
C

I want to create a duplicate file in bash that contains each line merged with the copy of next line, like:

A;B
B;C
C;

Answer 1

Using awk:

awk 'prev{ print prev ";" $0 }
     { prev = $0 }
     END { if (NR) print prev ";" }'

which with your input, gives

A;B
B;C
C;

Answer 2

Quick'n'dirty way (involves reading the file twice):

$ tail -n+2 file | paste -d';' file -
A;B
B;C
C;

Answer 3

$ sed 'x;G;s_\n_;_;1d;${p;x;s_$_;_;}' file
A;B
B;C
C;

What that sed expression is doing:

• x: save the incoming line in hold space, and retrieve the previous one
• G: append the new line (from hold space) to the old one
• s_\n_;_: replace line-break with a ;.
• 1d: if this is the first line, delete it (don't print it) and advance to next
• ${...;}: if this is the last line...
• p: first print the joined pair
• x: retrieve the final line
• s_$_;_: append final ;

Answer 4

Somewhat simpler sed solution without hold space:

sed '$!N;y/\n/;/;p;y/;/\n/;D' file

• $!N to join next line (if any; the $! is not needed with GNU sed when not in POSIX mode)
• y/\n/;/ replace the newline with ;
• print the resulting line
• y/;/\n/ to change back to newline, so with
• D you can get rid of the first line and continue with the next one

Answer 5

Same basic idea as the awk solution given by Torin:

$ perl -lne 'print "$last;$_" if defined $last; $last=$_;END{print "$last;" if $.}' file 
A;B
B;C
C;

Or, if you're into the whole brevity thing:

$ perl -lne'$.>1?print"$l;$_":1;$l=$_}{print"$l;"if$.' file 
A;B
B;C
C;

Answer 6

A Vim Solution

One can issue this command (credit to Conspicuous Compiler for suggesting this):

vim "+%s/\n\(.\+\)*/;\1\n\1" sample.txt

---

Alternatively and probably more customarily, start Vim and open the file and issue the ex command:

:%s/\n\(.\+\)*/;\1\n\1

Explanation:

Substitute the pattern:

• the new line character,\n
• a character group,\(.+\), which composes the entire next line. The quantifier which follows, *, just indicates that there can be zero or more matches

with the following:

• a semicolon
• followed by a reference to the character group, \1
• followed by the new line character, \n
• followed by a second reference to the character group, \1.

Before:

After:

Answer 7

$ perl -lp -0777e 's/\n(?=(.*\n?))/;$1/g'  input.txt