UNIX for Beginners Questions & Answers

# sed -n : no default print
sed -n '
# if line starts with 1, then
  /^1,/ {
# exchange with hold space
    x
# substitute embedded newlines with | and print if successful (in line 1 will not print)
    s/\n/|/gp
# jump to :L
    bL
  }
# else append to hold space
  H
  :L
# if last line then
  $ {
# exchange with hold space (or copy from hold space with g)
    x
# substitute embedded newlines with | and print
    s/\n/|/g; p
  }
' file

sed '
  :L
# append next line
# (Unix sed: N in the last line exits without default print, needs $b;N or $!N)
  $!N
# if the next line begins with 1, then
  /\n1,/ {
# print and delete the current line; D jumps to next cycle
    P;D
  }
# substitute embedded newline with |
  s/\n/|/
# if successful then jump to :L
  tL
# default print happens here
' file

sed -n '/^1/ {1h; 1! {x; s/\n//g; p; }}; /^2/ {s/^/|/; H; }; $ {x; s/\n//g; p; } ' file

{1h; 1!  ---->

1h

1!

 
 /^2/ {s/^/|/; H; ----> this part select line starting with 2 and appending pipe"|" at the starting.
H is putting this in holding area

 
 s/\n//g ---> this part replace newline char with empty line

 
 cat v
a\nb\nc\nd 
a b c d 
\n
xyx 
 xyz

sed 's/\n//g' v

line1

^line1$

line1\nline2

^line1\nline2$

sed -n '/^1/bL; {s/^/|/;H;}; ${:L;x;s/\n//gp;}' file