How can I swap two lines using sed (linux os)?

Question

Hello stack overflow,

Does anyone know how to replace 'line a' with 'line b' and 'line b' with 'line a' in a text file using the sed editor? I can see how to replace a line in the pattern space with a line that is in the hold space (i.e. /^Paco/x or /^Paco/g), but what if I want to take the line starting with Paco and replace it with the line starting with Vinh, and also take the line starting with Vinh and replace it with the line starting with Paco. Let's assume for starters that there is one line with Paco and one line with Vinh, and that the line Paco occurs before the line Vinh. Then we can move to the general case.

Thanks in advance,

David

Answer 1

A simple example from the GNU sed texinfo doc:

Note that on implementations other than GNU `sed' this script might
easily overflow internal buffers.

 #!/usr/bin/sed -nf

 # reverse all lines of input, i.e. first line became last, ...

 # from the second line, the buffer (which contains all previous lines)
 # is *appended* to current line, so, the order will be reversed
 1! G

 # on the last line we're done -- print everything
 $ p

 # store everything on the buffer again
 h

Answer 2

try this awk script.

s1="$1"
s2="$2"
awk -vs1="$s1" -vs2="$s2" '
{ a[++d]=$0 }
$0~s1{ h=$0;ind=d}
$0~s2{
 a[ind]=$0
 for(i=1;i<d;i++ ){ print a[i]}
 print h
 delete a;d=0;
}
END{ for(i=1;i<=d;i++ ){ print a[i] } }' file

output

$ cat file
1
2
3
4
5

$ bash test.sh 2 3
1
3
2
4
5

$ bash test.sh 1 4
4
2
3
1
5

Use sed (or not at all) for only simple substitution. Anything more complicated, use a programming language

Answer 3

#!/bin/sed -f
/^Paco/ {
:notdone
  N
  s/^\(Paco[^\n]*\)\(\n\([^\n]*\n\)*\)\(Vinh[^\n]*\)$/\4\2\1/
  t
  bnotdone
}

After matching /^Paco/ we read into the pattern buffer until s// succeeds (or EOF: the pattern buffer will be printed unchanged). Then we start over searching for /^Paco/.