linux shell: prepend or append text to next line after matching line -

using shell script in linux (bash or csh). find lines match given text pattern, , each line prepend or append (depending on application) text following line.

for example, file containing lines this:

header 1  12345  header 2  12345  header 1  12345  header 2  12345 

if searching "header 1", , append text "abcde", output this:

header 1  12345 abcde  header 2  12345  header 1  12345 abcde  header 2  12345 

i have cases want abcde preceding 12345.

i have been trying understand usage of sed purpose, feel stymied. pointers appreciated.

try command:

sed '/header 1/{n;s/$/ abcde/}' input.txt 

---

this logic:

for each line in 'input.txt'         if line matches /header 1/         read next line         append string ' abcde'     endif     endfor