Insert line after every match of a specific pattern in file, using ansible or unix commands

Question

I am trying to insert a line after each occurrence of a specific string in a text file. I tried with the lineinfile ansible module but it can only insert a line after the last or the first instance of the matched string in a file. Does anyone have any idea how I can achieve this using shell scripting or ansible playbook. I am using linux operating system so any unix command to solve this will help.

As shown below I am trying to insert mba after every line starting with foo:

Original txt file contents:

foo
ab
zz
foo='dummy'
cd
zz
foo='dats'
lm
xx

Required output:

foo
mba
ab
zz
foo='dummy'
mba
cd
zz
foo='dats'
mba
lm
xx

Answer 1

An awk-based solution might look as follows:

awk '{print} /^foo/{print "mba"}' input.txt

This will print the current line by default, but if it happens to start with foo, it will also print mba on an additional line before processing proceeds to the next line.

Answer 2

Q: "Insert 'mba' after every line starting with text 'foo'."

A: Use replace in Ansible, e.g. given the file

shell> cat foo.txt
foo
ab
zz
foo='dummy'
cd
zz
foo='dats'
lm
xx

the task

    - replace:
        path: foo.txt
        regexp: '^foo(.*)$'
        replace: |-
          foo\1
          mba

gives

shell> cat foo.txt
foo
mba
ab
zz
foo='dummy'
mba
cd
zz
foo='dats'
mba
lm
xx

---

The task below gives the same result and is idempotent

    - replace:
        path: foo.txt
        regexp: 'foo(.*)\n(?!mba)(.*)'
        replace: |-
          foo\1
          mba
          \2

Answer 3

A couple of choices:

• perl

  $ perl -pe 's/^foo.*/$&\nmg/' file
  foo
  mg
  ab
  zz
  foo='dummy'
  mg
  cd
  zz
  foo='dats'
  mg
  lm
  xx
  

• awk

  $ awk '1;/^foo/{print "mg" }' file
  foo
  mg
  ab
  zz
  foo='dummy'
  mg
  cd
  zz
  foo='dats'
  mg
  lm
  xx
  

Answer 4

awk

awk '/foo/ {$0 = $0"\nmba"}1' $file

sed

sed 's/foo.*/&\nmba/' $file

foo
mba
ab
zz
foo='dummy'
mba
cd
zz
foo='dats'
mba
lm
xx