Question

I would like to insert lines into a file in bash starting from a specific line.

Each line is a string which is an element of an array

line[0]="foo"
line[1]="bar"
...

and the specific line is ‘fields’

file="$(cat $myfile)"
for p in $file; do
    if [ "$p" = 'fields' ]
        then insertlines()     #<- here
    fi
done

Answer 1

This can be done with sed: sed 's/fields/fields\nNew Inserted Line/'

$ cat file.txt 
line 1
line 2 
fields
line 3
another line 
fields
dkhs

$ sed 's/fields/fields\nNew Inserted Line/' file.txt 
line 1
line 2 
fields
New Inserted Line
line 3
another line 
fields
New Inserted Line
dkhs

Use -i to save in-place instead of printing to stdout

sed -i 's/fields/fields\nNew Inserted Line/'

As a bash script:

#!/bin/bash

match='fields'
insert='New Inserted Line'
file='file.txt'

sed -i "s/$match/$match\n$insert/" $file