Insert lines in a file starting from a specific line - bash

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

Answer 2

Or anoter one example with the sed:
Prepare a test.txt file:
echo -e "line 1\nline 2\nline 3\nline 4" > /tmp/test.txt
cat /tmp/test.txt
line 1
line 2
line 3
line 4
Add a new line into the test.txt file:
sed -i '2 a line 2.5' /tmp/test.txt
# sed for in-place editing (-i) of the file: 'LINE_NUMBER a-ppend TEXT_TO_ADD'
cat /tmp/test.txt
line 1
line 2
line 2.5
line 3
line 4

Answer 3

This is definitely a case where you want to use something like sed (or awk or perl) rather than readling one line at a time in a shell loop. This is not the sort of thing the shell does well or efficiently.
You might find it handy to write a reusable function. Here's a simple one, though it won't work on fully-arbitrary text (slashes or regular expression metacharacters will confuse things):
function insertAfter # file line newText
{
local file="$1" line="$2" newText="$3"
sed -i -e "/^$line$/a"$'\\\n'"$newText"$'\n' "$file"
}
Example:
$ cat foo.txt
Now is the time for all good men to come to the aid of their party.
The quick brown fox jumps over a lazy dog.
$ insertAfter foo.txt \
"Now is the time for all good men to come to the aid of their party." \
"The previous line is missing 'bjkquvxz.'"
$ cat foo.txt
Now is the time for all good men to come to the aid of their party.
The previous line is missing 'bjkquvxz.'
The quick brown fox jumps over a lazy dog.
$

Answer 4

sed is your friend:
:~$ cat text.txt
foo
bar
baz
~$
~$ sed '/^bar/\na this is the new line/' text.txt > new_text.txt
~$ cat new_text.txt
foo
bar
this is the new line
baz
~$