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You can use Vim in Ex mode:

ex -s -c '1i|task goes here' -c x todo.txt

1. 1 select first line

2. i insert

3. x save and close

You can use the POSIX tool ex:

ex a.txt <<eof
1 insert
Sunday
.
xit
eof

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/ex.html

You can use Vim in Ex mode:

ex -sc '1i|task goes here' -cx todo.txt

1. 1 select first line

2. i insert

3. x save and close

You can use Vim in Ex mode:

ex -s -c '1i|task goes here' -c x todo.txt

1. 1 select first line

2. i insert

3. x save and close

You can use Vim in Ex mode:

ex -s -c '1i|task goes here' -c x alfa.txt

1. 1 select first line

2. i insert

3. x save and close

You can use Vim in Ex mode:

ex -sc '1i|task goes here' -cx todo.txt

1. 1 select first line

2. i insert

3. x save and close