Add/edit line text in file without open editor (linux command)

Question

I know how to add new text to a file, but how can I edit it?

Example: adding hello_world = 1 to test.txt using the following command:

echo "hello_world = 1" >> test.txt

But how can I change 1 to 0 or something else?

Answer 1

Using sed:

sed -i 's/1/0/g' test.txt

In general:

sed -i 's/oldstring/newstring/g' filename

See man sed for more info.

Answer 2

Through awk,

awk '{sub(/1/,"0")}1' infile > outfile

Example:

$ echo 'hello_world = 1' | awk '{sub(/1/,"0")}1'
hello_world = 0