Bash replace or create line in file

Question

What would be the best way of going about this.

If a line starts with a string, replace that whole line with new string If no line in the file starts with that string, append it to the file

Ex:

file.txt:

FOO=blah
BAR=bleh
GIN=blop

How can I replace the FOO line if it exists or append to the file if no line starts with FOO?

Possible [Duplicate] for [https://stackoverflow.com/questions/11145270/](https://stackoverflow.com/questions/11145270/how-to-replace-an-entire-line-in-a-text-file-by-line-number) — Nicolás Alarcón Rapela, Jun 15 '18 at 09:21
@NicolásAlarcónR. That's "by line number" so not a very good dupe. — tripleee, Jun 15 '18 at 09:30

score 0 · Answer 1 · answered Jun 15 '18 at 09:33

You can try this command,

if grep -q -F 'FOO' file.txt ; then sed -i 's/FOO/newline/g' file.txt; else echo newline >> file.txt; fi

or (depending or your version of sed)

if grep -q -F 'FOO' file.txt ; then sed -i '' 's/FOO/newline/g' file.txt; else echo newline >> file.txt; fi

1 Answers1