Delete newline characters between patterns foo I love Stack Exchange bar Some junk lines foo Welcome to the world of Bash bar foo Elon Musk rocks Yes, there is no `bar` complementing `foo` at the end of the file The required output is I love Stack Exchange Welcome to the world of Bash Elon Musk rocks So, remove all newline characters, between `^foo` and `^bar` or between `^foo` and the `end of file` where there is no `bar`

With `awk`:


awk '
$0 == "foo" {if (sep) print ""; sep = ""; inside = 1; next}
$0 == "bar" {inside = 0; next}
inside {printf "%s", sep $0; sep = " "}
END {if (sep) print ""}'


To match on lines with `foo` as the first word, replace `$0 == "foo"` with `$1 == "foo"`; to match on lines starting with `foo`, replace with `/^foo/` (short for `$0 ~ /^foo/`).