Shell

grep is on every Unix machine and handles one file at a time. ripgrep (rg) is built for a source tree: it walks directories in parallel, skips what .gitignore skips, and uses a regex engine that cannot go exponential. Both match lines against a pattern; the differences that matter are in what they search by default and how fast they get through a tree.

./script.sh, exec ./script.sh, . ./script.sh and ./script.sh & look alike and do four different things. What differs each time is whether a new process exists, whose environment changes, what $? holds afterwards, and who gets the terminal’s signals.