A simple reimplemenation of grep that the rust docs use to help devs learn
cargo run sun poem.txt
The above will look for, and print to stdout, all lines in poem.txt that contain sun.
The environment variable CASE_INSENSITIVE can be set to ignore case during searches.
The value of CASE_INSENSITIVE does not matter, it only matters that it is set.
The lesson for it can be found here Reading previous lessons is recommended.