# Source Code ## Encoding Epsil's JavaScript API accepts a string. A host reading an Epsil source file should decode it as UTF-8 and should write identifiers in [Unicode NFC form](https://www.unicode.org/reports/tr15/tr15-50.html), as required by the MathJSON symbol contract. The Epsil parser does not decode files or strip a byte-order mark. File I/O and decoding are the responsibility of the host. Inside a string literal, Unicode code points can also be written with [escape sequences](/epsil/literals/#escape-sequence). ## File Extension The conventional file extension is `.epsil`. ## MIME-type The project uses `text/epsil` as its media-type convention. It is not a registered IANA media type. ## Command line Installing `@cortex-js/compute-engine` provides the `epsil` command: ```shell epsil --eval "1 + 2" epsil program.epsil epsil --json program.epsil ``` With no file or `--eval`, `epsil` starts an interactive REPL when standard input is a terminal; otherwise it reads a program from standard input. The command applies a 10-second evaluation limit by default. Use `--time-limit ` to change it or `--time-limit 0` to disable it. Run `epsil --help` for the complete option list. See [Epsil CLI](/epsil/cli/) for installation, output modes, REPL commands, diagnostics, and exit-status behavior. ## Hashbang Comment A hashbang comment can appear at the absolute start of the source and is ignored by the Epsil parser. It can be used to run an executable source file through the installed command: ```epsil #!/usr/bin/env epsil ```