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12. Executing commands

We have already used stack exec multiple times in this guide. As you have likely already guessed, it allows you to run executables, but with a slightly modified environment.

The stack exec command

In particular: stack exec looks for executables on Stack's bin paths, and sets a few additional environment variables (like adding those paths to the PATH, and setting GHC_PACKAGE_PATH, which tells GHC which package databases to use).

If you want to see exactly what the modified environment looks like, try command:

stack exec env

The only issue is how to distinguish flags to be passed to Stack versus those for the underlying program. Thanks to the optparse-applicative library, Stack follows the Unix convention of -- to separate these. For example, command:

stack exec --package stm -- echo I installed the stm package via --package stm

yields output like:

Writing the configuration file for the implicit global project to:
.../global-project/stack.yaml. Note: You can change the snapshot via the
snapshot key there.
Using the latest snapshot lts-24.24.
I installed the stm package via --package stm

Flags worth mentioning:

  • --package foo can be used to force a package to be installed before running the given command.
  • --no-ghc-package-path can be used to stop the GHC_PACKAGE_PATH environment variable from being set. Some tools — notably Cabal (the tool) — do not behave well with that variable set.

You may also find it convenient to use stack exec to launch a subshell (substitute bash with your preferred shell) where your compiled executable is available at the front of your PATH. Command:

stack exec bash

The stack ghc and stack runghc commands

You will sometimes want to just compile (or run) a single Haskell source file, instead of creating an entire Cabal package for it. You can use stack exec ghc or stack exec runghc for that. As simple helpers, we also provide the stack ghc and stack runghc commands, for these common cases.