5. External Commands

cabal-install provides a system for external commands, akin to the ones used by tools like git or cargo.

If you execute cabal <cmd>, cabal-install will search the path for an executable named cabal-<cmd> and execute it. An error will be thrown in case the custom command is not found. The exit code of cabal when calling an external command is the same as the exit code of the command.

The name of the command is not passed as the first argument as is done in cargo, instead you will have to figure out the name via argv[0] as is the case in e.g. git.

The $CABAL_EXTERNAL_CABAL_PATH environment variable is set to the path of the cabal-install executable which invoked the subcommand.

It is strongly recommended that you implement your custom commands by calling the CLI via the $CABAL_EXTERNAL_CABAL_PATH variable rather than linking against the Cabal library. There is no guarantee that the subcommand will link against the same version of the Cabal library as cabal-install so it would lead to unexpected results and incompatibilities.

Historically, the cabal-install binary would pass the name of the executable which it is trying to invoke via the external command feature as the first argument to the executable itself. The main difference was that $CABAL_EXTERNAL_CABAL_PATH was called $CABAL, which means that you can stay compatible with both versions, depending on which variable is set.

Mind that if you were implementing external commands previously, you will not need to skip the first argument (the executable name) anymore.

cabal-install can also display the help message of the external command. When cabal help <cmd> is invoked, then cabal-<cmd> <cmd> --help will be called so your external command can display a help message.

For ideas or existing external commands, visit this Discourse thread.