Catkin creates and installs files for env-setting convenience. In the buildspace there are setup.zsh and setup.bash, these contain variable zsh and bash specific tweaks. They both import setup.sh, which contains bourne-shell variable settings.
env.sh, pointed to by the cmake variable CATKIN_ENV is special; it executes its arguments in the environment created by setup.sh and returns. Any custom commands executed by cmake should do so via this script. If executed without arguments, it will spawn a subshell (using the current login shell, or $SHELL) in the development environment.
Warning
That env.sh executes a subshell means that your shell initialization files will be read after the environment is set by catkin’s env hooks; this means that what these files want to do may get clobbered. If you want this entering- and exiting of subshells mechanism to work correctly, be careful what you do in your .bashrc or .zshrc. Understand what your shell does when it is invoked both interactively and non-interactively. The simpler mechanism is simply to ‘source’ the appropriate setup file, provided you are satisfied with the irreversible changes that these files make to your current environment. All depends on your workflow.
Environment hooks
Projects can, via the catkin_add_env_hooks() macro, add sh code to be executed by setup.sh (and by extension setup.bash and friends). If you need to add things to the environment, this is probably the place to do it. Don’t get fancy: the contents of these scripts must be interpretable by all members of the bourne shell family. Be safe and ensure that /bin/dash is okay with them.
NOTE: These environment hooks are only for variable settings. Shell aliases and functions
