Created MacOS login shell (markdown)

MacOS-login-shell.md

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+# Configure the terminal to run your shell as a login shell
+
+The stock shell configuration examples for MacOS assume that the shell in the terminal runs as a login shell.
+
+## Check if the current shell is a login shell
+
+* Bash:
+
+  ```bash
+  shopt -q login_shell && echo 'Login shell' || echo 'Not login shell'
+  ```
+* Zsh:
+
+  ```zsh
+  if [[ -o login ]]; then echo "login shell"; else echo "not login shell"; fi
+  ```
+
+* Fish:
+
+  ```fish
+  status --is-login; and echo "login shell"; or echo "not login shell"
+  ```
+
+## Set the shell to run as a login shell
+
+There are two possible ways here.
+
+### Set the shell as your login shell (recommended)
+
+* `System Preferences` > `Users and groups` > Open the padlock to allow changes >
+Right click on the corresponding user's name > `Advanced options` > Select the
+shell from the dropdown list next to `Login shell`
+
+* Then, in the Terminal app: `Preferences` > `General` > `Open shells with` > `Predefined login shell`
+
+### Configure the terminal to explicitly run a shell as a login shell
+
+* In the Terminal app: `Preferences` > `General` > `Open shells with` > `Command (full path)`
+
+  * Zsh: `/bin/zsh --login`
+  * Bash: `/bin/bash -l`
+  * Fish: `<path to fish executable> --login`