How to Use Pre/Post Hooks When Switching MongoDB Versions with m

m supports pre and post hooks that execute custom scripts automatically when you install or change MongoDB versions, enabling automation of setup and teardown tasks.

The m version manager for MongoDB (maintained in the aheckmann/m repository) provides a powerful hook system that runs arbitrary executable scripts at specific lifecycle events. This allows you to automate tasks like stopping services before a version switch or starting them afterward.

Understanding Pre and Post Hooks in m

Hooks in m are executable scripts registered to run either before (pre) or after (post) specific events. The system supports two primary events:

• install – Triggered when a new MongoDB version is downloaded and installed
• change – Triggered when switching the active MongoDB version (creating symlinks)

When registered, m stores the absolute path to your script in plain-text files located in the internal data directory ($M_DIR, defaulting to ~/.local/m). Hook files follow the naming convention pre_<event> and post_<event> (for example, pre_change or post_install).

Registering Hooks with m pre and m post Commands

The install_hook function in bin/m (lines 1735–1765) handles hook registration. When you run m pre <event> <script> or m post <event> <script>, m validates that the script path is absolute and executable before writing it to the hook file.

Hook Events and Validation

Valid events are strictly install or change. The system validates:

• The script path must be absolute (not relative)
• The script must have executable permissions
• Duplicate entries are automatically ignored

Storage Location

Hook files reside in $M_DIR, which derives from the M_PREFIX environment variable (default: $HOME/.local/m). A typical hook file path looks like ~/.local/m/pre_change.

How Hooks Execute During Version Switches

Hook execution occurs at specific points in the version activation flow defined in bin/m. The pre() and post() functions (lines 1812–1820 and 1826–1834) read their respective hook files line-by-line and execute each script.

Execution Points in the Activation Flow

When switching versions, m triggers hooks at these exact locations:

• Line 986: pre change runs immediately before creating symlinks to the new version
• Line 995: post change runs immediately after the symlinks are created

The same pattern applies to the install event, allowing you to run scripts before and after downloading and extracting a MongoDB binary.

Managing and Removing Hooks

The hook system provides commands to list and remove registered scripts.

Listing Registered Hooks

Run m pre <event> or m post <event> without specifying a script to display all registered hooks for that event. The list_pres and list_posts helper functions read and display the contents of the hook files.

Removing Hooks

Remove hooks using the rm keyword:

# Remove a specific post-change hook

m post change rm /home/user/scripts/cleanup.sh

# Remove all pre-install hooks

m pre install rm

The removal logic (lines 1756–1773 in bin/m) rewrites the hook file, skipping any line matching the target script path.

Practical Examples

Here are complete examples for automating MongoDB version switches with hooks.

Stopping Services Before Switching

Create a pre-change hook to gracefully stop mongod before the version changes:

#!/usr/bin/env bash

# /home/user/scripts/stop-mongod.sh

pkill -f mongod && echo "mongod stopped"

Register it:

chmod +x /home/user/scripts/stop-mongod.sh
m pre change /home/user/scripts/stop-mongod.sh

Starting Services After Switching

Create a post-change hook to start the new version:

#!/usr/bin/env bash

# /home/user/scripts/start-mongod.sh

"$M_BIN_DIR/mongod" --dbpath /data/db --fork --logpath /var/log/mongod.log

Register it:

chmod +x /home/user/scripts/start-mongod.sh
m post change /home/user/scripts/start-mongod.sh

Automating Installation Cleanup

Add a post-install hook to remove downloaded archives after installation:

#!/usr/bin/env bash

# /home/user/scripts/cleanup-install.sh

rm -f "$M_DIR/mongodb-*.tgz"
echo "Cleaned up installation archives"

chmod +x /home/user/scripts/cleanup-install.sh
m post install /home/user/scripts/cleanup-install.sh

Summary

• m provides pre and post hooks for the install and change events, allowing automatic script execution during MongoDB version management.
• Register hooks using m pre <event> <script> or m post <event> <script>; scripts must be absolute paths with executable permissions.
• Hooks execute at specific points in bin/m: pre change runs at line 986 before symlinking, and post change runs at line 995 after symlinking.
• Store hooks in $M_DIR (default ~/.local/m) in files named pre_<event> and post_<event>.
• Manage hooks by listing (m pre <event>) or removing (m pre <event> rm <script>) entries.

Frequently Asked Questions

What events support pre and post hooks in m?

m supports hooks for two events: install (triggered when downloading and installing a new MongoDB version) and change (triggered when switching the active MongoDB version). You can register both pre and post hooks for either event using m pre <event> <script> or m post <event> <script>.

Where does m store registered hook scripts?

m stores hook definitions in plain-text files inside the $M_DIR directory, which defaults to ~/.local/m unless overridden by the M_PREFIX environment variable. Each event gets two files: pre_<event> and post_<event> (for example, pre_change and post_change). These files contain absolute paths to the executable scripts, one per line.

How do I remove a specific hook without deleting all of them?

To remove a specific hook, run m pre <event> rm <script> or m post <event> rm <script>, replacing <script> with the exact absolute path of the script you want to remove. If you want to clear all hooks for an event, use m pre <event> rm or m post <event> rm without specifying a script path. The removal logic rewrites the hook file, excluding the targeted entry.

Can I use relative paths when registering hook scripts?

No, m requires absolute paths when registering hooks. The install_hook function in bin/m validates that the provided script path is absolute and that the file has executable permissions before writing it to the hook file. If you provide a relative path, the registration will fail. Always use the full path (e.g., /home/user/scripts/hook.sh rather than ./hook.sh).

{
  "mcpServers": {
    "instagit": {
      "command": "npx",
      "args": ["-y", "instagit@latest"]
    }
  }
}