How to Use Pre and Post Install Hooks to Run Custom Scripts When Installing MongoDB with m

The m MongoDB version manager executes custom scripts automatically before and after installation by reading executable absolute paths from $M_DIR/pre_install and $M_DIR/post_install files.

The aheckmann/m repository provides a lightweight Bash-based version manager for MongoDB that supports pre and post install hooks for automation. This hook system allows you to run arbitrary scripts during the installation lifecycle, enabling tasks like logging, environment validation, or notifications without manual intervention.

Understanding the Hook Architecture

Hooks in m are stored as plain-text lists of executable paths inside the m data directory ($M_DIR), which defaults to $HOME/.local/m. The system recognizes two lifecycle events for hooks: install (triggered when running m install <version> or m <version>) and change (triggered when switching active versions via m as <version>).

For the install event, m looks for two specific files:

• $M_DIR/pre_install – scripts to execute before downloading the MongoDB binary
• $M_DIR/post_install – scripts to execute after successful installation

According to the source code in bin/m, the validate_event function ensures only install and change are accepted as valid hook events. Each script path listed in these files must be executable (chmod +x) and specified as an absolute path to prevent execution of untrusted or ambiguous code.

Registering Pre and Post Install Hooks

You register hooks using the m pre and m post subcommands followed by the event type and absolute script path. The install_hook function (lines 335–395 in bin/m) handles appending validated paths to the appropriate hook file.

# Register a script to run before installation

m pre install /home/user/scripts/prepare_env.sh

# Register a script to run after installation

m post install /home/user/scripts/notify_complete.sh

Scripts must meet strict validation criteria enforced by the hook system. If a script is not executable or uses a relative path, m will reject the registration and exit with an error. This security measure ensures that only explicitly trusted code runs during the installation process.

Managing and Listing Existing Hooks

To view currently registered hooks, invoke the command without a script path. The list_pres() function (lines 40–48 in bin/m) and its counterpart list_posts() display the contents of the respective hook files:

# List all pre-install hooks

m pre install

# List all post-install hooks

m post install

Removing hooks follows a similar syntax. To delete a specific hook, append rm followed by the absolute path. To remove all hooks for an event, use rm without specifying a path:

# Remove a specific pre-install hook

m pre install rm /home/user/scripts/prepare_env.sh

# Remove ALL post-install hooks (empties the file)

m post install rm

Execution Flow During Installation

When you execute m 7.0.14 or m install 7.0.14, the main entry point in bin/m orchestrates the following sequence:

1. Pre-install phase: The pre() function (lines 12–20) reads $M_DIR/pre_install line-by-line and executes each script synchronously, waiting for completion before proceeding.
2. Installation phase: m downloads and extracts the requested MongoDB binary to $M_DIR/versions/.
3. Post-install phase: The post() function reads $M_DIR/post_install and executes each registered script, passing the installed version number as an argument.

Because hooks run synchronously, a failing pre-install hook will halt the installation process entirely, while a failing post-install hook will report errors but leave the installation intact.

Practical Example: Logging MongoDB Installations

The following workflow demonstrates setting up hooks that log installation timestamps and print completion messages:

# 1. Create the pre-install logging script

echo '#!/usr/bin/env bash' > $HOME/scripts/m_install_log.sh
echo 'date >> $HOME/.local/m/install.log' >> $HOME/scripts/m_install_log.sh
chmod +x $HOME/scripts/m_install_log.sh

# 2. Register as pre-install hook

m pre install $HOME/scripts/m_install_log.sh

# 3. Verify registration

m pre install

# 4. Create post-install notification script

echo '#!/usr/bin/env bash' > $HOME/scripts/m_install_done.sh
echo 'echo "MongoDB $1 installed successfully!"' >> $HOME/scripts/m_install_done.sh
chmod +x $HOME/scripts/m_install_done.sh
m post install $HOME/scripts/m_install_done.sh

# 5. Install a version to trigger both hooks

m 7.0.14

After running m 7.0.14, you will see the date appended to install.log before the download begins, followed by the success message "MongoDB 7.0.14 installed successfully!" after completion.

Summary

• Hook storage: Scripts are listed in $M_DIR/pre_install and $M_DIR/post_install as absolute paths.
• CLI management: Use m pre install <path> and m post install <path> to add hooks; use rm to remove them.
• Security requirements: Scripts must be executable and use absolute paths, enforced by the validate_event and install_hook functions in bin/m.
• Execution order: The pre() function runs before download, then installation occurs, then the post() function runs with the version number as an argument.
• Synchronous operation: m waits for each hook to complete before proceeding, ensuring deterministic installation behavior.

Frequently Asked Questions

Can I use relative paths like ~/scripts/hook.sh when registering hooks?

No. The install_hook function in bin/m explicitly requires absolute paths (e.g., /home/username/scripts/hook.sh) to prevent ambiguity about which script should execute. Relative paths will be rejected during registration.

Do hooks run for the change event as well as install?

Yes. The hook system supports both install and change events. When switching active MongoDB versions using m as <version>, m checks for pre_change and post_change files in $M_DIR and executes them using the same pre() and post() functions.

What happens if a pre-install hook fails or returns a non-zero exit code?

Because hooks execute synchronously, a failing pre-install hook (non-zero exit status) will halt the entire installation process immediately. The MongoDB binary will not be downloaded, and no post-install hooks will run. Post-install hook failures are logged but do not rollback the installation.

How can I view the raw hook files directly?

The hook files are plain text stored in your m data directory. You can inspect them directly with:

cat "$HOME/.local/m/pre_install"
cat "$HOME/.local/m/post_install"

These files contain one absolute script path per line, matching the output of m pre install and m post install.

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