How to Use the Command 'fswatch' (with examples)

fswatch is a powerful and flexible command-line tool designed to monitor file changes across platforms such as Linux, macOS, and BSD. It listens for changes in files and directories and can trigger specific actions when modifications are detected. Whether you’re a developer monitoring source files, a system administrator keeping an eye on configuration files, or just someone interested in tracking changes to important documents, fswatch provides a comprehensive solution for all these needs.

Use case 1: Run a Bash command on file creation, update, or deletion

Code:

fswatch path/to/file | xargs -n 1 bash_command

Motivation:

This use case is particularly useful for developers or system administrators who need to automate tasks in response to file changes. For example, if a configuration file is updated, an immediate action may be required to reload a service or compile new code automatically. This automation saves time and reduces the risk of manual errors.

Explanation:

• fswatch initiates the monitoring process on the specified path/to/file.
• The | character pipes the output from fswatch to another command.
• xargs takes input from fswatch and executes bash_command each time a change is detected.
• -n 1 ensures bash_command is executed once per input line.

Example Output:

Suppose bash_command is set to update a log file:

[INFO] File at path/to/file was modified
[INFO] File at path/to/file was modified

Use case 2: Watch one or more files and/or directories

Code:

fswatch path/to/file path/to/directory path/to/another_directory/**/*.js | xargs -n 1 bash_command

Motivation:

Monitoring multiple files and directories is essential in large projects where changes can occur in various locations simultaneously. Whether it’s JavaScript files in a directory or configuration settings spread across different folders, fswatch can handle these more complex monitoring needs effortlessly.

Explanation:

• fswatch is set to observe multiple paths: path/to/file, path/to/directory, and all .js files under path/to/another_directory, specified using the **/*.js wildcard for recursive file inclusion.
• As changes occur in the monitored locations, they are piped into xargs.
• xargs -n 1 bash_command executes the specified bash_command for each change detected, once per input line.

Example Output:

Upon watching changes in a set of JavaScript files:

[INFO] JavaScript file at path/to/another_directory/subdir/a.js was modified
[INFO] JavaScript file at path/to/another_directory/subdir/b.js was deleted

Use case 3: Print the absolute paths of the changed files

Code:

fswatch path/to/directory | xargs -n 1 -I {} echo {}

Motivation:

Sometimes, it’s valuable to have a record of files that were altered without needing to take immediate action. This might be useful for auditing purposes, maintaining records of file operations in a system, or simply for debugging when trying to trace the source of unexpected changes.

Explanation:

• fswatch monitors path/to/directory for any modifications.
• The output is directed through a pipe to xargs.
• xargs -n 1 -I {} specifies a placeholder {} that will be replaced with each detected event.
• echo {} displays the path of each changed file.

Example Output:

As files change in the given directory, it logs their paths:

/full/path/to/directory/file1.txt
/full/path/to/directory/file2.txt

Use case 4: Filter by event type

Code:

fswatch --event Updated|Deleted|Created path/to/directory | xargs -n 1 bash_command

Motivation:

Filtering events is critical when only certain types of changes matter. For instance, a team might only be concerned about deletions of important data files or the creation of new user accounts in a directory. With this filtering capability, fswatch can prioritize specific actions based on the occurrence of these targeted events.

Explanation:

• fswatch starts watching for changes within path/to/directory.
• The --event option filters changes based on specified types: Updated, Deleted, and Created.
• Upon detection of these specific event types, the results are piped to xargs.
• xargs -n 1 bash_command ensures that bash_command is invoked for each significant event.

Example Output:

If the specified event is a file update:

File /path/to/directory/file3.txt was updated

Conclusion:

The fswatch tool stands out with its ability to monitor files across platforms and execute commands based on varied change types. With these illustrative examples, users can harness fswatch for numerous applications, from automating workflow tasks and improving productivity to tracking important file modifications seamlessly.