At the risk of stating the obvious: macOS has its own way of doing things. When I wanted to run a scheduled script using Cron, the standard response seemed to be that Crontab is deprecated and to use launchd instead.

sigh

There are two main types of Jobs that can be run through launchd:

  • Agents - Which run as the currently logged in user
  • Daemons - Which run as any user specified (such as root)

Jobs are defined in terms of plist files. plist files are XML files which can be registered with the launchd system. There’s an excellent guide at launchd.info on how to setup all things launchd if you want to know more.

Below are the steps required to setup a simple launchd Agent to run a script at a given time each day.

Define the Job

Agent Job definitions are stored at ~/Library/LaunchAgents. The file name of Job definition file is of the form:

<NAMESPACE>.<SCRIPT>.plist

The NAMESPACE can be something unique to your computer such as a reversed domain name. SCRIPT is the name of your script.

Note: This is the format I use. Feel free to change this up as you like.

For example to create a backup script for a computer at machinex.ssanj.net, we would create the following plist:

~/Library/LaunchAgents/net.ssanj.machinex.backup.plist
Here is a simple Job definition for a script that should run daily at some time: 
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>Label</key>
        <string>NAMESPACE.SCRIPT</string>
        <key>WorkingDirectory</key>
        <string>YOUR_WORKING_DIRECTORY</string>
        <key>StandardOutPath</key>
        <string>PATH_TO_YOUR_STDOUT_FILE</string>
        <key>StandardErrorPath</key>
        <string>PATH_TO_YOUR_STDERR_FILE</string>
        <key>ProgramArguments</key>
        <array>
            <string>PATH_TO_YOUR_SCRIPT</string>
        </array>
        <key>StartCalendarInterval</key>
        <dict>
            <key>Hour</key>
            <integer>HOUR_TO_RUN_AT</integer>
            <key>Minute</key>
            <integer>MINUTE_WIHIN_HOUR_TO_RUN_AT</integer>
        </dict>
</dict>
</plist>
For example: 
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>Label</key>
        <string>net.ssanj.machinex.backup</string>
        <key>WorkingDirectory</key>
        <string>/Users/sanj/backups</string>
        <key>StandardOutPath</key>
        <string>/Users/sanj/backups/logs/backup.stdout</string>
        <key>StandardErrorPath</key>
        <string>/Users/sanj/backups/logs/backup.stderr</string>
        <key>ProgramArguments</key>
        <array>
            <string>/Users/sanj/backups/backup.sh</string>
        </array>
        <key>StartCalendarInterval</key>
        <dict>
            <key>Hour</key>
            <integer>10</integer>
            <key>Minute</key>
            <integer>30</integer>
        </dict>
        <key>EnvironmentVariables</key>
        <dict>
             <key>PATH</key>
             <string>/opt/homebrew/bin:/opt/homebrew/sbin:/Users/sanj/bin:/bin:/usr/bin:/usr/local/bin</string>
        </dict>
</dict>
</plist>

There are many more parameters that can be customised to suite your needs. Have a look at launchd.info for more.

Job Manipulation

You can manipulate Job characteristics using the launchctl command.

Activate Job

To activate the Job use the load subcommand:

launchctl load ~/Library/LaunchAgents/<NAMESPACE>.<SCRIPT>.plist

Verify Job

To verify the Job is activated use the list subcommand:

launchctl list <NAMESPACE>.<SCRIPT>

If this fails ensure you have loaded the correct Job and there are no issues with the plist file.

Deactivate Job

To deactivate the Job use the unload subcommand:

launchctl unload ~/Library/LaunchAgents/<NAMESPACE>.<SCRIPT>.plist

Troubleshooting

Ensure to check your StandardOutPath and StandardErrorPath files for any errors should your script fail to run as expected. Also check that your script is correctly defined in the plist file and has appropriate execution rights etc.

Godspeed